WEATHER TEORY

8/7/2019 WEATHER TEORY

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Whether preparing for a local flight or a longcross-country, flight-planning decisions based onweather can dramatically affect the safety of the flight.A solid understanding of weather theory provides thetools necessary to understand the reports and forecastsobtained from a Flight Service Station weatherspecialist and other aviation weather services.

This chapter is designed to help pilots acquire thebackground knowledge of weather principles necessary

to develop sound decision making skills relating toweather. It is important to note, however, that there isno substitute for experience.

NATURE OF THE ATMOSPHEREThe atmosphere is a mixture of gases that surround theEarth. This blanket of gases provides protection fromultraviolet rays as well as supporting human, animal,and plant life on the planet. Nitrogen accounts for 78percent of the gases that comprise the atmosphere,while oxygen makes up 21 percent. Argon, carbondioxide, and traces of other gases make up the remain-ing 1 percent. [Figure 10-1]

Within this envelope of gases, there are severalrecognizable layers of the atmosphere that are definednot only by altitude, but also by the specificcharacteristics of that level. [Figure 10-2]

The first layer, known as the troposphere , extendsfrom sea level up to 20,000 feet (8 km) over thenorthern and southern poles and up to 48,000 feet (14.5km) over the equatorial regions. The vast majority of weather, clouds, storms, and temperature variances

1% Trace Gases

21% Oxygen

78% Nitrogen

Figure 10-1. Composition of the atmosphere.

Troposphere—The layer of the atmosphere extending from the surfaceto a height of 20,000 to 60,000 feet depending on latitude.

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occur within this first layer of the atmosphere. Insidethe troposphere, the temperature decreases at a rate of about 2 ° Celsius every 1,000 feet of altitude gain, andthe pressure decreases at a rate of about 1 inch per1,000 feet of altitude gain. At the top of the troposphereis a boundary known as the tropopause , which trapsmoisture, and the associated weather, in thetroposphere. The altitude of the tropopause varies withlatitude and with the season of the year; therefore, ittakes on an elliptical shape, as opposed to round.Location of the tropopause is important because it iscommonly associated with the location of the

jetstream and possible clear air turbulence.

The atmospheric level above the tropopause is thestratosphere , which extends from the tropopause to a

height of about 160,000 feet (50 km). Little weatherexists in this layer and the air remains stable. At the topof the stratosphere is another boundary known as thestratopause, which exists at approximately 160,000feet. Directly above this is the mesosphere , whichextends to the mesopause boundary at about 280,000feet (85 km). The temperature in the mesospheredecreases rapidly with an increase in altitude and canbe as cold as –90 ° C. The last layer of the atmosphere isthe thermosphere . It starts above the mesosphere andgradually fades into outer space.

OXYGEN AND THE HUMAN BODYAs discussed earlier, nitrogen and other trace gasesmake up 79 percent of the atmosphere, while theremaining 21 percent is life sustaining, atmosphericoxygen. At sea level, atmospheric pressure is greatenough to support normal growth, activity, and life. At18,000 feet, however, the partial pressure of oxygen issignificantly reduced to the point that it adverselyaffects the normal activities and functioning of thehuman body. In fact, the reactions of the averageperson begin to be impaired at an altitude of about10,000 feet and for some people as low as 5,000 feet.The physiological reactions to oxygen deprivation areinsidious and affect people in different ways. Thesesymptoms range from mild disorientation to totalincapacitation, depending on body tolerance andaltitude.

Thermosphere

Mesosphere

Mesopause

Stratopause

Stratosphere

Troposphere

Tropopause

Figure 10-2. Layers of the atmosphere.

Tropopause—The boundary between the troposphere and thestratosphere which acts as a lid to confine most of the water vapor, andthe associated weather, to the troposphere.

Jetstream—A narrow band of wind with speeds of 100 to 200 m.p.h.usually associated with the tropopause.

Stratosphere—A layer of the atmosphere above the tropopauseextending to a height of approximately 160,000 feet.

Mesosphere—A layer of the atmosphere directly above the stratosphere.

Thermosphere—The last layer of the atmosphere that begins above themesosphere and gradually fades away into space.

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By using supplemental oxygen or cabin pressurizationsystems, pilots can fly at higher altitudes and overcomethe ill effects of oxygen deprivation.

SIGNIFICANCE OF ATMOSPHERICPRESSUREAt sea level, the atmosphere exerts pressure on the

Earth at a force of 14.7 pounds per square inch. Thismeans a column of air 1-inch square, extending fromthe surface up to the upper atmospheric limit, weighsabout 14.7 pounds. [Figure 10-3] A person standing atsea level also experiences the pressure of theatmosphere; however, the pressure is not a downwardforce, but rather a force of pressure over the entiresurface of the skin.

The actual pressure at a given place and time will differwith altitude, temperature, and density of the air. Theseconditions also affect aircraft performance, especiallywith regard to takeoff, rate of climb, and landings.

MEASUREMENT OF ATMOSPHERIC PRESSUREAtmospheric pressure is typically measured in inchesof mercury (in. Hg.) by a mercurial barometer. [Figure10-4] The barometer measures the height of a columnof mercury inside a glass tube. A section of the mercuryis exposed to the pressure of the atmosphere, whichexerts a force on the mercury. An increase in pressureforces the mercury to rise inside the tube; as pressuredrops, mercury drains out of the tube, decreasing theheight of the column. This type of barometer istypically used in a lab or weather observation station, isnot easily transported, and is a bit difficult to read.

An aneroid barometer is an alternative to a mercurialbarometer; it is easier to read and transport. [Figure10-5] The aneroid barometer contains a closed vessel,called an aneroid cell, that contracts or expands withchanges in pressure. The aneroid cell attaches to apressure indicator with a mechanical linkage to providepressure readings. The pressure sensing part of anaircraft altimeter is essentially an aneroid barometer. Itis important to note that due to the linkage mechanismof an aneroid barometer, it is not as accurate as amercurial barometer.

14.7 lbs.

Area =1Square Inch

Figure 10-3. One square inch of atmosphere weighsapproximately 14.7 pounds.

At sea level in astandard atmosphere,the weight of theatmosphere supportsa column of mercury29.92 inches high.

Heightof

Barometer29.92 inches

(760 mm)

Atmospheric Pressure

SeaLevel

29.92 in. Hg. = 1013.2 mb (hPa) = 14.7 lbs./in 2

Figure 10-4. Mercurial barometer.

Figure 10-5. Aneroid barometer.

Lower

Higher

SealedAneroid

Cell

AtmosphericPressure

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To provide a common reference for temperature andpressure the International Standard Atmosphere ( ISA )has been established. These standard conditions are thebasis for certain flight instruments and most airplaneperformance data. Standard sea level pressure isdefined as 29.92 in. Hg. at 59 ° F (15 ° C). Atmosphericpressure is also reported in millibars, with 1 inch of

mercury equaling approximately 34 millibars andstandard sea level equaling 1013.2 millibars. Typicalmillibar pressure readings range from 950.0 to 1040.0millibars. Constant pressure charts and hurricanepressure reports are written using millibars.

Since weather stations are located around the globe, alllocal barometric pressure readings are converted to asea level pressure to provide a standard for records andreports. To achieve this, each station converts itsbarometric pressure by adding approximately 1 inch of mercury for every 1,000 feet of elevation gain. Forexample, a station at 5,000 feet above sea level, with a

reading of 24.92 inches of mercury, reports a sea levelpressure reading of 29.92 inches. [Figure 10-6] Usingcommon sea level pressure readings helps ensureaircraft altimeters are set correctly, based on the currentpressure readings.

By tracking barometric pressure trends across a largearea, weather forecasters can more accurately predictmovement of pressure systems and the associatedweather. For example, tracking a pattern of risingpressure at a single weather station generally indicatesthe approach of fair weather. Conversely, decreasing orrapidly falling pressure usually indicates approachingbad weather and possibly, severe storms.

EFFECT OF ALTITUDE ON ATMOSPHERICPRESSUREAs altitude increases, pressure diminishes, as theweight of the air column decreases. On average, withevery 1,000 feet of altitude increase, the atmosphericpressure decreases 1 inch of mercury. This decrease inpressure (increase in density altitude) has a pronounced

effect on aircraft performance.

EFFECT OF ALTITUDE ON FLIGHTAltitude affects every aspect of flight from aircraftperformance to human performance. At higheraltitudes, with a decreased atmospheric pressure,takeoff and landing distances are increased, as areclimb rates.

When an aircraft takes off, lift must be developed bythe flow of air around the wings. If the air is thin, morespeed is required to obtain enough lift for takeoff;therefore, the ground run is longer. An aircraft thatrequires a 1,000-foot ground run at sea level willrequire almost double that at an airport 5,000 feetabove sea level [Figure 10-7]. It is also true that athigher altitudes, due to the decreased density of the air,aircraft engines and propellers are less efficient. Thisleads to reduced rates of climb and a greater ground runfor obstacle clearance.

Station Pressure(New Orleans) 29.92"

Standard Atmosphere

Station Pressure(Denver) 24.92"

New Orleans 29.92"Sea Level PressureDenver 29.92"

Figure 10-6. Station pressure is converted to, and reported in, sea level pressure.

ISA—International Standard Atmosphere: Standard atmosphericconditions consisting of a temperature of 59°F (15°C), and a baromet-ric pressure of 29.92 in. Hg. (1013.2 mb) at sea level. ISA values canbe calculated for various altitudes using standard lapse rate.

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EFFECT OF DIFFERENCES IN AIR DENSITYDifferences in air density caused by changes intemperature result in changes in pressure. This, in turn,creates motion in the atmosphere, both vertically andhorizontally, in the form of currents and wind. Motionin the atmosphere, combined with moisture, producesclouds and precipitation otherwise known as weather.

WINDPressure and temperature changes produce two kindsof motion in the atmosphere—vertical movement of ascending and descending currents, and horizontalmovement in the form of wind. Both types of motion inthe atmosphere are important as they affect the takeoff,landing, and cruise flight operations. More

important, however, is that these motions in theatmosphere, otherwise called atmospheric circulation,cause weather changes.

THE CAUSE OF ATMOSPHERICCIRCULATIONAtmospheric circulation is the movement of air aroundthe surface of the Earth. It is caused by uneven heatingof the Earth’s surface and upsets the equilibrium of theatmosphere, creating changes in air movement andatmospheric pressure. Because the Earth has a curved

surface that rotates on a tilted axis while orbiting thesun, the equatorial regions of the Earth receive a greateramount of heat from the sun than the polar regions. Theamount of sun that heats the Earth depends upon thetime of day, time of year, and the latitude of the specificregion. All of these factors affect the length of time andthe angle at which sunlight strikes the surface.

In general circulation theory, areas of low pressureexist over the equatorial regions, and areas of highpressure exist over the polar regions due to a differencein temperature. Solar heating causes air to become lessdense and rise in equatorial areas. The resulting lowpressure allows the high-pressure air at the poles toflow along the planet’s surface toward the equator. Asthe warm air flows toward the poles, it cools, becomingmore dense, and sinks back toward the surface. [Figure10-8] This pattern of air circulation is correct in theory;however, the circulation of air is modified by severalforces, most importantly the rotation of the Earth.

The force created by the rotation of the Earth is knownas Coriolis force. This force is not perceptible to us aswe walk around because we move so slowly and travelrelatively short distances compared to the size androtation rate of the Earth. However, it does significantlyaffect bodies that move over great distances, such as an

TAKEOFF DISTANCE

0°C0° CPRESS

ALTFT

PRESSALTFT

GRNDROLL

FT

GRNDROLL

FT

TOTAL FTTO CLEAR50 FT OBS

TOTAL FTTO CLEAR50 FT OBS

5000 1185 2125 6000 1305 2360 7000 1440 2635 8000 1590 2960

S.L. 745 1320 1000 815 1445 2000 895 1585

3000 980 1740 4000 1075 1920

1590 ft.

745 ft.

Pressure Altitude:8,000 ft.

Pressure Altitude:Sea Level

Figure 10-7. Takeoff distance increases with increased altitude.

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air mass or body of water. The Coriolis force deflectsair to the right in the Northern Hemisphere, causing itto follow a curved path instead of a straight line. Theamount of deflection differs depending on the latitude.It is greatest at the poles, and diminishes to zero at theequator. The magnitude of Coriolis force also differswith the speed of the moving body—the faster thespeed, the greater the deviation. In the NorthernHemisphere, the rotation of the Earth deflects movingair to the right and changes the general circulationpattern of the air.

The speed of the Earth’s rotation causes the generalflow to break up into three distinct cells in eachhemisphere. [Figure 10-9] In the Northern Hemisphere,the warm air at the equator rises upward from thesurface, travels northward, and is deflected eastwardby the rotation of the Earth. By the time it has traveledone-third of the distance from the equator to the NorthPole, it is no longer moving northward, but eastward.This air cools and sinks in a belt-like area at about 30 °

latitude, creating an area of high pressure as it sinkstoward the surface. Then it flows southward along thesurface back toward the equator. Coriolis force bendsthe flow to the right, thus creating the northeasterlytrade winds that prevail from 30 ° latitude to theequator. Similar forces create circulation cells thatencircle the Earth between 30 ° and 60° latitude, andbetween 60 ° and the poles. This circulation patternresults in the prevailing westerly winds in theconterminous United States.

Circulation patterns are further complicated byseasonal changes, differences between the surfaces of continents and oceans, and other factors.

Frictional forces caused by the topography of theEarth’s surface modify the movement of the air in theatmosphere. Within 2,000 feet of the ground, thefriction between the surface and the atmosphere slowsthe moving air. The wind is diverted from its pathbecause the frictional force reduces the Coriolis force.This is why the wind direction at the surface variessomewhat from the wind direction just a few thousandfeet above the Earth.

WIND PATTERNSAir flows from areas of high pressure into those of lowpressure because air always seeks out lower pressure.In the Northern Hemisphere, this flow of air from areasof high to low pressure is deflected to the right;producing a clockwise circulation around an area of high pressure. This is also known as anti-cycloniccirculation. The opposite is true of low-pressure areas;the air flows toward a low and is deflected to create acounter-clockwise or cyclonic circulation. [Figure10-10]

High-pressure systems are generally areas of dry,stable, descending air. Good weather is typicallyassociated with high-pressure systems for this reason.Conversely, air flows into a low-pressure area toreplace rising air. This air tends to be unstable, andusually brings increasing cloudiness and precipitation.Thus, bad weather is commonly associated with areasof low pressure.

A good understanding of high- and low-pressure windpatterns can be of great help when planning a flight,because a pilot can take advantage of beneficialtailwinds. [Figure 10-11] When planning a flight fromwest to east, favorable winds would be encountered

Surface Flow

Equator

Figure 10-8. Circulation pattern in a static environment.

60 ° N

30 ° N

60 ° S

30 ° S

0°

Figure 10-9. Three-cell circulation pattern due to the rotationof the Earth.

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along the northern side of a high-pressure system or thesouthern side of a low-pressure system. On the returnflight, the most favorable winds would be along thesouthern side of the same high-pressure system or thenorthern side of a low-pressure system. An addedadvantage is a better understanding of what type of weather to expect in a given area along a route of flightbased on the prevailing areas of highs and lows.

The theory of circulation and wind patterns is accuratefor large-scale atmospheric circulation; however, itdoes not take into account changes to the circulation ona local scale. Local conditions, geological features, andother anomalies can change the wind direction andspeed close to the Earth’s surface.

CONVECTIVE CURRENTSDifferent surfaces radiate heat in varying amounts.Plowed ground, rocks, sand, and barren land give off alarge amount of heat; water, trees, and other areas of vegetation tend to absorb and retain heat. The resultinguneven heating of the air creates small areas of localcirculation called convective currents.

Convective currents cause the bumpy, turbulent airsometimes experienced when flying at lower altitudesduring warmer weather. On a low altitude flight overvarying surfaces, updrafts are likely to occur overpavement or barren places, and downdrafts often occurover water or expansive areas of vegetation like agroup of trees. Typically, these turbulent conditions canbe avoided by flying at higher altitudes, even abovecumulus cloud layers. [Figure 10-12]

Convective currents are particularly noticeable in areaswith a landmass directly adjacent to a large body of

water, such as an ocean, large lake, or other appreciablearea of water. During the day, land heats faster thanwater, so the air over the land becomes warmer and lessdense. It rises and is replaced by cooler, denser airflowing in from over the water. This causes an onshorewind, called a sea breeze . Conversely, at night landcools faster than water, as does the corresponding air.In this case, the warmer air over the water rises and isreplaced by the cooler, denser air from the land,creating an offshore wind called a land breeze . Thisreverses the local wind circulation pattern. Convectivecurrents can occur anywhere there is an uneven heatingof the Earth’s surface. [Figure 10-13]

High

Low

Figure 10-10. Circulation pattern about areas of high and lowpressure.

High

Low

Figure 10-11. Favorable winds near a high-pressure system.

Sea Breeze—A coastal breeze blowing from sea to land caused by thetemperature difference when the land surface is warmer than the seasurface. The sea breeze usually occurs during the day.

Land Breeze—A coastal breeze flowing from land to sea caused by thetemperature difference when the sea surface is warmer than the adja-cent land. The land breeze usually occurs at night.

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Convection currents close to the ground can affect apilot’s ability to control the aircraft. On final approach,for example, the rising air from terrain devoid ofvegetation sometimes produces a ballooning effect thatcan cause a pilot to overshoot the intended landingspot. On the other hand, an approach over a large bodyof water or an area of thick vegetation tends to create asinking effect that can cause an unwary pilot to landshort of the intended landing spot. [Figure 10-14]

EFFECT OF OBSTRUCTIONS ON WINDAnother atmospheric hazard exists that can createproblems for pilots. Obstructions on the ground affectthe flow of wind and can be an unseen danger. Groundtopography and large buildings can break up the flowof the wind and create wind gusts that change rapidlyin direction and speed. These obstructions range frommanmade structures like hangars to large naturalobstructions, such as mountains, bluffs, or canyons. It

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Thermals

Figure 10-12. Convective turbulence avoidance.

Return Flow

Sea BreezeWarm Cool

WarmCool

Return Flow

Land Breeze

Figure 10-13. Sea breeze and land breeze wind circulation patterns.

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is especially important to be vigilant when flying in orout of airports that have large buildings or naturalobstructions located near the runway. [Figure 10-15]

The intensity of the turbulence associated with groundobstructions depends on the size of the obstacle and theprimary velocity of the wind. This can affect the

takeoff and landing performance of any aircraft and canpresent a very serious hazard. During the landing phaseof flight, an aircraft may “drop in” due to the turbulentair and be too low to clear obstacles during theapproach.

