Weather Theory Made Easy

The basics of aviation weather, including the cases of carious weather phenomena and how they can affect the safety of flight.

For more information on aviation weather, refer to Aviation Weather for Pilots(AC 00-6A) Aviation Weather Services (AC 00-45G) Internet Communications of Aviation Weather

and NOTAMS (00-62)

Introduction

The Atmosphere

• Composition– 78% Nitrogen– 21% Oxygen– 1% Inert

• Vertical Structure– Troposphere– Tropopause– Stratosphere– Mesosphere & thermosphere

Variations Diurnal Seasonal Variation with Latitude Topographic Altitude

Temperature

Atmospheric Pressure

• Measurement– Barometer

• Mercurial• Aneroid

• Variation– Altitude– Temperature

• Pressure Depiction on Chart– Isobars

Pressure Systems Highs

H surrounded by L Lows

L surrounded by H Ridge

Elongated are of H Trough

Elongated are of L Col

Neutral area between 2 H or 2 L

Atmospheric Pressure

Wind

• Convection• Pressure Gradient Force• Corriolis Force• Friction• Global Circulation Patterns• Jet Stream

Land & Sea Breezes Sea Breeze Land Breeze

Mountain/Valley Winds Valley Winds Mountain Winds

Katabatic Winds Wind Shear

Local/Small Scale Winds

Moisture

• Water Vapor– Relative Humidity– Dewpoint

• Change in State– Evaporation– Condensation– Freezing & Melting– Sublimation & Deposition

Super-Cooled Water? Exist at temps below freezing Freeze upon impact with an exposed object

Condensation Nuclei Microscopic particles Serves as a place for condensation to form Salt, dust, by-products of combustion

Moisture

Clouds/Cloud Formation

• Formation– Air moving over a cold surface– Air stagnating over a cold surface– Expansion cooling of upward moving air

• Classification– Fog– Low Clouds– Middle Clouds– High Clouds– Clouds with Extensive Vertical Development

Low Clouds Almost entirely of water Super cooled water, snow, ice Surface to ±6,500’

Middle Clouds Primarily of water Most is Super cooled Bases range from ±6,500’ to ±23,000’

Classification

Classification

• High Clouds– Almost entirely ice crystals– Base range ±16,500 to FL450

• Clouds with Extensive Vertical Development– Super cooled water above the freezing level– Tops may be ice crystals– Base range from ±1,000 to above 10K

Particle Growth Via added condensation or sublimation Via collision of particles

Precip vs. Cloud Thickness To produce significant precip, 4000’ thick Thicker the cloud, heavier the precip

Precipitation

Lapse Rate

The decrease in temperature with increasing altitude

Approx. 2˚C/1000’

Resists any upward OR downward development

Unstable may grow into a vertical or convective current

Atmospheric Stability

Temperature Inversion

An increase in temperature with an increase in altitude

Lapse rate is inverted

Usually confined to shallow layers of air

Visibility is often restricted

Unequal heating of the Earths surface

Overlying air

heated unevenl

y

Warm air pushed

aloft

Lapse rate will determin

e the rest

Temperatures Role in Stability

Moistures Role in Stability

Unsaturated• 3˚C/1000’• Unstable

Normal• 2˚C/1000’• Stable

Saturated

• 1.1˚ to 2.8˚C/1000’

• Unstable

Every physical process of weather is accompanied by or is the result of a _____________.

1. Movement of air2. Pressure differential3. Heat exchange

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What causes variations in altimeter settings between weather reporting points?

1. Unequal heating of the Earth’s surface

2. Variation of terrain elevation

3. Coriolis force

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The wind at 5,000 feet AGL is southwesterly while the surface wind is southerly. The difference is direction is primarily due to?

1. Stronger pressure gradient at higher altitudes

2. Friction between the wind & the surface

3. Stronger Coriolis force at the surface

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Convective circulation patterns associated with sea breezes are

caused by?

1. Warm, dense air moving inland from over the water

2. Water, absorbing & radiating heat faster than the land

3. Cool, dense air moving inland from over the water

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The development of thermals depends upon ___________.

1. A counterclockwise circulation of air

2. Temperature inversions3. Solar heating

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Air Masses

• What is an air mass?– Large body of air with fairly uniform

• Temperature• Moisture content

– Forms when air remains stationary and takes on the characteristics of the underlying surface

Temp & moisture content of the underlying surface

Tropical oceans & large deserts Middle latitudes are poor regions

Source Regions

Classifications

• Temp– Polar– Tropical

• Moisture content– Continental– Maritime

As it moves from its source region, it begins to change

Warming from below can cause instability Cooling from below may result in stability

May also cause poor visibility from fog and low clouds

Modification

Fronts

Boundaries between air masses

Change in temp May be abrupt or gradual

Change in wind direction or velocity Always shifts to the right in the Northern

Hemisphere Change in pressure