This same condition is even more noticeable whenflying in mountainous regions. [Figure 10-16] Whilethe wind flows smoothly up the windward side of themountain and the upward currents help to carry anaircraft over the peak of the mountain, the wind on theleeward side does not act in a similar manner. As the airflows down the leeward side of the mountain, the airfollows the contour of the terrain and is increasinglyturbulent. This tends to push an aircraft into the side of a mountain. The stronger the wind, the greater thedownward pressure and turbulence become.

Due to the effect terrain has on the wind in valleys orcanyons, downdrafts can be severe. Thus, a prudentpilot is well advised to seek out a mountain qualifiedflight instructor and get a mountain checkout beforeconducting a flight in or near mountainous terrain.

LOW-LEVEL WIND SHEARWind shear is a sudden, drastic change in windspeedand/or direction over a very small area. Wind shear cansubject an aircraft to violent updrafts and downdraftsas well as abrupt changes to the horizontal movementof the aircraft. While wind shear can occur at anyaltitude, low-level wind shear is especially hazardous

due to the proximity of an aircraft to the ground.Directional wind changes of 180 ° and speed changes of 50 knots or more are associated with low-level windshear. Low-level wind shear is commonly associatedwith passing frontal systems, thunderstorms, andtemperature inversions with strong upper level winds(greater than 25 knots).

Wind shear is dangerous to an aircraft for severalreasons. The rapid changes in wind direction andvelocity changes the wind’s relation to the aircraftdisrupting the normal flight attitude and performanceof the aircraft. During a wind shear situation, the effectscan be subtle or very dramatic depending on windspeedand direction of change. For example, a tailwind thatquickly changes to a headwind will cause an increasein airspeed and performance. Conversely, when aheadwind changes to a tailwind, the airspeed willrapidly decrease and there will be a correspondingdecrease in performance. In either case, a pilot must beprepared to react immediately to the changes tomaintain control of the aircraft.

Figure 10-14. Currents generated by varying surface conditions.

CoolSinking

Air

I n t e n d e d F l i g h t P a t h

WarmRising

Air

Wind Shear A sudden, drastic shift in windspeed, direction, or boththat may occur in the horizontal or vertical plane.

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In general, the most severe type of low-level windshear is associated with convective precipitation or rainfrom thunderstorms. One critical type of shearassociated with convective precipitation is known as amicroburst . A typical microburst occurs in a space of less than 1 mile horizontally and within 1,000 feetvertically. The lifespan of a microburst is about 15minutes during which it can produce downdrafts of upto 6,000 feet per minute. It can also produce ahazardous wind direction change of 45 knots or more,in a matter of seconds. When encountered close to theground, these excessive downdrafts and rapid changes

in wind direction can produce a situation in which it isdifficult to control the aircraft. [Figure 10-17] Duringan inadvertent takeoff into a microburst, the plane firstexperiences a performance-increasing headwind (#1),followed by performance-decreasing downdrafts (#2).Then the wind rapidly shears to a tailwind (#3), and canresult in terrain impact or flight dangerously close tothe ground (#4).

Figure 10-15. Turbulence caused by manmade obstructions.

Figure 10-16. Turbulence encountered over and around mountainous regions.

WIND

Microburst Α strong downdraft which normally occurs over horizontaldistances of 1 NM or less and vertical distances of less than 1,000 feet. Inspite of its small horizontal scale, an intense microburst could inducewindspeeds greater than 100 knots and downdrafts as strong as 6,000feet per minute.

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Microbursts are often difficult to detect because theyoccur in a relatively confined area. In an effort to warnpilots of low-level wind shear, alert systems have beeninstalled at several airports around the country. A seriesof anemometers, placed around the airport, form a netto detect changes in windspeeds. When windspeedsdiffer by more than 15 knots, a warning for wind shearis given to pilots. This system is known as the low-levelwind shear alert system, or LLWAS.

It is important to remember that wind shear can affectany flight and any pilot at any altitude. While windshear may be reported, it often remains undetected andis a silent danger to aviation. Always be alert to thepossibility of wind shear, especially when flying in andaround thunderstorms and frontal systems.

WIND AND PRESSURE REPRESENTATIONON SURFACE WEATHER MAPSSurface weather maps provide information aboutfronts, areas of high and low pressure, and surfacewinds and pressures for each station. This type of weather map allows pilots to see the locations of frontsand pressure systems, but more importantly, it depictsthe wind and pressure at the surface for each location.For more information on surface analysis and weatherdepiction charts see Chapter 11.

Wind conditions are reported by an arrow attached tothe station location circle. [Figure 10-18] The stationcircle represents the head of the arrow, with the arrowpointing in the direction from which the wind isblowing. Winds are described by the direction fromwhich they blow, thus a northwest wind means that thewind is blowing from the northwest toward thesoutheast. The speed of the wind is depicted by barbs

or pennants placed on the wind line. Each barbrepresents a speed of 10 knots, while half a barb isequal to 5 knots and a pennant is equal to 50 knots.

The pressure for each station is recorded on the weatherchart and is shown in millibars. Isobars are lines drawnon the chart to depict areas of equal pressure. Theselines result in a pattern that reveals the pressuregradient or change in pressure over distance. [Figure10-19] Isobars are similar to contour lines on atopographic map that indicate terrain altitudes andslope steepness. For example, isobars that are closelyspaced indicate a steep wind gradient and strong windsprevail. Shallow gradients, on the other hand, arerepresented by isobars that are spaced far apart, and areindicative of light winds. Isobars help identify low- andhigh-pressure systems as well as the location of ridges,troughs, and cols. A high is an area of high pressure

Strong Downdraft

Increasing Headwind Increasing Tailwind

Outflow Outflow

1

2

34

Figure 10-17. Effect of a microburst wind.

NW / 5 kts SW / 20 kts

E / 35 kts N / 50 kts W / 105 kts

Calm

Examples of wind speed and direction plots

Figure 10-18. Depiction of winds on a surface weather chart.

Isobars—Lines which connect points of equal barometric pressure.

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surrounded by lower pressure; a low is an area of low

pressure surrounded by higher pressure. A ridge is anelongated area of high pressure, and a trough is anelongated area of low pressure. A col is the intersectionbetween a ridge and a trough, or an area of neutralitybetween two highs or two lows.

Isobars furnish valuable information about winds in thefirst few thousand feet above the surface. Close to theground, wind direction is modified by the surface andwindspeed decreases due to friction with thesurface. At levels 2,000 to 3,000 feet above thesurface, however, the speed is greater and the directionbecomes more parallel to the isobars. Therefore, the

surface winds are shown on the weather map as well asthe winds at a slightly higher altitude.

Generally, the wind 2,000 feet above the ground will be20° to 40 ° to the right of surface winds, and the wind-speed will be greater. The change of wind direction isgreatest over rough terrain and least over flat surfaces,such as open water. In the absence of winds aloftinformation, this rule of thumb allows for a roughestimate of the wind conditions a few thousand feetabove the surface.

ATMOSPHERIC STABILITYThe stability of the atmosphere depends on its ability toresist vertical motion. A stable atmosphere makesvertical movement difficult, and small verticaldisturbances dampen out and disappear. In an unstableatmosphere, small vertical air movements tend tobecome larger, resulting in turbulent airflow andconvective activity. Instability can lead to significantturbulence, extensive vertical clouds, and severeweather.

Rising air expands and cools due to the decrease in airpressure as altitude increases. The opposite is true of descending air; as atmospheric pressure increases, thetemperature of descending air increases as it iscompressed. Adiabatic heating , or adiabatic cooling ,are the terms used to describe this temperature change.

The adiabatic process takes place in all upward anddownward moving air. When air rises into an area of

lower pressure, it expands to a larger volume. As themolecules of air expand, the temperature of the airlowers. As a result, when a parcel of air rises, pressuredecreases, volume increases, and temperaturedecreases. When air descends, the opposite is true. Therate at which temperature decreases with an increase inaltitude is referred to as its lapse rate. As air ascendsthrough the atmosphere, the average rate oftemperature change is 2°C (3.5°F) per 1,000 feet.

Since water vapor is lighter than air, moisture decreasesair density, causing it to rise. Conversely, as moisturedecreases, air becomes denser and tends to sink. Sincemoist air cools at a slower rate, it is generally less sta-ble than dry air since the moist air must rise higherbefore its temperature cools to that of the surroundingair. The dry adiabatic lapse rate (unsaturated air) is 3°C(5.4°F) per 1,000 feet. The moist adiabatic lapse ratevaries from 1.1°C to 2.8°C (2°F to 5°F) per 1,000 feet.

The combination of moisture and temperaturedetermine the stability of the air and the resultingweather. Cool, dry air is very stable and resists verticalmovement, which leads to good and generally clearweather. The greatest instability occurs when the air ismoist and warm, as it is in the tropical regions in thesummer. Typically, thunderstorms appear on a daily

basis in these regions due to the instability of thesurrounding air.

Isobars

Closely spaced isobars meana steep pressure gradient andstrong winds.

552

558

564

570

576

582

Widely spaced isobarsmean a shallow pressuregradient and relativelylight winds.

Figure 10-19. Isobars reveal the pressure gradient of an areaof high- or low-pressure areas.

Adiabatic heating—A process of heating dry air through compression.As air moves downward it is compressed, resulting in a temperatureincrease.

Adiabatic cooling—A process of cooling the air through expansion. Forexample, as air moves upward, it expands with the reduction of atmos-pheric pressure and cools as it expands.

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INVERSIONAs air rises and expands in the atmosphere, thetemperature decreases. There is an atmosphericanomaly that can occur, however, that changes thistypical pattern of atmospheric behavior. When thetemperature of the air rises with altitude, a temperatureinversion exists. Inversion layers are commonly

shallow layers of smooth, stable air close to the ground.The temperature of the air increases with altitude to acertain point, which is the top of the inversion. The airat the top of the layer acts as a lid, keeping weather andpollutants trapped below. If the relative humidity of theair is high, it can contribute to the formation of clouds,fog, haze, or smoke, resulting in diminished visibilityin the inversion layer.

Surface based temperature inversions occur on clear,cool nights when the air close to the ground is cooledby the lowering temperature of the ground. The airwithin a few hundred feet of the surface becomes

cooler than the air above it. Frontal inversions occurwhen warm air spreads over a layer of cooler air, orcooler air is forced under a layer of warmer air.

MOISTURE AND TEMPERATUREThe atmosphere, by nature, contains moisture in theform of water vapor. The amount of moisture present inthe atmosphere is dependent upon the temperature of the air. Every 20 ° F increase in temperature doubles theamount of moisture the air can hold. Conversely, adecrease of 20 ° F cuts the capacity in half.

Water is present in the atmosphere in three states:liquid, solid, and gaseous. All three forms can readilychange to another, and all are present within thetemperature ranges of the atmosphere. As waterchanges from one state to another, an exchange of heattakes place. These changes occur through the processesof evaporation , sublimation , condensation ,deposition , melting, or freezing. However, water vaporis added into the atmosphere only by the processes of evaporation and sublimation.

Evaporation is the changing of liquid water to watervapor. As water vapor forms, it absorbs heat from thenearest available source. This heat exchange is knownas the latent heat of evaporation. A good example of this is when the body’s perspiration evaporates. The neteffect is a cooling sensation as heat is extracted fromthe body. Similarly, sublimation is the changing of icedirectly to water vapor, completely bypassing theliquid stage. Though dry ice is not made of water, butrather carbon dioxide, it demonstrates the principle of sublimation, when a solid turns directly into vapor.

RELATIVE HUMIDITYHumidity refers to the amount of water vapor presentin the atmosphere at a given time. Relative humidity is

the actual amount of moisture in the air compared tothe total amount of moisture the air could hold at thattemperature. For example, if the current relativehumidity is 65 percent, the air is holding 65 percent of the total amount of moisture that it is capable ofholding at that temperature and pressure. While muchof the western United States rarely sees days of high

humidity, relative humidity readings of 75 to 90percent are not uncommon in the southern UnitedStates during warmer months. [Figure 10-20]

TEMPERATURE/DEWPOINT RELATIONSHIPThe relationship between dewpoint and temperaturedefines the concept of relative humidity. The dewpoint,given in degrees, is the temperature at which the air canhold no more moisture. When the temperature of theair is reduced to the dewpoint, the air is completelysaturated and moisture begins to condense out of theair in the form of fog, dew, frost, clouds, rain, hail, orsnow.

As moist, unstable air rises, clouds often form at thealtitude where temperature and dewpoint reach thesame value. When lifted, unsaturated air cools at a rateof 5.4 ° F per 1,000 feet and the dewpoint temperaturedecreases at a rate of 1 ° F per 1,000 feet. This results ina convergence of temperature and dewpoint at a rate of 4.4 ° F. Apply the convergence rate to the reportedtemperature and dewpoint to determine the height of the cloud base.

Given:

Temperature (T) = 85°FDewpoint (DP) = 71°FConvergence Rate (CR) = 4.4°T – DP = Temperature Dewpoint Spread (TDS)TDS ÷ CR = XX × 1,000 feet = height of cloud base AGL

Example:

85ºF – 71ºF = 14ºF14ºF ÷ 4.4°F = 3.183.18 × 1,000 = 3,180 feet AGLThe height of the cloud base is 3,180 feet AGL.

Inversion—An increase in temperature with altitude.

Evaporation—The transformation of a liquid to a gaseous state, such asthe change of water to water vapor.

Sublimation Process by which a solid is changed to a gas without goingthrough the liquid state.

Condensation A change of state of water from a gas (water vapor) to aliquid.

Deposition The direct transformation of a gas to a solid state, in whichthe liquid state is bypassed. Some sources use the term sublimation todescribe this process instead of deposition.

Dewpoint—The temperature at which air reaches a state where it canhold no more water.

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Explanation:

With an outside air temperature (OAT) of 85 ° F at thesurface, and dewpoint at the surface of 71 ° F, the spreadis 14°. Divide the temperature dewpoint spread by theconvergence rate of 4.4 ° F, and multiply by 1,000 todetermine the approximate height of the cloud base.

METHODS BY WHICH AIR REACHES THESATURATION POINTIf air reaches the saturation point while temperatureand dewpoint are close together, it is highly likely thatfog, low clouds, and precipitation will form. There arefour methods by which air can reach the completesaturation point. First, when warm air moves over a

cold surface, the air’s temperature drops and reachesthe saturation point. Second, the saturation point maybe reached when cold air and warm air mix. Third,when air cools at night through contact with the coolerground, air reaches its saturation point. The fourthmethod occurs when air is lifted or is forced upward inthe atmosphere.

As air rises, it uses heat energy to expand. As a result,the rising air loses heat rapidly. Unsaturated air losesheat at a rate of 3.0 ° C (5.4 ° F) for every 1,000 feet of

altitude gain. No matter what causes the air to reach itssaturation point, saturated air brings clouds, rain, andother critical weather situations.

DEW AND FROSTOn cool, calm nights, the temperature of the groundand objects on the surface can cause temperatures of the surrounding air to drop below the dewpoint. Whenthis occurs, the moisture in the air condenses anddeposits itself on the ground, buildings, and otherobjects like cars and aircraft. This moisture is known asdew and sometimes can be seen on grass in themorning. If the temperature is below freezing, themoisture will be deposited in the form of frost. Whiledew poses no threat to an aircraft, frost poses a definite

flight safety hazard. Frost disrupts the flow of air overthe wing and can drastically reduce the production of lift. It also increases drag, which, when combined withlowered lift production, can eliminate the ability to takeoff. An aircraft must be thoroughly cleaned and free of frost prior to beginning a flight.

FOGFog, by definition, is a cloud that begins within 50 feetof the surface. It typically occurs when the temperatureof air near the ground is cooled to the air’s dewpoint.

A cubic meter of air with 17g of water vaporat 20 ° C is at saturation, or 100% relativehumidity. Any further cooling will causecondensation (fog, clouds, dew) to form.Thus, 20 ° C is the dewpoint for this situation.

If the temperature is loweredto 10 ° C, the air can only hold9g of water vapor, and 8g ofwater will condense as waterdroplets. The relative humiditywill still be at 100%.

If the same cubic meter of air warmsto 30 ° C, the 17g of water vapor willproduce a relative humidity of 56%.(17g is 56% of the 30g the air couldhold at this temperature.)

CCC

Figure 10-20.The relationship between relative humidity, temperature, and dewpoint.

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At this point, water vapor in the air condenses andbecomes visible in the form of fog. Fog is classifiedaccording to the manner in which it forms and isdependent upon the current temperature and theamount of water vapor in the air.

On clear nights, with relatively little to no wind

present, radiation fog may develop. [Figure 10-21]Usually, it forms in low-lying areas like mountainvalleys. This type of fog occurs when the ground coolsrapidly due to terrestrial radiation, and the surroundingair temperature reaches its dewpoint. As the sun risesand the temperature increases, radiation fog will liftand eventually burn off. Any increase in wind will alsospeed the dissipation of radiation fog. If radiation fog isless than 20 feet thick, it is known as ground fog.

When a layer of warm, moist air moves over a coldsurface, advection fog is likely to occur. Unlikeradiation fog, wind is required to form advection fog.Winds of up to 15 knots allow the fog to form andintensify; above a speed of 15 knots, the fog usuallylifts and forms low stratus clouds. Advection fog iscommon in coastal areas where sea breezes can blowthe air over cooler landmasses.

In these same coastal areas, upslope fog is likely aswell. Upslope fog occurs when moist, stable air isforced up sloping land features like a mountain range.This type of fog also requires wind for formation andcontinued existence. Upslope and advection fog, unlikeradiation fog, may not burn off with the morning sun,but instead can persist for days. They also can extendto greater heights than radiation fog.

Steam fog, or sea smoke, forms when cold, dry airmoves over warm water. As the water evaporates, itrises and resembles smoke. This type of fog is commonover bodies of water during the coldest times of theyear. Low-level turbulence and icing are commonlyassociated with steam fog.

Ice fog occurs in cold weather when the temperature ismuch below freezing and water vapor forms directlyinto ice crystals. Conditions favorable for its formationare the same as for radiation fog except for coldtemperature, usually –25°F or colder. It occurs mostlyin the arctic regions, but is not unknown in middlelatitudes during the cold season.

CLOUDSClouds are visible indicators and are often indicative of future weather. For clouds to form, there must beadequate water vapor and condensation nuclei, as wellas a method by which the air can be cooled. When theair cools and reaches its saturation point, the invisiblewater vapor changes into a visible state. Through theprocesses of deposition (also referred to assublimation) and condensation, moisture condenses orsublimates onto miniscule particles of matter like dust,salt, and smoke known as condensation nuclei . Thenuclei are important because they provide a means for

the moisture to change from one state to another.

Cloud type is determined by its height, shape, andbehavior. They are classified according to the height of their bases as low, middle, or high clouds, as well asclouds with vertical development. [Figure 10-22]

Low clouds are those that form near the Earth’s surfaceand extend up to 6,500 feet AGL. They are madeprimarily of water droplets, but can includesupercooled water droplets that induce hazardousaircraft icing. Typical low clouds are stratus,stratocumulus, and nimbostratus. Fog is also classifiedas a type of low cloud formation. Clouds in this familycreate low ceilings, hamper visibility, and can changerapidly. Because of this, they influence flight planningand can make VFR flight impossible.

Middle clouds form around 6,500 feet AGL and extendup to 20,000 feet AGL. They are composed of water,ice crystals, and supercooled water droplets . Typicalmiddle-level clouds include altostratus andaltocumulus. These types of clouds may beencountered on cross-country flights at higheraltitudes. Altostratus clouds can produce turbulenceand may contain moderate icing. Altocumulus clouds,which usually form when altostratus clouds arebreaking apart, also may contain light turbulence andicing.

Figure 10-21. Radiation fog.

Condensation Nuclei—Small particles of solid matter in the air onwhich water vapor condenses.

Supercooled Water Droplets—Water droplets that have been cooledbelow the freezing point, but are still in a liquid state.

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High clouds form above 20,000 feet AGL and usuallyform only in stable air. They are made up of ice crystalsand pose no real threat of turbulence or aircraft icing.Typical high-level clouds are cirrus, cirrostratus, andcirrocumulus.

Clouds with extensive vertical development arecumulus clouds that build vertically into toweringcumulus or cumulonimbus clouds. The bases of theseclouds form in the low to middle cloud base region butcan extend into high altitude cloud levels. Toweringcumulus clouds indicate areas of instability in theatmosphere, and the air around and inside them isturbulent. These types of clouds often developinto cumulonimbus clouds or thunderstorms.Cumulonimbus clouds contain large amounts of

moisture and unstable air, and usually producehazardous weather phenomena such as lightning, hail,tornadoes, gusty winds, and wind shear. Theseextensive vertical clouds can be obscured by othercloud formations and are not always visible from theground or while in flight. When this happens, theseclouds are said to be embedded, hence the term,embedded thunderstorms.

Cloud classification can be further broken down intospecific cloud types according to the outward

appearance and cloud composition. Knowing theseterms can help identify visible clouds.

The following is a list of cloud classifications:

• Cumulus—Heaped or piled clouds.

• Stratus—Formed in layers.

• Cirrus—Ringlets; fibrous clouds; also high-levelclouds above 20,000 feet.

• Castellanus—Common base with separatevertical development; castle-like.

• Lenticularus—Lens shaped; formed overmountains in strong winds.

• Nimbus—Rain bearing clouds.• Fracto—Ragged or broken.

• Alto—Meaning high; also middle-level cloudsexisting at 5,000 to 20,000 feet.

To pilots, the cumulonimbus cloud is perhaps the mostdangerous cloud type. It appears individually or ingroups and is known as either an air mass or orographicthunderstorm. Heating of the air near the Earth’ssurface creates an air mass thunderstorm; the upslope

Cumulonimbus

Cumulus

Clouds withVertical

Development

CirrocumulusCirrus

Cirrostratus

High Clouds

Middle Clouds

Low Clouds

Stratocumulus

NimbostratusStratus

20,000 AGL

6,500 AGL

Altocumulus

Altostratus

Figure 10-22. Basic cloud types.

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motion of air in the mountainous regions causesorographic thunderstorms. Cumulonimbus clouds thatform in a continuous line are nonfrontal bands ofthunderstorms or squall lines.

Since rising air currents cause cumulonimbus clouds,they are extremely turbulent and pose a significant

hazard to flight safety. For example, if an aircraft entersa thunderstorm, the aircraft could experience updraftsand downdrafts that exceed 3,000 feet per minute. Inaddition, thunderstorms can produce large hailstones,damaging lightning, tornadoes, and large quantities of water, all of which are potentially hazardous to aircraft.

A thunderstorm makes its way through three distinctstages before dissipating. It begins with the cumulusstage, in which lifting action of the air begins. Ifsufficient moisture and instability are present, theclouds continue to increase in vertical height.Continuous, strong updrafts prohibit moisture from

falling. The updraft region grows larger than theindividual thermals feeding the storm. Withinapproximately 15 minutes, the thunderstorm reachesthe mature stage, which is the most violent time periodof the thunderstorm’s life cycle. At this point, drops of moisture, whether rain or ice, are too heavy for thecloud to support and begin falling in the form of rain orhail. This creates a downward motion of the air. Warm,rising air; cool, precipitation-induced descending air;and violent turbulence all exist within and near thecloud. Below the cloud, the down-rushing air increasessurface winds and decreases the temperature. Once the

vertical motion near the top of the cloud slows down,the top of the cloud spreads out and takes on ananvil-like shape. At this point, the storm enters thedissipating stage. This is when the downdrafts spreadout and replace the updrafts needed to sustain thestorm. [Figure 10-23]

It is impossible to fly over thunderstorms in lightaircraft. Severe thunderstorms can punch through thetropopause and reach staggering heights of 50,000 to60,000 feet depending on latitude. Flying underthunderstorms can subject aircraft to rain, hail,damaging lightning, and violent turbulence. A goodrule of thumb is to circumnavigate thunderstorms by atleast 5 nautical miles (NM) since hail may fall for milesoutside of the clouds. If flying around a thunderstormis not an option, stay on the ground until it passes.

CEILINGA ceiling , for aviation purposes, is the lowest layer of

clouds reported as being broken or overcast, or thevertical visibility into an obscuration like fog or haze.Clouds are reported as broken when five-eighths toseven-eighths of the sky is covered with clouds.Overcast means the entire sky is covered with clouds.Current ceiling information is reported by the aviationroutine weather report (METAR) and automatedweather stations of various types.

Ceiling—The height above the Earth’s surface of the lowest layer of clouds reported as broken or overcast, or the vertical visibility into anobscuration.

40

30

20

10

5

0

A l t i t u d e

( T h o u s a n

d s o

f f e e

t )

32 ° F 0° C

Equilibrium Level

32 ° F 32 ° F0° C

3-5 mi.Cumulus Stage

5-10 mi.Mature Stage

5-7 mi.Dissipating Stage

Horizontal Distance

0° C

Figure 10-23. Life cycle of a thunderstorm.

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VISIBILITYClosely related to cloud cover and reported ceilings isvisibility information. Visibility refers to the greatesthorizontal distance at which prominent objects can beviewed with the naked eye. Current visibility is alsoreported in METAR and other aviation weather reports,as well as automated weather stations. Visibility

information, as predicted by meteorologists, isavailable during a preflight weather briefing.

PRECIPITATIONPrecipitation refers to any form of water particles thatform in the atmosphere and fall to the ground. It has aprofound impact on flight safety. Depending on theform of precipitation, it can reduce visibility, createicing situations, and affect landing and takeoffperformance of an aircraft.

Precipitation occurs because water or ice particles inclouds grow in size until the atmosphere can no longer

support them. It can occur in several forms as it fallstoward the Earth, including drizzle, rain, ice pellets,hail, and ice.

Drizzle is classified as very small water droplets,smaller than 0.02 inches in diameter. Drizzle usuallyaccompanies fog or low stratus clouds. Water dropletsof larger size are referred to as rain. Rain that fallsthrough the atmosphere but evaporates prior to strikingthe ground is known as virga. Freezing rain andfreezing drizzle occur when the temperature of thesurface is below freezing; the rain freezes on contactwith the cooler surface.

If rain falls through a temperature inversion, it mayfreeze as it passes through the underlying cold air andfall to the ground in the form of ice pellets. Ice pelletsare an indication of a temperature inversion and thatfreezing rain exists at a higher altitude. In the case of hail, freezing water droplets are carried up and downby drafts inside clouds, growing larger in size as theycome in contact with more moisture. Once the updraftscan no longer hold the freezing water, it falls to theEarth in the form of hail. Hail can be pea-sized, or itcan grow as large as 5 inches in diameter, larger than asoftball.

Snow is precipitation in the form of ice crystals thatfalls at a steady rate or in snow showers that begin,change in intensity, and end rapidly. Falling snow alsovaries in size, being very small grains or large flakes.Snow grains are the equivalent of drizzle in size.

Precipitation in any form poses a threat to safety of flight. Often, precipitation is accompanied by lowceilings and reduced visibility. Aircraft that have ice,snow, or frost on their surfaces must be carefully

cleaned prior to beginning a flight because of thepossible airflow disruption and loss of lift. Rain cancontribute to water in the fuel tanks. Precipitation cancreate hazards on the runway surface itself, makingtakeoffs and landings difficult, if not impossible, due tosnow, ice, or pooling water and very slick surfaces.

AIR MASSESAir masses are large bodies of air that take on thecharacteristics of the surrounding area, or sourceregion. A source region is typically an area in which theair remains relatively stagnant for a period of days orlonger. During this time of stagnation, the air masstakes on the temperature and moisture characteristicsof the source region. Areas of stagnation can be foundin polar regions, tropical oceans, and dry deserts. Airmasses are classified based on their region oforigination:

• Polar or Tropical

• Maritime or Continental

A continental polar air mass forms over a polar regionand brings cool, dry air with it. Maritime tropical airmasses form over warm tropical waters like theCaribbean Sea and bring warm, moist air. As the airmass moves from its source region and passes overland or water, the air mass is subjected to the varyingconditions of the land or water, and these modify thenature of the air mass. [Figure 10-24]

An air mass passing over a warmer surface will be

warmed from below, and convective currents form,causing the air to rise. This creates an unstable air masswith good surface visibility. Moist, unstable air causescumulus clouds, showers, and turbulence to form.Conversely, an air mass passing over a colder surfacedoes not form convective currents, but instead creates astable air mass with poor surface visibility. The poorsurface visibility is due to the fact that smoke, dust, andother particles cannot rise out of the air mass and areinstead trapped near the surface. A stable air mass canproduce low stratus clouds and fog.

FRONTSAs air masses move across bodies of water and land,they eventually come in contact with another air masswith different characteristics. The boundary layerbetween two types of air masses is known as a front.An approaching front of any type always meanschanges to the weather are imminent.

Air Mass—An extensive body of air having fairly uniform properties of temperature and moisture.

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There are four types of fronts, which are namedaccording to the temperature of the advancing air as itrelates to the temperature of the air it is replacing.[Figure 10-25]

• Warm Front

• Cold Front

• Stationary Front

• Occluded Front

Any discussion of frontal systems must be temperedwith the knowledge that no two fronts are the same.However, generalized weather conditions areassociated with a specific type of front that helpsidentify the front.

WARM FRONTA warm front occurs when a warm mass of airadvances and replaces a body of colder air. Warm frontsmove slowly, typically 10 to 25 miles per hour (m.p.h.).The slope of the advancing front slides over the top of the cooler air and gradually pushes it out of the area.Warm fronts contain warm air that often has very highhumidity. As the warm air is lifted, the temperaturedrops and condensation occurs.

Generally, prior to the passage of a warm front,cirriform or stratiform clouds, along with fog, can beexpected to form along the frontal boundary. Inthe summer months, cumulonimbus clouds(thunderstorms) are likely to develop. Light tomoderate precipitation is probable, usually in the formof rain, sleet, snow, or drizzle, punctuated by poorvisibility. The wind blows from the south-southeast,and the outside temperature is cool or cold, withincreasing dewpoint. Finally, as the warm frontapproaches, the barometric pressure continues to falluntil the front passes completely.

North American air mass source regions. Notestandard air mass abbreviations: arctic (A), continentalpolar (cP), maritime polar (mP), continental tropical(cT), and maritime tropical (mT).cP

A

mP

mP

mT

mTmT

cT

Figure 10-24. Air mass source regions.

(red/blue)*

Cold Front (blue)*

Symbols for Surface Fronts and Other SignificantLines Shown on the Surface Analysis Chart

Warm Front (red)*

(purple)*

Table A

Occluded Front

Stationary Front

* Note : Fronts may be black and white or color, dependingon their source. Also, fronts shown in color code will notnecessarily show frontal symbols.

Figure 10-25. Common chart symbology to depict weatherfront location.

Warm Front—The boundary between two air masses where warm air isreplacing cold air.

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During the passage of a warm front, stratiform cloudsare visible and drizzle may be falling. The visibility isgenerally poor, but improves with variable winds. Thetemperature rises steadily from the inflow of relativelywarmer air. For the most part, the dewpoint remainssteady and the pressure levels off.

After the passage of a warm front, stratocumulusclouds predominate and rain showers are possible. Thevisibility eventually improves, but hazy conditions mayexist for a short period after passage. The wind blowsfrom the south-southwest. With warming temperatures,the dewpoint rises and then levels off. There isgenerally a slight rise in barometric pressure, followedby a decrease of barometric pressure.

FLIGHT TOWARD AN APPROACHING WARMFRONTBy studying a typical warm front, much can be learnedabout the general patterns and atmospheric conditionsthat exist when a warm front is encountered in flight.Figure 10-26 depicts a warm front advancing eastwardfrom St. Louis, Missouri, toward Pittsburgh,Pennsylvania.

At the time of departure from Pittsburgh, the weather isgood VFR with a scattered layer of cirrus clouds at15,000 feet. As the flight progresses westward toColumbus and closer to the oncoming warm front, the

clouds deepen and become increasingly stratiform inappearance with a ceiling of 6,000 feet. The visibilitydecreases to 6 miles in haze with a falling barometricpressure. Approaching Indianapolis, the weatherdeteriorates to broken clouds at 2,000 feet with 3 milesvisibility and rain. With the temperature and dewpointthe same, fog is likely. At St. Louis, the sky is overcast

with low clouds and drizzle and the visibility is 1 mile.Beyond Indianapolis, the ceiling and visibility wouldbe too low to continue VFR. Therefore, it would bewise to remain in Indianapolis until the warm front hadpassed, which might require a day or two.

COLD FRONTA cold front occurs when a mass of cold, dense, andstable air advances and replaces a body of warmer air.Cold fronts move more rapidly than warm fronts,progressing at a rate of 25 to 30 m.p.h. However,extreme cold fronts have been recorded moving atspeeds of up to 60 m.p.h. A typical cold front moves in

a manner opposite that of a warm front; because it is sodense, it stays close to the ground and acts like asnowplow, sliding under the warmer air and forcing theless dense air aloft. The rapidly ascending air causes

COLD AIR

WA R M A I R

ST. LOUIS INDIANAPOLIS COLUMBUS PITTSBURGH200 MILES 400 MILES 600 MILES

NIMBOSTRATUS

ALTOSTRATUS

CIRROSTRATUS

CIRRUS

METAR KSTL 1950Z 21018KT 1SM –RA 0VC010 18/18 A2960

METAR KCMH 1950Z 13018KT 6SM HZ 0VC060 14/10 A2990

METAR KPIT 1950Z 13012KT 10SM SCT150 12/01 A3002

METAR KIND 1950Z 16012KT 3SM RA BKN020 15/15 A2973St. Louis

Indianapolis

1005

1002

999

999 1002 1005 1008 1011 1014

1008 1011 1014 1017

1017

Columbus Pittsburgh

651

1065

593

59 56

606

50

53103420

06802010

40 12526

16618

Figure 10-26. Warm front cross-section with surface weather chart depiction and associated METAR.

Cold Front—The boundary between two air masses where cold air isreplacing warm air.

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the temperature to decrease suddenly, forcing thecreation of clouds. The type of clouds that formdepends on the stability of the warmer air mass. A coldfront in the Northern Hemisphere is normally orientedin a northeast to southwest manner and can be severalhundred miles long, encompassing a large area of land.

Prior to the passage of a typical cold front, cirriformor towering cumulus clouds are present, andcumulonimbus clouds are possible. Rain showers andhaze are possible due to the rapid development of clouds. The wind from the south-southwest helps toreplace the warm temperatures with the relative colderair. A high dewpoint and falling barometric pressureare indicative of imminent cold front passage.

As the cold front passes, towering cumulus orcumulonimbus clouds continue to dominate the sky.Depending on the intensity of the cold front, heavy rainshowers form and might be accompanied by lightning,

thunder, and/or hail. More severe cold fronts can alsoproduce tornadoes. During cold front passage, thevisibility will be poor, with winds variable and gusty,and the temperature and dewpoint drop rapidly. Aquickly falling barometric pressure bottoms out duringfrontal passage, then begins a gradual increase.

After frontal passage, the towering cumulus andcumulonimbus clouds begin to dissipate to cumulusclouds with a corresponding decrease in theprecipitation. Good visibility eventually prevails withthe winds from the west-northwest. Temperaturesremain cooler and the barometric pressure continues torise.

FAST-MOVING COLD FRONTFast-moving cold fronts are pushed by intense pressuresystems far behind the actual front. The frictionbetween the ground and the cold front retards themovement of the front and creates a steeper frontalsurface. This results in a very narrow band of weather,concentrated along the leading edge of the front. If thewarm air being overtaken by the cold front is relativelystable, overcast skies and rain may occur for somedistance ahead of the front. If the warm air is unstable,scattered thunderstorms and rain showers may form. Acontinuous line of thunderstorms, or a squall line, mayform along or ahead of the front. Squall lines present aserious hazard to pilots as squall type thunderstormsare intense and move quickly. Behind a fast movingcold front, the skies usually clear rapidly and the frontleaves behind gusty, turbulent winds and coldertemperatures.

FLIGHT TOWARD AN APPROACHING COLDFRONTLike warm fronts, not all cold fronts are the same.Examining a flight toward an approaching cold front,

pilots can get a better understanding of the type ofconditions that can be encountered in flight. Figure10-27 shows a flight from Pittsburgh, Pennsylvania,toward St. Louis, Missouri.

At the time of departure from Pittsburgh, the weather isVFR with 3 miles visibility in smoke and a scattered

layer of clouds at 3,500 feet. As the flight progresseswestward to Columbus and closer to the oncoming coldfront, the clouds show signs of vertical developmentwith a broken layer at 2,500 feet. The visibility is 6miles in haze with a falling barometric pressure.Approaching Indianapolis, the weather has deterioratedto overcast clouds at 1,000 feet, and 3 miles visibilitywith thunderstorms and heavy rain showers. At St.Louis, the weather gets better with scattered clouds at1,000 feet and a 10 mile visibility.

A pilot using sound judgment based on the knowledgeof frontal conditions, would most likely remain in

Indianapolis until the front had passed. Trying to flybelow a line of thunderstorms or a squall line ishazardous and foolish, and flight over the top of oraround the storm is not an option. Thunderstorms canextend up to well over the capability of small airplanesand can extend in a line for 300 to 500 miles.

COMPARISON OF COLD AND WARM FRONTSWarm fronts and cold fronts are very different in natureas are the hazards associated with each front. They varyin speed, composition, weather phenomenon, andprediction. Cold fronts, which move at 20 to 35 m.p.h.,move very quickly in comparison to warm fronts,

which move at only 10 to 25 m.p.h. Cold fronts alsopossess a steeper frontal slope. Violent weather activityis associated with cold fronts and the weather usuallyoccurs along the frontal boundary, not in advance.However, squall lines can form during the summermonths as far as 200 miles in advance of a severe coldfront. Whereas warm fronts bring low ceilings, poorvisibility, and rain, cold fronts bring sudden storms,gusty winds, turbulence, and sometimes hail ortornadoes.

Cold fronts are fast approaching with little or nowarning, and they make a complete weather change in

just a few hours. The weather clears rapidly afterpassage and drier air with unlimited visibilities prevail.Warm fronts, on the other hand, provide advancewarning of their approach and can take days to passthrough a region.

WIND SHIFTSWind around a high-pressure system rotates in aclockwise fashion, while low-pressure winds rotate in acounter-clockwise manner. When two high-pressuresystems are adjacent, the winds are almost in direct

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opposition to each other at the point of contact. Frontsare the boundaries between two areas of pressure, andtherefore, wind shifts are continually occurring withina front. Shifting wind direction is most pronounced inconjunction with cold fronts.

STATIONARY FRONTWhen the forces of two air masses are relatively equal,the boundary or front that separates them remainsstationary and influences the local weather for days.This front is called a stationary front . The weatherassociated with a stationary front is typically a mixturethat can be found in both warm and cold fronts.

OCCLUDED FRONTAn occluded front occurs when a fast-moving coldfront catches up with a slow-moving warm front. Asthe occluded front approaches, warm front weather

prevails, but is immediately followed by cold frontweather. There are two types of occluded fronts thatcan occur, and the temperatures of the colliding frontalsystems play a large part in defining the type of frontand the resulting weather. A cold front occlusion occurswhen a fast-moving cold front is colder than the airahead of the slow-moving warm front. When thisoccurs, the cold air replaces the cool air and forces thewarm front aloft into the atmosphere. Typically, thecold front occlusion creates a mixture of weather foundin both warm and cold fronts, providing the air is

relatively stable. A warm front occlusion occurs whenthe air ahead of the warm front is colder than the air of the cold front. When this is the case, the cold front ridesup and over the warm front. If the air forced aloft bythe warm front occlusion is unstable, the weather willbe more severe than the weather found in a cold frontocclusion. Embedded thunderstorms, rain, and fog arelikely to occur.

Figure 10-28 depicts a cross-section of a typical coldfront occlusion. The warm front slopes over theprevailing cooler air and produces the warm front typeweather. Prior to the passage of the typical occludedfront, cirriform and stratiform clouds prevail, light toheavy precipitation is falling, visibility is poor,dewpoint is steady, and barometric pressure isfalling. During the passage of the front, nimbostratusand cumulonimbus clouds predominate, and towering

cumulus may also be possible. Light to heavyprecipitation is falling, visibility is poor, winds arevariable, and the barometric pressure is leveling off.After the passage of the front, nimbostratus andaltostratus clouds are visible, precipitation isdecreasing and clearing, and visibility is improving.Stationary Front—A boundary between two air masses that arerelatively balanced.

Occluded Front—A frontal occlusion occurs when a fast-moving coldfront catches up with a slow-moving warm front. The difference intemperature within each frontal system is a major factor in determiningwhether a cold or warm front occlusion occurs.

1011

1008100510051008

1011

1011 1011 1014

1014

C O L D A I R

METAR KSTL 1950Z 30018KT 10SM SCT010 08/02 A2979

METAR KIND 1950Z 20024KT 3SM +TSRA

OVC 010 24/23 A2974

METAR KCMH 1950Z 20012KT 6SM HZ BKN025 25/24 A2983

METAR KPIT 1950Z 20012KT 3SM FU SCT035 24/22 A2989


WARM AIR

CUMULONIMBUS

St. Louis Indianapolis Columbus Pittsburgh

4642

10

0661033

74 0713

71 776

7375

370

35

48

12

102122

10

25

Figure 10-27. Cold front cross-section with surface weather chart depiction and associated METAR.

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10-23

St. Louis IndianapolisColumbus Pittsburgh

1023

1020101710141011100610051002999999100210051006

1011

1011 1011 1014 1017 1020 1023

COLD AIR

WARM AIR

C O L D A I R

CUMULONIMBUS

NIMBOSTRATUS

ALTOSTRATUS

CIRROSTRATUS

CIRRUS

METAR KSTL 1950Z 31023G40KT 8SM SCT035 05/M03 A2976

METAR KCMH 1950Z 16017KT 2SM BR 0VC080 11/10 A2970

METAR KPIT 1950Z 13012KT 75M BKN130 08/04 A3012

METAR KIND 1950Z 29028G45KT 1/2 SM TSRAGR

VV005 18/16 A2970


428 3207626

52251

47740

2

058 142834

2002066

6212

Figure 10-28. Occluded front cross-section with a weather chart depiction and associated METAR.

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11-2

weather reports (PIREPs). Using radio telemetry,radiosonde observations are made by soundingballoons from which weather data is received twicedaily. These upper air observations providetemperature, humidity, pressure, and wind data forheights up to and above 100,000 feet. In addition tothis, pilots provide vital information regarding upper

air weather observations. Pilots remain the onlyreal-time source of information regarding turbulence,icing, and cloud heights, which is gathered from pilotsin flight, through the filing of pilot weather reports orPIREPs. Together, pilot reports and radiosondeobservations provide information on upper airconditions important for flight planning. Many U.S.and international airlines have equipped their aircraftwith instrumentation that automatically transmitsin-flight weather observations through the DataLink system to the airline dispatcher who disseminates thedata to appropriate weather forecasting authorities.

RADAR OBSERVATIONSWeather observers use three types of radar to provideinformation about precipitation, wind, and weathersystems. The WSR-88D NEXRAD radar, commonlycalled Doppler radar, provides in-depth observationsthat inform surrounding communities of impendingweather. FAAterminal doppler weather radar (TDWR),installed at some major airports around the country,also aids in providing severe weather alerts andwarnings to airport traffic controllers. Terminal radarensures pilots are aware of wind shear, gust fronts, andheavy precipitation, all of which are dangerous toarriving and departing aircraft. The third type of radar

commonly used in the detection of precipitation is theFAA airport surveillance radar. This radar is usedprimarily to detect aircraft; however, it also detects thelocation and intensity of precipitation which is used toroute aircraft traffic around severe weather in an airportenvironment.

S ERVICE OUTLETSService outlets are government or private facilities thatprovide aviation weather services. Several differentgovernment agencies, including the Federal AviationAdministration (FAA), National Oceanic andAtmospheric Administration (NOAA), and theNational Weather Service (NWS) work in conjunctionwith private aviation companies to provide differentmeans of accessing weather information.

FAA FLIGHT SERVICE STATIONThe FAA Flight Service Station (FSS) is the primarysource for preflight weather information. A preflightweather briefing from an automated FSS (AFSS) canbe obtained 24 hours a day by calling 1-800-WXBRIEF almost anywhere in the U.S. In areas not servedby an AFSS, National Weather Service facilities may

provide pilot weather briefings. Telephone numbers forNWS facilities and additional numbers forFSSs/AFSSs can be found in the Airpor t/Facility

Directory (A/FD) or in the U.S. Government section of the telephone book.

Flight Service Stations also provide in-flight weather

briefing services, as well as scheduled and unscheduledweather broadcasts. An FSS may also furnish weatheradvisories to flights within the FSS region of authority.

TRANSCRIBED INFORMATIONBRIEFING SERVICE (TIBS)The Transcribed Information Briefing Service (TIBS)is a service which is prepared and disseminated byselected Automated Flight Service Stations. It providescontinuous telephone recordings of meteorological andaeronautical information. Specifically, TIBS providesarea and route briefings, airspace procedures, andspecial announcements. It is designed to be a

preliminary briefing tool and is not intended to replacea standard briefing from an FSS specialist.

The TIBS service is available 24 hours a day and isupdated when conditions change, but it can only beaccessed by a TOUCH-TONE © phone. The phonenumbers for the TIBS service are listed in the A/FD.

DIRECT USER ACCESSTERMINAL SERVICE (DUATS)The Direct User Access Terminal Service, which isfunded by the FAA, allows any pilot with a currentmedical certificate to access weather information andfile a flight plan via computer. Two methods of accessare available to connect with DUATS. The first is onthe Internet through DynCorp at http://www.duats.comor Data Transformation Corporation athttp://www.duat.com. The second method requires amodem and a communications program supplied by aDUATS provider. To access the weather informationand file a flight plan by this method, pilots use a tollfree telephone number to connect the user’s computerdirectly to the DUATS computer. The current vendorsof DUATS service and the associated phone numbersare listed in Chapter 7 of the Aeronautical Information

Manual (AIM).

ENROUTE FLIGHT ADVISORY SERVICEA service specifically designed to provide timelyenroute weather information upon pilot request isknown as the enroute flight advisory service (EFAS),or Flight Watch. EFAS provides a pilot with weatheradvisories tailored to the type of flight, route, and cruis-ing altitude. EFAS can be one of the best sources forcurrent weather information along the route of flight.

A pilot can usually contact an EFAS specialist from 6a.m. to 10 p.m. anywhere in the conterminous U.S. and

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Puerto Rico. The common EFAS frequency, 122.0MHz, is established for pilots of aircraft flying between5,000 feet AGL and 17,500 feet MSL.

HAZARDOUS IN-FLIGHT WEATHERADVISORY (HIWAS)HIWAS is a national program for broadcasting haz-

ardous weather information continuously over selectednavaids. The broadcasts include advisories such asAIRMETS , SIGMETS , convective SIGMETS , andurgent PIREPs . These broadcasts are only a summaryof the information, and pilots should contact an FSS orEFAS for detailed information. Navaids that haveHIWAS capability are depicted on sectional charts withan “H” in the upper right corner of the identificationbox. [Figure 11-1]

TRANSCRIBED WEATHER BROADCAST(TWEB)A transcribed weather broadcast is a weather reporttransmitted continuously over selected navaids. On asectional chart, a “T” in the upper right-hand corner of the navaid box indicates TWEB availability. TWEBweather usually consists of route-orientated data

including route forecasts, forecast outlook, winds aloft,and other selected weather reports for an area within 50nautical miles (NM) of the FSS or for a 50-mile widecorridor along a specific route. A TWEB forecast isvalid for 12 hours and is updated four times a day.

WEATHER BRIEFINGSPrior to every flight, pilots should gather allinformation vital to the nature of the flight. Thisincludes an appropriate weather briefing obtained froma specialist at an FSS, AFSS, or NWS.

For weather specialists to provide an appropriateweather briefing, they need to know which of the threetypes of briefings is needed—a standard briefing, anabbreviated briefing, or an outlook briefing. Otherhelpful information is whether the flight is visual flightrule (VFR) or instrument flight rule (IFR), aircraftidentification and type, departure point, estimated timeof departure (ETD), flight altitude, route of flight,

destination, and estimated time en route (ETE).

This information is recorded in the flight plan system,and a note is made regarding the type of weatherbriefing provided. If necessary, it can be referencedlater to file or amend a flight plan. It is also used whenan aircraft is overdue or is reported missing.

STANDARD BRIEFINGA standard briefing is the most complete report andprovides the overall weather picture. This type ofbriefing should be obtained prior to the departure of any flight and should be used during flight planning. Astandard briefing provides the following information insequential order if it is applicable to the route of flight.

1. Adverse Conditions— This includes informa-tion about adverse conditions that mayinfluence a decision to cancel or alter the routeof flight. Adverse conditions includessignificant weather, such as thunderstorms oraircraft icing, or other important items such asairport closings.

2. VFR Flight NOT RECOMMENDED— If the weather for the route of flight is belowVFR minimums, or if it is doubtful the flightcould be made under VFR conditions due tothe forecast weather, the briefer may state thatVFR is not recommended. It is the pilot’sdecision whether or not to continue the flightunder VFR, but this advisory should beweighed carefully.

3. Synopsis— The synopsis is an overview of thelarger weather picture. Fronts and majorweather systems that affect the general areaare provided.

H

Symbol Indicates HIWAS

Figure 11-1. HIWAS availability is shown on sectional chart.

AIRMET—In-flight weather advisory concerning moderate icing,moderate turbulence, sustained winds of 30 knots or more at thesurface, and widespread areas of ceilings less than 1,000 feet and/orvisibility less than 3 miles.

SIGMET—An in-flight weather advisory that is considered significantto all aircraft. SIGMET criteria include severe icing, severe andextreme turbulence, duststorms, sandstorms, volcanic eruptions, andvolcanic ash that lower visibility to less than 3 miles.

Convective SIGMET—A weather advisory concerning convectiveweather significant to the safety of all aircraft. Convective SIGMETsare issued for tornadoes, lines of thunderstorms, thunderstorms over awide area, embedded thunderstorms, wind gusts to 50 knots or greater,and/or hail 3 / 4 inch in diameter or greater.

Urgent PIREP—Any pilot report that contains any of the followingweather phenomena: tornadoes, funnel clouds, or waterspouts; severeor extreme turbulence, including clear air turbulence; severe icing; hail;volcanic ash; or low-level wind shear.

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4. Current Conditions— This portion of thebriefing contains the current ceilings,visibility, winds, and temperatures. If thedeparture time is more than 2 hours away,current conditions will not be included in thebriefing.

5. En Route Forecast— The en route forecast isa summary of the weather forecast for theproposed route of flight.

6. Destination Forecast— The destinationforecast is a summary of the expected weatherfor the destination airport at the estimated timeof arrival (ETA).

7. Winds and Temperatures Aloft— Winds andtemperatures aloft is a report of the winds atspecific altitudes for the route of flight.However, the temperature information is

provided only on request.

8. Notices to Airmen— This portion suppliesNOTAM information pertinent to the route of flight which has not been published in theNotice to Airmen publication. PublishedNOTAM information is provided during thebriefing only when requested.

9. ATC Delays— This is an advisory of anyknown air traffic control (ATC) delays thatmay affect the flight.

10. Other Information— At the end of thestandard briefing, the FSS specialist willprovide the radio frequencies needed to open aflight plan and to contact en route flightadvisory service (EFAS). Any additionalinformation requested is also provided at thistime.

ABBREVIATED BRIEFINGAn abbreviated briefing is a shortened version of thestandard briefing. It should be requested when adeparture has been delayed or when specific weatherinformation is needed to update the previous briefing.When this is the case, the weather specialist needs to

know the time and source of the previous briefing sothe necessary weather information will not be omittedinadvertently.

OUTLOOK BRIEFINGAn outlook briefing should be requested when aplanned departure is 6 or more hours away. It providesinitial forecast information that is limited in scope dueto the timeframe of the planned flight. This type of briefing is a good source of flight planning informationthat can influence decisions regarding route of flight,altitude, and ultimately the go, no-go decision. A

follow-up briefing prior to departure is advisable sincean outlook briefing generally only containsinformation based on weather trends and existingweather in geographical areas at or near the departureairport.

AVIATION WEATHER REPORTS

Aviation weather reports are designed to give accuratedepictions of current weather conditions. Each reportprovides current information that is updated atdifferent times. Some typical reports are aviationroutine weather reports (METAR), pilot weatherreports (PIREPs), and radar weather reports (SDs).

AVIATION ROUTINE WEATHER REPORT(METAR)An aviation routine weather report, or METAR, is anobservation of current surface weather reported in astandard international format. While the METAR codehas been adopted worldwide, each country is allowed

to make modifications to the code. Normally, thesedifferences are minor but necessary to accommodatelocal procedures or particular units of measure. Thisdiscussion of METAR will cover elements used in theUnited States.

Example:

METAR KGGG 161753Z AUTO 14021G26 3/4SM+TSRA BR BKN008 OVC012CB 18/17 A2970 RMKPRESFR

A typical METAR report contains the following

information in sequential order:

1. Type of Report— There are two types of METAR reports. The first is the routineMETAR report that is transmitted every hour.The second is the aviation selected specialweather report (SPECI). This is a specialreport that can be given at any time to updatethe METAR for rapidly changing weatherconditions, aircraft mishaps, or other criticalinformation.

2. Station Identifier— Each station is identified

by a four-letter code as established by theInternational Civil Aviation Organization(ICAO). In the 48 contiguous states, a uniquethree-letter identifier is preceded by the letter“K.” For example, Gregg County Airport inLongview, Texas, is identified by the letters“KGGG ,” K being the country designationand GGG being the airport identifier. In otherregions of the world, including Alaska andHawaii, the first two letters of the four-letterICAO identifier indicate the region, country,

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11-5

or state. Alaska identifiers always begin withthe letters “PA” and Hawaii identifiers alwaysbegin with the letters “PH.” A list of stationidentifiers can be found at an FSS or NWSoffice.

3. Date and Time of Report— The date and time

(161753Z ) are depicted in a six-digit group.The first two digits of the six-digit group arethe date. The last four digits are the time of theMETAR, which is always given inCoordinated Universal Time (UTC). A “Z” isappended to the end of the time to denote thetime is given in Zulu time (UTC) as opposedto local time.

4. Modifier— Modifiers denote that the METARcame from an automated source or that thereport was corrected. If the notation “ AUTO ”is listed in the METAR, the report came from

an automated source. It also lists “AO1” or“AO2” in the remarks section to indicate thetype of precipitation sensors employed at theautomated station.

When the modifier “ COR ” is used, itidentifies a corrected report sent out to replacean earlier report that contained an error.

Example:

METAR KGGG 161753Z COR

5. Wind— Winds are reported with five digits(14021 ) unless the speed is greater than 99knots, in which case the wind is reported withsix digits. The first three digits indicate thedirection the wind is blowing in tens of degrees. If the wind is variable, it is reportedas “VRB.” The last two digits indicate thespeed of the wind in knots (KT) unless thewind is greater than 99 knots, in which case itis indicated by three digits. If the winds aregusting, the letter “G” follows the windspeed(G26 ). After the letter “G,” the peak gustrecorded is provided. If the wind varies morethan 60° and the windspeed is greater than 6

knots, a separate group of numbers, separatedby a “V,” will indicate the extremes of thewind directions.

6. Visibility— The prevailing visibility ( 3/4 SM )is reported in statute miles as denoted by theletters “SM.” It is reported in both miles andfractions of miles. At times, RVR, or runwayvisual range is reported following the

prevailing visibility. RVR is the distance apilot can see down the runway in a movingaircraft. When RVR is reported, it is shownwith an R, then the runway number followedby a slant, then the visual range in feet. Forexample, when the RVR is reported asR17L/1400FT, it translates to a visual range of

1,400 feet on runway 17 left.

7. Weather— Weather can be broken down intotwo different categories: qualifiers andweather phenomenon (+TSRA BR ). First, thequalifiers of intensity, proximity, and thedescriptor of the weather will be given. Theintensity may be light (-), moderate ( ), orheavy (+). Proximity only depicts weatherphenomena that are in the airport vicinity. Thenotation “VC” indicates a specific weatherphenomenon is in the vicinity of 5 to 10 milesfrom the airport. Descriptors are used to

describe certain types of precipitation andobscurations. Weather phenomena may bereported as being precipitation, obscurations,and other phenomena such as squalls or funnelclouds. Descriptions of weather phenomena asthey begin or end, and hailstone size are alsolisted in the remarks sections of the report.[Figure 11-2]

8. Sky Condition— Sky condition ( BKN008OVC012CB ) is always reported in thesequence of amount, height, and type orindefinite ceiling/height (vertical visibility).The heights of the cloud bases are reportedwith a three-digit number in hundreds of feetabove the ground. Clouds above 12,000 feetare not detected or reported by an automatedstation. The types of clouds, specificallytowering cumulus (TCU) or cumulonimbus(CB) clouds, are reported with their height.Contractions are used to describe the amountof cloud coverage and obscuring phenomena.The amount of sky coverage is reported ineighths of the sky from horizon to horizon.[Figure 11-3]

9. Temperature and Dewpoint— The airtemperature and dewpoint are always given indegrees Celsius ( 18/17 ). Temperatures below0°C are preceded by the letter “M” to indicateminus.

10. Altimeter Setting— The altimeter setting isreported as inches of mercury in a four-digitnumber group ( A2970 ). It is always precededby the letter “A.” Rising or falling pressuremay also be denoted in the remarks sections as“PRESRR” or “PRESFR” respectively.

Zulu Time—A term used in aviation for coordinated universal time(UTC) which places the entire world on one time standard.

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11-6

11. Remarks— Comments may or may not appearin this section of the METAR. The informationcontained in this section may include winddata, variable visibility, beginning and endingtimes of particular phenomenon, pressureinformation, and various other informationdeemed necessary. An example of a remark regarding weather phenomenon that does notfit in any other category would be: OCNLLTGICCG. This translates as occasionallightning in the clouds, and from cloud toground. Automated stations also use theremarks section to indicate the equipmentneeds maintenance. The remarks sectionalways begins with the letters “ RMK .”

Example:

METAR BTR 161753Z 14021G26 3/4SM -RA BRBKN008 OVC012 18/17 A2970 RMK PRESFR

Explanation:Type of Report: ...............Routine METARLocation: ........................Baton Rouge, LouisianaDate: ...............................16 th day of the monthTime: ..............................1753 ZuluModifier: ........................None shownWind Information: ..........Winds 140 ° at 21 knots

gusting to 26 knotsVisibility: ........................3/4 statute mileWeather: .........................light rain and mistSky Conditions: ..............Skies broken 800 feet,

overcast 1,200Temperature: ..................Temperature 18 ° C, dewpoint

17° CAltimeter: .......................29.70 in. Hg.Remarks: ........................Barometric pressure is

falling.

Qualifier Weather Phenomena

Intensityor

Proximity1

Descriptor

2

Precipitation

3

Obscuration

4

Other

5

- Light

Moderate (no qualifier)

+ Heavy

VC in the vicinity

MI Shallow

BC Patches

DR Low Drifting

BL Blowing

SH Showers

TS Thunderstorms

FZ Freezing

PR Partial

DZ Drizzle

RA Rain

SN Snow

SG Snow grains

IC Ice Crystals (diamond dust)

PL Ice Pellets

GR Hail

GS Small hail or snow pellets

UP *Unknown Precipitation

BR Mist

FG Fog

FU Smoke

DU Dust

SA Sand

HZ Haze

PY Spray

VA Volcanic ash

PO Dust/sand whirls

SQ Squalls

FC Funnel cloud

+FC Tornado or Waterspout

SS Sandstorm

DS Dust storm

The weather groups are constructed by considering columns 1-5 in this table, in sequence;i.e., intensity, followed by descriptor, followed by weather phenomena; i.e., heavy rain showers(s)is coded as +SHRA. * Automated stations only

Figure 11-2. Descriptors and weather phenomena used in a typical METAR.

Figure 11-3. Reportable contractions for sky condition.

Sky Cover Less than 1/8(Clear)

1/8 - 2/8(Few)

3/8 - 4/8(Scattered)

5/8 - 7/8(Broken)

8/8 or Overcast(Overcast)

ContractionSKCCLRFEW

FEW SCT BKN OVC

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11-7

PILOT WEATHER REPORTS (PIREPs)Pilot weather reports provide valuable informationregarding the conditions as they actually exist in theair, which cannot be gathered from any other source.Pilots can confirm the height of bases and tops of clouds, locations of wind shear and turbulence, and thelocation of in-flight icing. If the ceiling is below 5,000

feet, or visibility is at or below 5 miles, ATC facilitiesare required to solicit PIREPs from pilots in the area.When unexpected weather conditions are encountered,pilots are encouraged to make a report to an FSS orATC. When a pilot weather report is filed, the ATCfacility or FSS will add it to the distribution system tobrief other pilots and provide in-flight advisories.

PIREPs are easy to file and a standard reporting formoutlines the manner in which they should be filed.Figure 11-4 shows the elements of a PIREP form. Itemnumbers one through five are required informationwhen making a report, as well as at least one weather

phenomenon encountered. PIREPs are normallytransmitted as an individual report, but may be

appended to a surface report. Pilot reports are easilydecoded and most contractions used in the reports areself-explanatory.

Example:

UA/OV GGG 090025/ M 1450/ FL 060/ TP C182/ SK080 OVC/ WX FV 04R/ TA 05/ WV 270030/ TB LGT/RM HVY RAIN

Explanation:Type: ...............................Routine pilot reportLocation: ........................25 NM out on the 090°

radial, Gregg County VORTime: ..............................1450 ZuluAltitude or Flight Level: 6,000 feetAircraft Type: .................Cessna 182Sky Cover: ......................8,000 overcastVisibility/Weather: ..........4 miles in rainTemperature: ..................5° CelsiusWind: ..............................270° at 30 knotsTurbulence: ....................LightIcing: ..............................None reportedRemarks: ........................Rain is heavy.

Figure 11-4. PIREP encoding and decoding.

Pilot Weather Report—A report, generated by pilots, concerningmeteorological phenomenon encountered in flight.

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RADAR WEATHER REPORTS (SD)Areas of precipitation and thunderstorms are observedby radar on a routine basis. Radar weather reports areissued by radar stations at 35 minutes past the hour,with special reports issued as needed.

Radar weather reports provide information on the type,

intensity, and location of the echo top of theprecipitation. [Figure 11-5] These reports may alsoinclude direction and speed of the area of precipitationas well as the height and base of the precipitation inhundreds of feet MSL. RAREPs are especiallyvaluable for preflight planning to help avoid areas of severe weather. However, radar only detects objects inthe atmosphere that are large enough to be consideredprecipitation. Cloud bases and tops, ceilings, andvisibility are not detected by radar.

A typical RAREP will include:

• Location identifier and time of radar observation.

• Echo pattern:

1. Line (LN)—A line of precipitationechoes at least 30 miles long, at least fourtimes as long as it is wide, and at least 25 per-cent coverage within the line.

2. Area (AREA)—A group of echoes of similar type and not classified as a line.

3. Single Cell (CELL)—A singleisolated convective echo such as a rain shower.

• Area coverage in tenths.

• Type and intensity of weather.

• Azimuth, referenced to true north, and range, innautical miles, from the radar site, of pointsdefining the echo pattern. For lines and areas,there will be two azimuth and range sets thatdefine the pattern. For cells, there will be only oneazimuth and range set.

• Dimension of echo pattern—The dimension of anecho pattern is given when the azimuth and rangedefine only the center line of the pattern.

• Cell movement—Movement is only coded forcells; it will not be coded for lines or areas.

• Maximum top of precipitation and location.

Maximum tops may be coded with the symbols“MT” or “MTS.” If it is coded with “MTS,” itmeans that satellite data as well as radarinformation was used to measure the top of theprecipitation.

• If the word “AUTO” appears in the report, itmeans the report is automated from WSR-88Dweather radar data.

• The last section is primarily used to prepare radarsummary charts, but can be used during preflightto determine the maximum precipitation intensitywithin a specific grid box. The higher the number,the greater the intensity. Two or more numbersappearing after a grid box reference, such asPM34, indicates precipitation in consecutive gridboxes.

Example:

TLX 1935 LN 8 TRW++ 86/40 199/11520W C2425 MTS 570 AT 159/65 AUTO^MO1 NO2 ON3 PM34 QM3 RL2=

11-8

Radar is operating normally but thereare no echoes being detected.

SYMBOL MEANING

R Rain

RW Rain Shower

S Snow

SW Snow Shower

T Thunderstorm

SYMBOL INTENSITY

- Light

(none) Moderate

+ Heavy

++ Very Heavy

X Intense

XX Extreme

CONTRACTION OPERATIONAL STATUS

PPINE

PPINA

PPIOM

AUTO

Radar observation is not available.

Radar is inoperative or out of service.

Automated radar report from WSR-88D.

Figure 11-5. Radar weather report codes.

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Explanation:

The radar report gives the following information: Thereport is automated from Oklahoma City and was madeat 1935 UTC. The echo pattern for this radar reportindicates a line of echos covering 8/10ths of the area.Thunderstorms and very heavy rain showers areindicated. The next set of numbers indicate the azimuththat defines the echo (86° at 40 NM and 199° at 115NM). Next, the dimension of this echo is given as 20nautical miles wide (10 nautical miles on either side of the line defined by the azimuth and range). The cellswithin the line are moving from 240 ° at 25 knots. Themaximum top of the precipitation, as determined byradar and satellite, is 57,000 feet and it is located on the159 ° radial, 65 NM out. The last line indicates theintensity of the precipitation, for example in grid QMthe intensity is 3 or heavy precipitation. (1 is light and6 is extreme.)

AVIATION FORECASTS

Observed weather condition reports are often used inthe creation of forecasts for the same area. A variety of different forecast products are produced and designedto be used in the preflight planning stage. The printedforecasts that pilots need to be familiar with are theterminal aerodrome forecast (TAF), aviation areaforecast (FA), in-flight weather advisories (SIGMET,AIRMET), and the winds and temperatures aloftforecast (FD).

TERMINAL AERODROMEFORECASTS (TAF)A terminal aerodrome forecast is a report establishedfor the 5 statute mile radius around an airport. TAFreports are usually given for larger airports. Each TAFis valid for a 24-hour time period, and is updated fourtimes a day at 0000Z, 0600Z, 1200Z, and 1800Z. TheTAF utilizes the same descriptors and abbreviations asused in the METAR report.

The terminal forecast includes the followinginformation in sequential order:

1. Type of Report— A TAF can be either aroutine forecast (TAF) or an amended forecast

(TAF AMD).

2. ICAO Station Identifier— The stationidentifier is the same as that used in a METAR.

3. Date and Time of Origin— Time and date of TAF origination is given in the six-numbercode with the first two being the date, the lastfour being the time. Time is always given inUTC as denoted by the Z following thenumber group.

4. Valid Period Date and Time— The validforecast time period is given by a six-digitnumber group. The first two numbers indicatethe date, followed by the two-digit beginningtime for the valid period, and the last twodigits are the ending time.

5. Forecast Wind— The wind direction andspeed forecast are given in a five-digit numbergroup. The first three indicate the direction of the wind in reference to true north. The lasttwo digits state the windspeed in knots asdenoted by the letters “KT.” Like the METAR,winds greater than 99 knots are given in threedigits.

6. Forecast Visibility— The forecast visibility isgiven in statute miles and may be in wholenumbers or fractions. If the forecast is greaterthan 6 miles, it will be coded as “P6SM.”

7. Forecast Significant Weather— Weatherphenomenon is coded in the TAF reports in thesame format as the METAR. If no significantweather is expected during the forecast timeperiod, the denotation “NSW” will be includedin the “becoming” or “temporary” weathergroups.

8. Forecast Sky Condition— Forecast sky con-ditions are given in the same manner as theMETAR. Only cumulonimbus (CB) clouds areforecast in this portion of the TAF report asopposed to CBs and towering cumulus in theMETAR.

9. Forecast Change Group— For anysignificant weather change forecast to occurduring the TAF time period, the expectedconditions and time period are included in thisgroup. This information may be shown asFrom (FM), Becoming (BECMG), andTemporary (TEMPO). “From” is used when arapid and significant change, usually within anhour, is expected. “Becoming” is used when agradual change in the weather is expected overa period of no more than 2 hours. “Temporary”is used for temporary fluctuations of weather,expected to last for less than an hour.

10. Probability Forecast— The probabilityforecast is given percentage that describes theprobability of thunderstorms and precipitationoccurring in the coming hours. This forecast isnot used for the first 6 hours of the 24-hourforecast.

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Example:

TAFKPIR 111130Z 111212 15012KT P6SM BKN090TEMPO 1214 5SM BRFM1500 16015G25KT P6SM SCT040 BKN250FM0000 14012KT P6SM BKN080 OVC150 PROB400004 3SM TSRA BKN030CBFM0400 1408KT P6SM SCT040 OVC080 TEMPO0408 3SM TSRA OVC030CBBECMG 0810 32007KT=

Explanation:

Routine TAF for Pierre, South Dakota … on the 11 th dayof the month, at 1130Z … valid for 24 hours from 1200Zon the 11 th to 1200Z on the 12 th … wind from 150 ° at 12knots … visibility greater than 6 statute miles … brokenclouds at 9,000 feet … temporarily, between 1200Z and1400Z, visibility 5 statute miles in mist … from 1500Zwinds from 160 ° at 15 knots, gusting to 25 knotsvisibility greater than 6 statute miles … clouds scatteredat 4,000 feet and broken at 25,000 feet … from 0000Zwind from 140 ° at 12 knots … visibility greater than 6statute miles … clouds broken at 8,000 feet, overcast at15,000 feet … between 0000Z and 0400Z, there is 40percent probability of visibility 3 statute miles …

thunderstorm with moderate rain showers … cloudsbroken at 3,000 feet with cumulonimbus clouds … from0400Z … winds from 140 ° at 8 knots … visibility greaterthan 6 miles … clouds at 4,000 scattered and overcast at8,000 … temporarily between 0400Z and 0800Z …

visibility 3 miles … thunderstorms with moderate rainshowers … clouds overcast at 3,000 feet withcumulonimbus clouds … becoming between 0800Z and1000Z … wind from 320 ° at 7 knots … end of report (=).

AREA FORECASTS (FA)The aviation area forecast (FA) gives a picture of clouds, general weather conditions, and visualmeteorological conditions (VMC) expected over alarge area encompassing several states. There are sixareas for which area forecasts are published in thecontiguous 48 states. Area forecasts are issued threetimes a day and are valid for 18 hours. This type of

forecast gives information vital to en route operationsas well as forecast information for smaller airports thatdo not have terminal forecasts.

Area forecasts are typically disseminated in foursections and include the following information:

1. Header— This gives the location identifier of the source of the FA, the date and time of issuance, the valid forecast time, and the areaof coverage.

Example:

DFWC FA 120945SYNOPSIS AND VFR CLDS/WXSYNOPSIS VALID UNTIL 130400CLDS/WX VALID UNTIL 122200…OTLK VALID122200-130400OK TX AR LA MS AL AND CSTL WTRS

Explanation:

The area forecast shows information given by DallasFort Worth, for the region of Oklahoma, Texas,Arkansas, Louisiana, Mississippi, and Alabama, aswell as a portion of the gulf coastal waters. It wasissued on the 12 th day of the month at 0945. Thesynopsis is valid from the time of issuance until 0400hours on the 13 th. VFR clouds and weather informationon this area forecast is valid until 2200 hours on the12 th and the outlook is valid until 0400 hours on the13 th.

2. Precautionary Statements— IFR conditions,mountain obscurations, and thunderstormhazards are described in this section.Statements made here regarding height aregiven in MSL, and if given otherwise, AGL orCIG (ceiling) will be noted.

Example:

SEE AIRMET SIERRA FOR IFR CONDS ANDMTN OBSCN.TS IMPLY SEV OR GTR TURB SEV ICE LLWSAND IFR CONDS.NON MSL HGTS DENOTED BYAGL OR CIG.

Explanation:

The area forecast covers VFR clouds and weather, sothe precautionary statement warns that AIRMET Sierrashould be referenced for IFR conditions and mountainobscuration. The code TS indicates the possibility of thunderstorms and implies there may be an occurrence

of severe or greater turbulence, severe icing, low-levelwind shear, and IFR conditions. The final line of theprecautionary statement alerts the user that heights, forthe most part, are mean sea level (MSL). Those that arenot MSL will be above ground level (AGL) or ceiling(CIG).

3. Synopsis— The synopsis gives a briefsummary identifying the location andmovement of pressure systems, fronts, andcirculation patterns.

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six regions of forecast, states, regional areas, andcommon geographical features.

IN-FLIGHT WEATHER ADVISORIESIn-flight weather advisories, which are provided to enroute aircraft, are forecasts that detail potentiallyhazardous weather. These advisories are also availableto pilots prior to departure for flight planning purposes.An in-flight weather advisory is issued in the form of either an AIRMET, SIGMET, or Convective SIGMET.

AIRMAN’S METEOROLOGICAL INFORMATION(AIRMET)AIRMETs (WAs) are examples of in-flight weatheradvisories that are issued every 6 hours withintermediate updates issued as needed for a particulararea forecast region. The information contained in anAIRMET is of operational interest to all aircraft, butthe weather section concerns phenomena consideredpotentially hazardous to light aircraft and aircraft withlimited operational capabilities.

An AIRMET includes forecast of moderate icing,moderate turbulence, sustained surface winds of 30knots or greater, widespread areas of ceilings less than1,000 feet and/or visibilities less than 3 miles, andextensive mountain obscurement.

Each AIRMET bulletin has a fixed alphanumericdesignator, numbered sequentially for easyidentification, beginning with the first issuance of theday. SIERRA is the AIRMET code used to denoteinstrument flight rules (IFR) and mountainobscuration; TANGO is used to denote turbulence,strong surface winds, and low-level wind shear; andZULU is used to denote icing and freezing levels.

Example:

DFWT WA 241650AIRMET TANGO UPDT 3 FOR TURBC… STGSFC WINDS AND LLWS VALID UNTIL 242000

AIRMET TURBC… OK TX…UPDTFROM OKC TO DFW TO SAT TO MAF TO CDSTO OKC OCNL MDT TURBC BLO 60 DUE TO

STG AND GUSTY LOW LVL WINDS. CONDSCONTG BYD 2000Z

Explanation:

This AIRMET was issued by Dallas Fort Worth on the24 th day of the month, at 1650 Zulu time. On this thirdupdate, the AIRMET Tango is issued for turbulence,strong surface winds, and low-level wind shear until2000 Zulu on the same day. The turbulence section of the AIRMET is an update for Oklahoma and Texas. It

defines an area from Oklahoma City to Dallas, Texas,to San Antonio, to Midland, Texas, to Childress, Texas,to Oklahoma City that will experience occasionalmoderate turbulence below 6,000 feet due to strong andgusty low-level winds. It also notes that theseconditions are forecast to continue beyond 2000 Zulu.

SIGNIFICANT METEOROLOGICALINFORMATION (SIGMET)SIGMETs (WSs) are in-flight advisories concerningnon-convective weather that is potentially hazardous toall aircraft. They report weather forecasts that includesevere icing not associated with thunderstorms, severeor extreme turbulence or clear air turbulence (CAT) notassociated with thunderstorms, dust storms orsandstorms that lower surface or in-flight visibilities tobelow 3 miles, and volcanic ash.

SIGMETs are unscheduled forecasts that are valid for4 hours, but if the SIGMET relates to hurricanes, it isvalid for 6 hours.

A SIGMET is issued under an alphabetic identifier,from November through Yankee, excluding Sierra andTango. The first issuance of a SIGMET is designated asa UWS, or Urgent Weather SIGMET. Re-issuedSIGMETs for the same weather phenomenon aresequentially numbered until the weather phenomenonends.

Example:

SFOR WS 100130SIGMET ROME02 VALID UNTIL 100530OR WAFROM SEA TO PDT TO EUG TO SEAOCNL MOGR CAT BTN 280 AND 350 EXPCDDUE TO JTSTR.CONDS BGNG AFT 0200Z CONTG BYD 0530Z .

Explanation:

This is SIGMET Romeo 2, the second issuance for thisweather phenomenon. It is valid until the 10th day of the month at 0530 Zulu time. This SIGMET is forOregon and Washington, for a defined area fromSeattle to Portland to Eugene to Seattle. It calls foroccasional moderate or greater clear air turbulencebetween 28,000 and 35,000 feet due to the location of the jetstream. These conditions will be beginning after0200 Zulu and will continue beyond the forecast scopeof this SIGMET of 0530 Zulu.

CONVECTIVE SIGNIFICANT METEOROLOGICALINFORMATION (WST)A Convective SIGMET (WST) is an in-flight weatheradvisory issued for hazardous convective weather thataffects the safety of every flight. Convective SIGMETs

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are issued for severe thunderstorms with surface windsgreater than 50 knots, hail at the surface greater thanor equal to 3/4 inch in diameter, or tornadoes.They are also issued to advise pilots ofembedded thunderstorms, lines of thunderstorms,or thunderstorms with heavy or greater precipitationthat affect 40 percent or more of a 3,000 square foot or

greater region.

Convective SIGMETs are issued for each area of thecontiguous 48 states but not Alaska or Hawaii.Convective SIGMETs are issued for the eastern (E),western (W), and central (C) United States. Each reportis issued at 55 minutes past the hour, but special reportscan be issued during the interim for any reason. Eachforecast is valid for 2 hours. They are numberedsequentially each day from 1-99, beginning at 00 Zulutime. If no hazardous weather exists, the ConvectiveSIGMET will still be issued; however, it will state“CONVECTIVE SIGMET…. NONE.”

Example:

MKCC WST 221855CONVECTIVE SIGMET 21CVALID UNTIL 2055KS OK TXVCNTY GLD-CDS LINENO SGFNT TSTMS RPRTDLINE TSTMS DVLPG BY 1955Z WILL MOV EWD30-35 KT THRU 2055ZHAIL TO 2 IN PSBL

Explanation:

This Convective SIGMET provides the followinginformation: The WST indicates this report is aConvective SIGMET. The current date is the 22 nd of the month and it was issued at 1855 Zulu. It isConvective SIGMET number 21C, indicating that it isthe 21st consecutive report issued for the central UnitedStates. This report is valid for 2 hours until 2055 Zulutime. The Convective SIGMET is for an area fromKansas to Oklahoma to Texas, in the vicinity of a linefrom Goodland, Kansas, to Childress, Texas. Nosignificant thunderstorms are being reported, but a lineof thunderstorms will develop by 1955 Zulu time andwill move eastward at a rate of 30-35 knots through2055 Zulu. Hail up to 2 inches in size is possible withthe developing thunderstorms.

WINDS AND TEMPERATURE ALOFTFORECAST (FD)Winds and temperatures aloft forecasts provide windand temperature forecasts for specific locations in thecontiguous United States, including network locationsin Hawaii and Alaska. The forecasts are made twice aday based on the radiosonde upper air observationstaken at 0000Z and 1200Z.

Through 12,000 feet are true altitudes and above18,000 feet are pressure altitudes. Wind direction isalways in reference to true north and windspeed isgiven in knots. The temperature is given in degreesCelsius. No winds are forecast when a given level iswithin 1,500 feet of the station elevation. Similarly,temperatures are not forecast for any station within

2,500 feet of the station elevation.

If the windspeed is forecast to be greater than 100 knotsbut less than 199 knots, the computer adds 50 to thedirection and subtracts 100 from the speed. To decodethis type of data group, the reverse must beaccomplished. For example, when the data appears as“731960,” subtract 50 from the 73 and add 100 to the19, and the wind would be 230 ° at 119 knots with atemperature of –60 ° C. If the windspeed is forecast tobe 200 knots or greater, the wind group is coded as 99knots. For example, when the data appears as “7799,”subtract 50 from 77 and add 100 to 99, and the wind is

270°

at 199 knots or greater. When the forecastwindspeed is calm or less than 5 knots, the data groupis coded “9900,” which means light and variable.[Figure 11-7]

Explanation of figure 11-7:

The heading indicates that this FD was transmitted onthe 15th of the month at 1640Z and is based on the 1200Zulu radiosonde. The valid time is 1800 Zulu on thesame day and should be used for the period between1700Z and 2100Z. The heading also indicates that thetemperatures above 24,000 feet MSL are negative.Since the temperatures above 24,000 feet are negative,the minus sign is omitted.

A 4-digit data group shows the wind direction inreference to true north, and the windspeed in knots. The

elevation at Amarillo, TX (AMA) is 3,605 feet, so thelowest reportable altitude is 6,000 feet for the forecastwinds. In this case, “2714” means the wind is forecastto be from 270 ° at a speed of 14 knots.

A 6-digit group includes the forecast temperature aloft.The elevation at Denver (DEN) is 5,431 feet, so thelowest reportable altitude is 9,000 feet for the windsand temperature forecast. In this case, “2321-04”indicates the wind is forecast to be from 230 ° at a speedof 21 knots with a temperature of –4 ° C.

FD KWBC 151640BASED ON 151200Z DATAVALID 151800Z FOR USE 1700-2100ZTEMPS NEGATIVE ABV 24000FD 3000 6000 9000 12000 18000 24000 30000AMA 2714 2725+00 2625-04 2531-15 2542-27 265842DEN 2321-04 2532-08 2434-19 2441-31 235347

Figure 11-7. Winds and temperatures aloft forecast.

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WEATHER CHARTSWeather charts are graphic charts that depict current orforecast weather. They provide an overall picture of theUnited States and should be used in the beginningstages of flight planning. Typically, weather chartsshow the movement of major weather systems andfronts. Surface analysis, weather depiction, and radarsummary charts are sources of current weatherinformation. Significant weather prognostic chartsprovide an overall forecast weather picture.

SURFACE ANALYSIS CHARTThe surface analysis chart, depicts an analysis of thecurrent surface weather. [Figure 11-8] This chart is acomputer prepared report that is transmitted every 3hours and covers the contiguous 48 states and adjacentareas. A surface analysis chart shows the areas of highand low pressure, fronts, temperatures, dewpoints,wind directions and speeds, local weather, and visualobstructions.

Surface weather observations for reporting pointsacross the United States are also depicted on this chart.Each of these reporting points is illustrated by a stationmodel. [Figure 11-9] A station model will include:

• Type of Observation— A round model indicatesan official weather observer made theobservation. A square model indicates the

observation is from an automated station.Stations located offshore give data from ships,buoys, or offshore platforms.

• Sky Cover— The station model depicts total skycover and will be shown as clear, scattered,broken, overcast, or obscured/partially obscured.

• Clouds— Cloud types are represented by specificsymbols. Low cloud symbols are placed beneaththe station model, while middle and high cloudsymbols are placed directly above the stationmodel. Typically, only one type of cloud will bedepicted with the station model.

• Sea Level Pressure— Sea level pressure given inthree digits to the nearest tenth of a millibar. For1000 mbs or greater, prefix a 10 to the threedigits. For less than 1000 mbs, prefix a 9 to thethree digits.

• Pressure Change/Tendency— Pressure changein tenths of millibars over the past 3 hours. Thisis depicted directly below the sea level pressure.

• Precipitation— A record of the precipitation thathas fallen over the last 6 hours to the nearesthundredth of an inch.

• Dewpoint— Dewpoint is given in degreesFahrenheit.

Figure 11-8. Surface analysis chart.

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• Present Weather— Over 100 different weathersymbols are used to describe the current weather.

• Temperature— Temperature is given in degreesFahrenheit.

• Wind— True direction of wind is given by thewind pointer line, indicating the direction fromwhich the wind is coming. A short barb is equalto 5 knots of wind, a long barb is equal to 10knots of wind, and a pennant is equal to 50 knots.

WEATHER DEPICTION CHARTA weather depiction chart details surface conditions asderived from METAR and other surface observations.

The weather depiction chart is prepared andtransmitted by computer every 3 hours beginning at0100 Zulu time, and is valid at the time of the plotteddata. It is designed to be used for flight planning bygiving an overall picture of the weather across theUnited States. [Figure 11-10]

This type of chart typically displays major fronts orareas of high and low pressure. The weather depictionchart also provides a graphic display of IFR, VFR, andMVFR (marginal VFR) weather. Areas of IFRconditions (ceilings less than 1,000 feet and visibilityless than 3 miles) are shown by a hatched area outlinedby a smooth line. MVFR regions (ceilings 1,000 to3,000 feet, visibility 3 to 5 miles) are shown by a

Figure 11-9. Sample station model and weather chart symbols.

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non-hatched area outlined by a smooth line. Areas of VFR (no ceiling or ceiling greater than 3,000 feet andvisibility greater than 5 miles) are not outlined.

Weather depiction charts show a modified stationmodel that provides sky conditions in the form of totalsky cover, cloud height or ceiling, weather, andobstructions to visibility, but does not include winds orpressure readings like the surface analysis chart. Abracket ( ] ) symbol to the right of the station indicatesthe observation was made by an automated station. Adetailed explanation of a station model is depicted inthe previous discussion of surface analysis charts.

RADAR SUMMARY CHARTA radar summary chart is a graphically depictedcollection of radar weather reports (SDs). [Figure11-11] The chart is published hourly at 35 minutes pastthe hour. It displays areas of precipitation as well asinformation regarding the characteristics of theprecipitation. [Figure 11-12] A radar summary chartincludes:

• No information— If information is not reported,the chart will say “NA.” If no echoes aredetected, the chart will say “NE.”

• Precipitation intensity contours— Intensity canbe described as one of six levels and is shown onthe chart by three contour intervals.

• Height of tops— The heights of the echo tops aregiven in hundreds of feet MSL.

• Movement of cells— Individual cell movementis indicated by an arrow pointing in the directionof movement. The speed of movement in knots isthe number at the top of the arrow head. “LM”indicates little movement.

• Type of precipitation— The type of precipitationis marked on the chart using specific symbols.These symbols are not the same as used on theMETAR charts.

• Echo configuration— Echoes are shown asbeing areas, cells, or lines.

• Weather watches— Severe weather watch areasfor tornadoes and severe thunderstorms aredepicted by boxes outlined with heavy dashedlines.

The radar summary chart is a valuable tool for preflightplanning. It does, however, contain several limitationsfor the usage of the chart. This chart depicts only areasof precipitation. It will not show areas of clouds andfog with no appreciable precipitation, or the height of the tops and bases of the clouds. Radar summary chartsare a depiction of current precipitation and should beused in conjunction with current METAR and weatherforecasts.

Figure 11-10. Weather depiction chart.

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Figure 11-11. Radar summary chart.

Figure 11-12. Intensity levels and contours, and precipitation type symbols.

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SIGNIFICANT WEATHER PROGNOSTICCHARTS

Significant Weather Prognostic Charts are available forlow-level significant weather from the surface toFL240 (24,000 feet), also referred to as the 400millibar level, and high-level significant weather from

FL250 to FL600 (25,000 to 60,000 feet). The primaryconcern of this discussion is the low-level significantweather prognostic chart.

The low-level chart comes in two forms: the 12- and24-hour forecast chart, and the 36 and 48 surface onlyforecast chart. The first chart is a four-panel chart thatincludes 12- and 24-hour forecasts for significantweather and surface weather. Charts are issued fourtimes a day at 0000Z, 0600Z, 1200Z, and 1800Z. Thevalid time for the chart is printed on the lower left-handcorner of each panel.

The upper two panels show forecast significant

weather, which may include nonconvective turbulence,freezing levels, and IFR or MVFR weather. Areas of moderate or greater turbulence are enclosed in dashedlines. Numbers within these areas give the height of theturbulence in hundreds of feet MSL. Figures below theline show the anticipated base, while figures above theline show the top of the zone of turbulence. Also shownon this panel are areas of VFR, IFR, and MVFR. IFRareas are enclosed by solid lines, MVFR areas areenclosed by scalloped lines, and the remaining,unenclosed area is designated VFR. Zigzag lines andthe letters “SFC” indicate freezing levels in that area

are at the surface. Freezing level height contours forthe highest freezing level are drawn at 4,000-footintervals with dashed lines.

The lower two panels show the forecast surfaceweather and depicts the forecast locations andcharacteristics of pressure systems, fronts, and

precipitation. Standard symbols are used to show frontsand pressure centers. Direction of movement of thepressure center is depicted by an arrow. The speed, inknots, is shown next to the arrow. In addition, areas of forecast precipitation and thunderstorms are outlined.Areas of precipitation that are shaded indicate at leastone-half of the area is being affected by the precipita-tion. Unique symbols indicate the type of precipitationand the manner in which it occurs.

Figure 11-13 depicts a typical significant weatherprognostic chart as well as the symbols typically usedto depict precipitation. Prognostic charts are anexcellent source of information for preflight planning;

however, this chart should be viewed in light of currentconditions and specific local area forecasts.

The 36- and 48-hour significant weather prognosticchart is an extension of the 12- and 24-hour forecast. Itprovides information regarding only surface weatherforecasts and includes a discussion of the forecast. Thischart is issued only two times a day. It typicallycontains forecast positions and characteristics ofpressure patterns, fronts, and precipitation. An exampleof a 36- and 48-hour surface prognostic chart is shownin figure 11-14.

Figure 11-13. Significant weather prognostic chart.

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Figure 11-14. 36- and 48-hour surface prognostic chart.

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12-1

Each time a pilot operates an airplane, the flight nor-mally begins and ends at an airport. An airport maybe a small sod field or a large complex utilized by aircarriers. This chapter discusses airport operationsand identifies features of an airport complex, as wellas provides information on operating on or in thevicinity of an airport.

TYPES OF AIRPORTSThere are two types of airports.

• Controlled Airport

• Uncontrolled Airport

CONTROLLED AIRPORTA controlled airport has an operating control tower. Airtraffic control (ATC) is responsible for providing forthe safe, orderly, and expeditious flow of air traffic atairports where the type of operations and/or volume of traffic requires such a service. Pilots operating from acontrolled airport are required to maintain two-wayradio communication with air traffic controllers, and toacknowledge and comply with their instructions.

Pilots must advise ATC if they cannot comply with theinstructions issued and request amended instructions.

A pilot may deviate from an air traffic instruction in anemergency, but must advise ATC of the deviation assoon as possible.

UNCONTROLLED AIRPORTAn uncontrolled airport does not have an operatingcontrol tower. Two-way radio communications are notrequired, although it is a good operating practice forpilots to transmit their intentions on the specified fre-quency for the benefit of other traffic in the area. Figure12-1 lists recommended communication procedures.

More information on radio communications will bediscussed later in this chapter.

S OURCES FOR AIRPORT DATAWhen a pilot flies into a different airport, it isimportant to review the current data for that airport.This data can provide the pilot with information,such as communication frequencies, services avail-able, closed runways, or airport construction. Threecommon sources of information are:

• Aeronautical Charts

• Airport/Facility Directory (A/FD)

• Notices to Airmen (NOTAMs)

AERONAUTICAL CHARTSAeronautical charts provide specific information onairports. Chapter 14 contains an excerpt from an aero-nautical chart and an aeronautical chart legend, whichprovides guidance on interpreting the information onthe chart.

AIRPORT/FACILITY DIRECTORYThe Airport/Facility Directory (A/FD) provides themost comprehensive information on a given airport. Itcontains information on airports, heliports, and sea-

plane bases that are open to the public. The A/FDs arecontained in seven books, which are organized byregions. These A/FDs are revised every 8 weeks.Figure 12-2 contains an excerpt from a directory. For acomplete listing of information provided in an A/FDand how the information may be decoded, refer to the“Directory Legend Sample” located in the front of eachA/FD.

In the back of each A/FD, there is information such asspecial notices, parachute jumping areas, and facility

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FACILITY ATAIRPORT

FREQUENCY USECOMMUNICATION/BROADCAST PROCEDURES

OUTBOUND INBOUNDPRACTICE

INSTRUMENTAPPROACH

UNICOM(No Tower or FSS)

Communicate with UNICOM station on publishedCTAF frequency (122.7, 122.8, 122.725, 122.975,or 123.0). If unable to contact UNICOM station,use self-announce procedures on CTAF.

Before taxiing andbefore taxiing on therunway for departure.

10 miles out.Entering downwind,base, and final.Leaving the runway.

No Tower, FSS,or UNICOM

Self-announce on MULTICOM frequency 122.9 Departing finalapproach fix(name) or onfinal approachsegment inbound.

Approachcompleted/ terminated.

No Tower in operation,FSS open

Communicate with FSS on CTAF frequency.

FSS closed(No Tower)

Self-announce on CTAF.

Self-announce on CTAF.Tower or FSSnot in operation









Figure 12-1. Recommended communication procedures.

Figure 12-2. Airport/Facility Directory excerpt.

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telephone numbers. It would be helpful to review anA/FD to become familiar with the information itcontains.

NOTICES TO AIRMENNotices to Airmen (NOTAMs) provide the most currentinformation available. They provide time-critical infor-mation on airports and changes that affect the nationalairspace system and are of concern to instrument flightrule (IFR) operations. NOTAM information is classifiedinto three categories. These are NOTAM-D or distant,NOTAM-L or local, and flight data center (FDC)NOTAMs. NOTAM-Ds are attached to hourly weatherreports and are available at flight service stations(AFSS/FSS). NOTAM-Ls include items of a local nature,such as taxiway closures or construction near a runway.These NOTAMs are maintained at the FSS nearest theairport affected. NOTAM-Ls must be requested from anFSS other than the one nearest the local airport for whichthe NOTAM was issued. FDC NOTAMs are issued by

the National Flight Data Center and contain regulatoryinformation, such as temporary flight restrictions or anamendment to instrument approach procedures. TheNOTAM-Ds and FDC NOTAMs are contained in theNotices to Airmen publication, which is issued every 28days. Prior to any flight, pilots should check for anyNOTAMs that could affect their intended flight.

AIRPORT MARKINGS AND SIGNSThere are markings and signs used at airports, whichprovide directions and assist pilots in airport opera-tions. Some of the most common markings and signswill be discussed. Additional information may be foundin the Aeronautical Information Manual (AIM).

RUNWAY MARKINGSRunway markings vary depending on the type of oper-ations conducted at the airport. Figure 12-3 shows arunway that is approved as a precision instrumentapproach runway and also shows some other commonrunway markings. A basic VFR runway may only havecenterline markings and runway numbers.

Since aircraft are affected by the wind during takeoffsand landings, runways are laid out according to the

local prevailing winds. Runway numbers are in refer-ence to magnetic north. Certain airports have two oreven three runways laid out in the same direction.These are referred to as parallel runways and are distin-guished by a letter being added to the runway number.Examples are runway 36L (left), 36C (center), and 36R(right).

Another feature of some runways is a displaced thresh-old. A threshold may be displaced because of anobstruction near the end of the runway. Although this

portion of the runway is not to be used for landing, itmay be available for taxiing, takeoff, or landing rollout.

Some airports may have a blast pad/stopway area. Theblast pad is an area where a propeller or jet blast candissipate without creating a hazard. The stopway areais paved in order to provide space for an airplane to

decelerate and stop in the event of an aborted takeoff.These areas cannot be used for takeoff or landing.

TAXIWAY MARKINGSAirplanes use taxiways to transition from parkingareas to the runway. Taxiways are identified by a con-tinuous yellow centerline stripe. A taxiway mayinclude edge markings to define the edge of the taxi-way. This is usually done when the taxiway edge doesnot correspond with the edge of the pavement. If anedge marking is a continuous line, the paved shoulderis not intended to be used by an airplane. If it is adashed marking, an airplane may use that portion of the pavement. Where a taxiway approaches a runway,there may be a holding position marker. These consistof four yellow lines (two solid and two dashed). Thesolid lines are where the airplane is to hold. At somecontrolled airports, holding position markings may befound on a runway. They are used when there areintersecting runways, and air traffic control issuesinstructions such as “cleared to land—hold short of runway 30.”

OTHER MARKINGSSome of the other markings found on the airportinclude vehicle roadway markings, VOR receivercheckpoint markings, and non-movement area bound-

ary markings.

Vehicle roadway markings are used when necessary todefine a pathway for vehicle crossing areas that are alsointended for aircraft. These markings usually consist of a solid white line to delineate each edge of the roadwayand a dashed line to separate lanes within the edges of the roadway.

A VOR receiver checkpoint marking consists of apainted circle with an arrow in the middle. The arrow isaligned in the direction of the checkpoint azimuth. Thisallows pilots to check aircraft instruments with naviga-

tional aid signals.

A non-movement area boundary marking delineates amovement area under air traffic control. These mark-ings are yellow and located on the boundary betweenthe movement and non-movement area. They normallyconsist of two yellow lines (one solid and one dashed).

AIRPORT SIGNSThere are six types of signs that may be found at air-ports. The more complex the layout of an airport, the

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more important the signs become to pilots. Figure 12-4shows examples of signs, their purpose, and appropri-ate pilot action. The six types of signs are:

• Mandatory Instruction Signs —have a redbackground with a white inscription. These signsdenote an entrance to a runway, a critical area, ora prohibited area.

• Location Signs —are black with yellow inscrip-tion and a yellow border and do not have arrows.They are used to identify a taxiway or runwaylocation, to identify the boundary of the runway,

or identify an instrument landing system (ILS)critical area.

• Direction Signs —have a yellow backgroundwith black inscription. The inscription identifiesthe designation of the intersecting taxiway(s)leading out of an intersection.

• Destination Signs —have a yellow backgroundwith black inscription and also contain arrows.These signs provide information on locatingthings, such as runways, terminals, cargo areas,and civil aviation areas.

1 9

A

A

4

LEGENDIn-Pavement Runway Guard Lights

Elevated Runway Guard Lights

Stop BarCenterline/Lead-On Lights

Clearance Bar LightsPosition Marking4

Stop Bar/ ILS Hold

VehicleLanes

(Zipper Style)

SurfacePaintedRunwayMarking

Taxiway EdgeMarking

(Do Not Cross)

Taxiway/ Taxiway

HoldMarking

HoldMarkingFor Land

And HoldShortOperations

T e r m i n a l

} Non-Movement Area

Figure 12-3. Selected airport markings and surface lighting.

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• Information Signs —have a yellow backgroundwith black inscription. These signs are used toprovide the pilot with information on such thingsas areas that cannot be seen from the controltower, applicable radio frequencies, and noiseabatement procedures. The airport operator deter-mines the need, size, and location of these signs.

• Runway Distance Remaining Signs —have ablack background with white numbers. Thenumbers indicate the distance of the remainingrunway in thousands of feet.

AIRPORT LIGHTINGThe majority of airports have some type of lightingfor night operations. The variety and type of lightingsystems depends on the volume and complexity of operations at a given airport. Airport lighting is stan-

dardized so that airports use the same light colors forrunways and taxiways.

AIRPORT BEACONAirport beacons help a pilot identify an airport at night.The beacons are operated from dusk till dawn andsometimes they are turned on if the ceiling is less than1,000 feet and/or the ground visibility is less than 3statute miles (visual flight rules minimums). However,there is no requirement for this, so a pilot has theresponsibility of determining if the weather is VFR.

The beacon has a vertical light distribution to make itmost effective from 1-10° above the horizon, althoughit can be seen well above or below this spread. The bea-con may be an omnidirectional capacitor-dischargedevice, or it may rotate at a constant speed, which pro-duces the visual effect of flashes at regular intervals.The combination of light colors from an airport beaconindicates the type of airport. [Figure 12-5]

Some of the most common beacons are:

• Flashing white and green for civilian landairports.

• Flashing white and yellow for a water airport.

• Flashing white, yellow, and green for a heliport.

• Two quick white flashes followed by a greenflash identifies a military airport.

4-22

8-APCH

ILS

4

B

22

J

L

22

MIL

Hold short of runway on taxiway

Exit boundary of ILS critical area

Defines directions to takeoff runways

Defines directions for arriving aircraft

Provides remaining runway lengthin 1,000 feet increments

Indicates taxiway does not continue

Defines direction & designation of exit taxiwayfrom runway

Runway Exit: .

Defines direction & designation of intersectingtaxiway(s)

Taxiway Direction:

Hold short of aircraft on approach

Hold short of ILS approach critical area

Identifies paved areas where aircraft entry isprohibited

Identifies taxiway on which aircraft is located

Identifies runway on which aircraft is located

Taxiway/Runway Hold Position:

Runway Approach Hold Position:

ILS Critical Area Hold Position:

Exit boundary of runway protected areas

Runway Safety Area/Obstacle FreeZone Boundary:

ILS Critical Area Boundary:

No Entry:

Taxiway Location:

Outbound Destination:

Inbound Destination:

Taxiway Ending MarkerRunway Location:

Runway Distance Remaining

TYPE OF SIGN AND ACTION OR PURPOSE TYPE OF SIGN AND ACTION OR PURPOSE

AIRPORT SIGN SYSTEMS

26-8 Runway/Runway Hold Position:

Hold short of intersecting runway

A LGDirection Sign Array:

Identifies location in conjunction withmultiple intersecting taxiways

Figure 12-4. Airport signs.

WhiteWhite

White

GreenGreen

Figure 12-5. Airport rotating beacons.

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APPROACH LIGHT SYSTEMSApproach light systems are primarily intended toprovide a means to transition from instrument flightto visual flight for landing. The system configura-tion depends on whether the runway is a precision ornonprecision instrument runway. Some systemsinclude sequenced flashing lights, which appear to

the pilot as a ball of light traveling toward the run-way at high speed. Approach lights can also aidpilots operating under VFR at night.

VISUAL GLIDESLOPE INDICATORSVisual glideslope indicators provide the pilot withglidepath information that can be used for day or nightapproaches. By maintaining the proper glidepath asprovided by the system, a pilot should have adequateobstacle clearance and should touch down within aspecified portion of the runway.

VISUAL APPROACH SLOPE INDICATORVisual approach slope indicator (VASI) installationsare the most common visual glidepath systems in use.The VASI provides obstruction clearance within 10°of the runway extended runway centerline, and to 4nautical miles (NM) from the runway threshold.

A VASI consists of light units arranged in bars. Thereare 2-bar and 3-bar VASIs. The 2-bar VASI has nearand far light bars and the 3-bar VASI has near, middle,and far light bars. Two-bar VASI installations provideone visual glidepath which is normally set at 3°. The3-bar system provides two glidepaths with the lower

glidepath normally set at 3° and the upper glidepathone-fourth degree above the lower glidepath.

The basic principle of the VASI is that of color differen-tiation between red and white. Each light unit projects abeam of light having a white segment in the upper partof the beam and a red segment in the lower part of thebeam. The lights are arranged so the pilot will see thecombination of lights shown in figure 12-6 to indicatebelow, on, or above the glidepath.

OTHER GLIDEPATH SYSTEMSA precision approach path indicator (PAPI) uses lightssimilar to the VASI system except they are installed ina single row, normally on the left side of the runway.[Figure 12-7]

A tri-color system consists of a single light unit projecting a three-color visual approach path. A below theglidepath indication is red, on the glidepath color isgreen, and above the glidepath is indicated by amber.When descending below the glidepath, there is a smallarea of dark amber. Pilots should not mistake this areafor an “above the glidepath” indication. [Figure 12-8]

There are also pulsating systems, which consist of asingle light unit projecting a two-color visual approachpath. A below the glidepath indication is shown by asteady red light, slightly below is indicated by pulsat-ing red, on the glidepath is indicated by a steady whitelight, and a pulsating white light indicates above the

glidepath. [Figure 12-9]

RUNWAY LIGHTINGThere are various lights that identify parts of the runwaycomplex. These assist a pilot in safely making a takeoff or landing during night operations.

RUNWAY END IDENTIFIER LIGHTSRunway end identifier lights (REIL) are installed atmany airfields to provide rapid and positive identifi-

High(More Than

3.5 Degrees)

Slightly High(3.2 Degrees)

On Glidepath(3 Degrees)

Slightly Low(2.8 Degrees)

Low(Less Than

2.5 Degrees)

Below Glidepath

Near Bar

Far Bar

On Glidepath Above Glidepath

Figure 12-6. 2-Bar VASI system.

Figure 12-7. Precision approach path indicator.

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cation of the approach end of a particular runway. Thesystem consists of a pair of synchronized flashinglights located laterally on each side of the runwaythreshold. REILs may be either omnidirectional orunidirectional facing the approach area.

RUNWAY EDGE LIGHTSRunway edge lights are used to outline the edges of runways at night or during low visibility conditions.These lights are classified according to the intensitythey are capable of producing. They are classified ashigh intensity runway lights (HIRL), medium intensityrunway lights (MIRL), or low intensity runway lights(LIRL). The HIRL and MIRL have variable intensitysettings. These lights are white, except on instrumentrunways, where amber lights are used on the last 2,000feet or half the length of the runway, whichever is less.The lights marking the end of the runway are red.

IN-RUNWAY LIGHTINGTouchdown zone lights (TDZL), runway centerlinelights (RCLS), and taxiway turnoff lights are installed

on some precision runways to facilitate landing underadverse visibility conditions. TDZLs are two rows of transverse light bars disposed symmetrically about therunway centerline in the runway touchdown zone.RCLS consists of flush centerline lights spaced at 50foot intervals beginning 75 feet from the landingthreshold. Taxiway turnoff lights are flush lights, whichemit a steady green color.

CONTROL OF AIRPORT LIGHTINGAirport lighting is controlled by air traffic controllersat controlled airports. At uncontrolled airports, thelights may be on a timer, or where an FSS is located atan airport, the FSS personnel may control the light-ing. A pilot may request various light systems beturned on or off and also request a specified intensity,

if available, from ATC or FSS personnel. At selecteduncontrolled airports, the pilot may control the light-ing by using the radio. This is done by selecting aspecified frequency and clicking the radio micro-phone. For information on pilot controlled lighting at

Amber

Green

Red

Amber A b o

v e G l i d e p a t h

O n G l i d e p a t h

B e l o w G l i d e p a t h

Figure 12-8. Tri-color visual approach slope indicator.

PULSATING WHITEULS TING WHITE

PULSATING RED

STEADY RED S l i g h t l y B e l o w G l i d e p a t h

B e l o w G l i d e p a t h

Threshold

PULSATING WHITE

A b o v e G l i d

e p a t h STEADY WHITE

O n G l i d e p a t h

Figure 12-9. Pulsating visual approach slope indicator.

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various airports, refer to the Airport /Facil ity Directory . [Figure 12-10]

TAXIWAY LIGHTSOmnidirectional taxiway lights outline the edges of thetaxiway and are blue in color. At many airports, theseedge lights may have variable intensity settings thatmay be adjusted by an air traffic controller when

deemed necessary or when requested by the pilot.Some airports also have taxiway centerline lights thatare green in color.

OBSTRUCTION LIGHTSObstructions are marked or lighted to warn pilots of their presence during daytime and nighttime condi-tions. Obstruction lighting can be found both on andoff an airport to identify obstructions. They may bemarked or lighted in any of the following conditions.

• Red Obstruction Lights —either flash or emit a

steady red color during nighttime operations, andthe obstructions are painted orange and white fordaytime operations.

• High Intensity White Obstruction Light —flashes high intensity white lights during thedaytime with the intensity reduced for nighttime.

• Dual Lighting —is a combination of flashing redbeacons and steady red lights for nighttime oper-ation, and high intensity white lights for daytimeoperations.

WIND DIRECTION INDICATORSIt is important for a pilot to know the direction of thewind. At facilities with an operating control tower,this information is provided by ATC. Informationmay also be provided by FSS personnel located at aparticular airport or by requesting information on acommon traffic advisory frequency (CTAF) at air-ports that have the capacity to receive and broadcaston this frequency.

When none of these services is available, it is possibleto determine wind direction and runway in use byvisual wind indicators. A pilot should check these windindicators even when information is provided on theCTAF at a given airport because there is no assurancethat the information provided is accurate.

Wind direction indicators include a wind sock, windtee, or tetrahedron. These are usually located in a cen-tral location near the runway and may be placed in thecenter of a segmented circle, which will identify thetraffic pattern direction, if it is other than the standardleft-hand pattern. [Figures 12-11 and 12-12]

The wind sock is a good source of information since itnot only indicates wind direction, but allows the pilotto estimate the wind velocity and gusts or factor. Thewind sock extends out straighter in strong winds andwill tend to move back and forth when the wind is

gusty. Wind tees and tetrahedrons can swing freely, andwill align themselves with the wind direction. The windtee and tetrahedron can also be manually set to alignwith the runway in use; therefore, a pilot should alsolook at the wind sock, if available.

RADIO COMMUNICATIONSOperating in and out of a controlled airport, as wellas in a good portion of the airspace system, requiresthat an aircraft have two-way radio communicationcapability. For this reason, a pilot should be knowl-edgeable of radio station license requirements andradio communications equipment and procedures.

RADIO LICENSEThere is no license requirement for a pilot operatingin the United States; however, a pilot who operatesinternationally is required to hold a restrictedradiotelephone permit issued by the FederalCommunications Commission (FCC). There is alsono station license requirement for most general avia-tion aircraft operating in the United States. A stationlicense is required however for an aircraft which isoperating internationally, which uses other than a veryhigh frequency (VHF) radio, and which meets othercriteria.

RADIO EQUIPMENTIn general aviation, the most common types of radiosare VHF. A VHF radio operates on frequenciesbetween 118.0 and 136.975 and is classified as 720 or760 depending on the number of channels it canaccommodate. The 720 and 760 uses .025 spacing(118.025, 118.050) with the 720 having a frequencyrange up to 135.975 and the 760 going up to 136.975.VHF radios are limited to line of sight transmissions;

7 times within 5seconds



Highest intensity available

Medium or lower intensity(Lower REIL or REIL off)

Lowest intensity available(Lower REIL or REIL off)

KEY MIKE FUNCTION

Figure 12-10. Radio control runway lighting.

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12-9

therefore, aircraft at higher altitudes are able to trans-mit and receive at greater distances.

Using proper radio phraseology and procedures will

contribute to a pilot’s ability to operate safely andefficiently in the airspace system. A review of thePilot/Controller Glossary contained in the

Aeronautical Information Manual (AIM) will assist apilot in the use and understanding of standard termi-nology. The AIM also contains many examples of radio communications, which should be helpful.

The International Civil Aviation Organization (ICAO)has adopted a phonetic alphabet, which should be usedin radio communications. When communicating with

ATC, pilots should use this alphabet to identify theiraircraft. [Figure 12-13]

LOST COMMUNICATION PROCEDURESIt is possible that a pilot might experience a malfunc-tion of the radio. This might cause the transmitter,receiver, or both to become inoperative. If a receiverbecomes inoperative and a pilot needs to land at a con-trolled airport, it is advisable to remain outside orabove Class D airspace until the direction and flow of traffic is determined. A pilot should then advise thetower of the aircraft type, position, altitude, andintention to land. The pilot should continue, enter thepattern, report a position as appropriate, and watch

TETRAHEDRON

WIND TEE

WIND SOCKOR CONE

Application of Traffic Pattern

Indicators

Legend:

Recommended Standard Left-HandTraffic pattern (depicted)(Standard Right-hand TrafficPattern would be the opposite)

DOWNWIND

DEPARTURE

FINAL

D E P A

R T U R

E

BASE

DEPARTURE

RUNWAY

HAZARD ORPOPULATED AREA

LANDING DIRECTION

INDICATOR

WINDCONE

LANDING RUNWAY(OR LANDING STRIP)INDICATORS

SEGMENTED CIRCLE

TRAFFIC PATTERN INDICATORS

CROSS- WIND

E N T R

Y

Figure 12-12. Segmented circle and airport traffic pattern.

Figure 12-11.Wind direction indicators.

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for light signals from the tower. Light signal colorsand their meanings are contained in figure 12-14.

If the transmitter becomes inoperative, a pilot shouldfollow the previously stated procedures and also moni-tor the appropriate air traffic control frequency. Duringdaylight hours air traffic control transmissions may beacknowledged by rocking the wings, and at night byblinking the landing light.

When both receiver and transmitter are inoperative, thepilot should remain outside of Class D airspace untilthe flow of traffic has been determined and then enterthe pattern and watch for light signals.

If a radio malfunctions prior to departure, it is advisableto have it repaired, if possible. If this is not possible, a

call should be made to air traffic control and the pilotshould request authorization to depart without two-wayradio communications. If authorization is given todepart, the pilot will be advised to monitor the appro-priate frequency and/or watch for light signals asappropriate.

AIR TRAFFIC CONTROL SERVICESBesides the services provided by FSS as discussed inChapter 11, there are numerous other services providedby ATC. In many instances a pilot is required to have con-tact with air traffic control, but even when not required, apilot will find it helpful to request their services.

PRIMARY RADARRadar is a method whereby radio waves are transmittedinto the air and are then received when they have beenreflected by an object in the path of the beam. Range is

AlfaBravoCharlieDelta

EchoFoxtrotGolf HotelIndiaJulietKiloLimaMikeNovemberOscarPapaQuebecRomeoSierra

TangoUniformVictorWhiskeyXrayYankeeZuluOneTwoThreeFourFiveSixSevenEightNineZero

(AL-FAH)(BRAH-VOH)(CHAR-LEE) OR (SHAR-LEE)(DELL-TAH)

(ECK-OH)(FOKS-TROT)(GOLF)(HOH-TEL)(IN-DEE-AH)(JEW-LEE-ETT)(KEY-LOH)(LEE-MAH)(MIKE)(NO-VEM-BER)(OSS-CAH)(PAH-PAH)(KEH-BECK)(ROW-ME-OH)(SEE-AIR-RAH)

(TANG-GO)(YOU-NEE-FORM) OR (OO-NEE-FORM)(VIK-TAH)(WISS-KEY)(ECKS-RAY)(YANG-KEY)(ZOO-LOO)(WUN)(TOO)(TREE)(FOW-ER)(FIFE)(SIX)(SEV-EN)(AIT)(NIN-ER)(ZEE-RO)

TELEPHONY PHONIC (PRONUNCIATION)

ABCD

EFGHIJKLMNOPQRS

TUVWXYZ1234567890

CHARACTER MORSE CODE

•--•••-•-•-••

•••-•--••••••••----•-•-••---•---•--•--•-•-••••

-••-•••-•---••--•----•••----••---•••--••••-•••••-••••--•••---••----•-----

Figure 12-13. Phonetic alphabet.

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determined by measuring the time it takes (at the speedof light) for the radio wave to go out to the object andthen return to the receiving antenna. The direction of adetected object from a radar site is determined by theposition of the rotating antenna when the reflected por-tion of the radio wave is received.

Modern radar is very reliable and there are seldomoutages. This is due to reliable maintenance andimproved equipment. There are, however, some limi-tations which may affect air traffic control servicesand prevent a controller from issuing advisories con-cerning aircraft which are not under their control andcannot be seen on radar.

The characteristics of radio waves are such that theynormally travel in a continuous straight line unless theyare “bent” by atmospheric phenomena such as temper-ature inversions, reflected or attenuated by denseobjects such as heavy clouds and precipitation, orscreened by high terrain features.

AIR TRAFFIC CONTROL RADARBEACON SYSTEMThe air traffic control radar beacon system (ATCRBS) isoften referred to as “secondary surveillance radar.” Thissystem consists of three components and helps in allevi-ating some of the limitations associated with primaryradar. The three components are an interrogator,transponder, and radarscope. The advantages of ATCRBSare the reinforcement of radar targets, rapid target identi-fication, and a unique display of selected codes.

TRANSPONDERThe transponder is the airborne portion of the second-ary surveillance radar system and a system with whicha pilot should be familiar. The ATCRBS cannot displaythe secondary information unless an aircraft isequipped with a transponder. A transponder is alsorequired to operate in certain controlled airspace.Airspace is discussed in chapter 13.

A transponder code consists of four numbers fromzero to seven (4,096 possible codes). There are somestandard codes, or ATC may issue a four-digit code toan aircraft. When a controller requests a code or func-tion on the transponder, the word “squawk” may beused. Figure 12-15 lists some standard transponderphraseology.

RADAR TRAFFIC INFORMATION SERVICERadar equipped air traffic control facilities provideradar assistance to VFR aircraft provided the aircraftcan communicate with the facility and are within radarcoverage. This basic service includes safety alerts, traf-fic advisories, limited vectoring when requested, andsequencing at locations where this procedure has beenestablished. In addition to basic radar service, terminalradar service area (TRSA) has been implemented atcertain terminal locations. The purpose of this serviceis to provide separation between all participating VFRaircraft and all IFR aircraft operating within the TRSA.Class C service provides approved separation betweenIFR and VFR aircraft, and sequencing of VFR aircraftto the primary airport. Class B service providesapproved separation of aircraft based on IFR, VFR,

LIGHT GUN SIGNALS

COLOR ANDTYPE OF SIGNAL

MOVEMENT OF VEHICLES,EQUIPMENT AND PERSONNEL

AIRCRAFT ONTHE GROUND

AIRCRAFTIN FLIGHT

STEADYGREEN

FLASHINGGREEN

STEADY RED

FLASHING RED

FLASHING WHITE

ALTERNATINGRED AND GREEN

Cleared to cross,proceed or go

Cleared for takeoff Cleared to land

Not applicable Cleared for taxi Return for landing(to be followed by steadygreen at the proper time)

STOP STOP Give way to other aircraftand continue circling

Clear the taxiway/runway Taxi clear of therunway in use

Airport unsafe, donot land

Return to starting point on airport Return to starting pointon airport

Not applicable

Exercise Extreme Caution!!!! Exercise Extreme Caution!!!! Exercise Extreme Caution!!!!

Figure 12-14. Light gun signals.

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and/or weight, and sequencing of VFR arrivals to theprimary airport(s).

ATC issues traffic information based on observed radartargets. The traffic is referenced by azimuth from theaircraft in terms of the 12-hour clock. Also the distancein nautical miles, direction in which the target is mov-ing, and the type and altitude of the aircraft, if known,are given. An example would be: “Traffic 10 o’clock 5miles east bound, Cessna 152, 3,000 feet.” The pilotshould note that traffic position is based on the aircrafttrack, and that wind correction can affect the clock

position at which a pilot locates traffic. [Figure 12-16]

WAKE TURBULENCEAll aircraft generate a wake while in flight. This distur-bance is caused by a pair of counter-rotating vorticestrailing from the wingtips. The vortices from larger air-craft pose problems to encountering aircraft. The wake of these aircraft can impose rolling moments exceeding theroll-control authority of the encountering aircraft. Also,the turbulence generated within the vortices can damage

aircraft components and equipment if encountered atclose range. For this reason, a pilot must envision thelocation of the vortex wake and adjust the flightpathaccordingly.

During ground operations and during takeoff, jet-engine blast (thrust stream turbulence) can causedamage and upsets at close range. For this reason,pilots of small aircraft should consider the effects of

TRACK TRACK

Traffic information would be issued to the pilot of aircraft "A"as 12 o'clock. The actual position of the traffic as seen bythe pilot of aircraft "A" would be 2 o'clock. Traffic informationissued to aircraft "B" would also be given as 12 o'clock, butin this case, the pilot of "B" would see traffic at 10 o'clock.

Figure 12-16. Traffic advisories.

SQUAWK (number)

IDENT

SQUAWK (number) and IDENT

SQUAWK STANDBY

SQUAWK LOW/NORMAL

SQUAWK ALTITUDE

STOP ALTITUDE SQUAWK

STOP S QU AWK ( mo de i n u se )

STOP SQUAWK

SQUAWK MAYDAY

SQUAWK VFR

Opera te radar beacon t ransponder on des igna ted code in MODE A/3

Engage the "IDENT" fea ture (mi li ta ry I /P) of the t ransponder.

O pe ra te t ra ns po nd er o n sp ec if ie d c od e i n MO DE A/ 3 a nd e nga gethe "IDENT" (mili tary I/P) feature.

Switch transponder to standby posit ion.

Opera te t ransponder on low or normal sensi tivi ty as speci fied .Transponder i s opera ted in "NORMAL" pos it ion unless ATCspecifies " LOW" ("ON" is used inste ad o f "NORMAL" a s amaster control label on some types of t ransponders).

Activate MODE C with automatic alt itude reporting.

Turn off alt itude reporting switch and continue transmitt ingMO DE C f ra mi ng p ul se s. I f yo ur e qu ip me nt d oe s n ot h ave t hi scapabil ity, turn off MODE C.

Swi tch off speci fied mode. (Used for mil it ary a ircraf t when thecontrol le r i s unaware of mi li ta ry service requi rements for thea ircraf t to continue opera tion on another MODE.)

Switch off transponder.

Opera te t ransponder in the emergency pos it ion (MODE A Code7 70 0 fo r c iv il t ra ns po nd er. MO DE 3 Co de 7 70 0 a nd e me rg en cyfeature for military transponder.)

O pe ra te ra da r b ea co n t ra ns po nd er o n Co de 1 20 0 i n t he M OD EA/ 3, o r o the r a pp ro pri at e V FR c od e.

RADAR BEACON PHRASEOLOGY

Figure 12-15. Transponder phraseology.

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12-13

jet -engine blast and mainta in adequate separa tion.Also, pilots of larger aircraft should consider theeffects of their aircraft’s jet-engine blast on other air-craft and equipment on the ground.

VORTEX GENERATIONLift is generated by the creation of a pressure differentialover the wing surface. The lowest pressure occurs overthe upper wing surface, and the highest pressure underthe wing. This pressure differential triggers the rollup of the airflow aft of the wing resulting in swirling airmasses trailing downstream of the wingtips. After therollup is completed, the wake consists of two counter-rotating cylindrical vortices. Most of the energy is withina few feet of the center of each vortex, but pilots shouldavoid a region within about 100 feet of the vortex core.[Figure 12-17]

VORTEX STRENGTHThe strength of the vortex is governed by the weight,speed, and shape of the wing of the generating aircraft.The vortex characteristics of any given aircraft can alsobe changed by the extension of flaps or other wing con-figuration devices as well as by a change in speed. Thegreatest vortex strength occurs when the generating air-craft is heavy, clean, and slow.

VORTEX BEHAVIORTrailing vortices have certain behavioral characteristicsthat can help a pilot visualize the wake location and takeavoidance precautions.

Vortices are generated from the moment an aircraftleaves the ground, since trailing vortices are the by-product of wing lift. The vortex circulation is outward,upward, and around the wingtips when viewed fromeither ahead or behind the aircraft. Tests have shownthat vortices remain spaced a bit less than a wingspanapart, drifting with the wind, at altitudes greater than awingspan from the ground. Tests have also shown thatthe vortices sink at a rate of several hundred feet perminute, slowing their descent and diminishing instrength with time and distance behind the generatingaircraft. [Figure 12-18]

When the vortices of larger aircraft sink close to theground (within 100 to 200 feet), they tend to move lat-erally over the ground at a speed of 2 or 3 knots. Acrosswind will decrease the lateral movement of theupwind vortex and increase the movement of the down-wind vortex. A tailwind condition can move the vorticesof the preceding aircraft forward into the touchdownzone.

VORTEX AVOIDANCE PROCEDURES

• Landing behind a larger aircraft on the samerunway—stay at or above the larger aircraft’sapproach flightpath and land beyond its touch-down point.

• Landing behind a larger aircraft on a parallelrunway closer than 2,500 feet—consider thepossibility of drift and stay at or above the largeraircraft’s final approach flightpath and note itstouchdown point.

• Landing behind a larger aircraft on crossing run-way—cross above the larger aircraft’s flightpath.

Figure 12-17. Vortex generation.

Touchdown Rotation

Wake Ends Wake Begins

Figure 12-18. Vortex behavior.

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12-14

• Landing behind a departing aircraft on the samerunway—land prior to the departing aircraft’srotating point.

• Landing behind a larger aircraft on a crossingrunway—note the aircraft’s rotation point and if past the intersection, continue and land prior tothe intersection. If the larger aircraft rotates priorto the intersection, avoid flight below its flight-path. Abandon the approach unless a landing isensured well before reaching the intersection.

• Departing behind a large aircraft, rotate prior tothe large aircraft’s rotation point and climb aboveits climb path until turning clear of the wake.

• For intersection takeoffs on the same runway, bealert to adjacent larger aircraft operations, partic-ularly upwind of the runway of intended use. If an intersection takeoff clearance is received,avoid headings that will cross below the largeraircraft’s path.

• If departing or landing after a large aircraft execut-ing a low approach, missed approach, or touch andgo landing (since vortices settle and move laterallynear the ground, the vortex hazard may exist alongthe runway and in the flightpath, particularly in aquartering tailwind), it is prudent to wait 2 minutesprior to a takeoff or landing.

• En route it is advisable to avoid a path belowand behind a large aircraft, and if a large aircraftis observed above on the same track, change theaircraft position laterally and preferablyupwind.

COLLISION AVOIDANCETitle 14 of the Code of Federal Regulations (14 CFR)part 91 has established right-of-way rules, minimumsafe altitudes, and VFR cruising altitudes to enhanceflight safety. The pilot can contribute to collision avoid-ance by being alert and scanning for other aircraft. Thisis particularly important in the vicinity of an airport.

Effective scanning is accomplished with a series of short, regularly spaced eye movements that bring suc-cessive areas of the sky into the central visual field.Each movement should not exceed 10°, and eachshould be observed for at least 1 second to enabledetection. Although back and forth eye movementsseem preferred by most pilots, each pilot shoulddevelop a scanning pattern that is most comfortable andthen adhere to it to assure optimum scanning.

Even if entitled to the right-of-way, a pilot should giveway if it is felt another aircraft is too close.

CLEARING PROCEDURESThe following procedures and considerationsshould assist a pilot in collision avoidance undervarious situations.

• Before Takeoff —Prior to taxiing onto a runwayor landing area in preparation for takeoff, pilotsshould scan the approach area for possible land-ing traffic, executing appropriate maneuvers toprovide a clear view of the approach areas.

• Climbs and Descents —During climbs anddescents in flight conditions which permit visualdetection of other traffic, pilots should executegentle banks left and right at a frequency whichpermits continuous visual scanning of the air-space.

• Straight and Level —During sustained peri-ods of straight-and-level flight, a pilot shouldexecute appropriate clearing procedures atperiodic intervals.

• Traffic Patterns —Entries into traffic patternswhile descending should be avoided.

• Traffic at VOR Sites —Due to converging traffic,sustained vigilance should be maintained in thevicinity of VORs and intersections.

• Training Operations —Vigilance should bemaintained and clearing turns should be madeprior to a practice maneuver. During instruction,the pilot should be asked to verbalize the clearingprocedures (call out “clear left, right, above, andbelow”).

High-wing and low-wing aircraft have their respectiveblind spots. High-wing aircraft should momentarilyraise their wing in the direction of the intended turn andlook for traffic prior to commencing the turn. Low-wing aircraft should momentarily lower the wing.

RUNWAY INCURSION AVOIDANCEIt is important to give the same attention to operat-ing on the surface as in other phases of flights.Proper planning can prevent runway incursions andthe possibility of a ground collision. A pilot shouldbe aware of the airplane’s position on the surface atall times and be aware of other aircraft and vehicleoperations on the airport. At times controlled air-ports can be busy and taxi instructions complex. In

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this situation it may be advisable to write down taxiinstructions. The following are some practices tohelp prevent a runway incursion.

• Read back all runway crossing and/or holdinstructions.

• Review airport layouts as part of preflightplanning and before descending to land, andwhile taxiing as needed.

• Know airport signage.

• Review Notices to Airmen (NOTAM) for informa-tion on runway/taxiway closures and constructionareas.

• Request progressive taxi instructions from ATCwhen unsure of the taxi route.

• Check for traffic before crossing any Runway

Hold Line and before entering a taxiway.

• Turn on aircraft lights and the rotating beacon orstrobe lights while taxing.

• When landing, clear the active runway as soon aspossible, then wait for taxi instructions beforefurther movement.

• Study and use proper phraseology in order tounderstand and respond to ground controlinstructions.

• Write down complex taxi instructions at unfamil-iar airports.

For more detailed information, refer to AdvisoryCircular (AC) 91-73, Part 91 Pilot and FlightcrewProcedures During Taxi Operations and Part 135

Single-Pilot Operations.

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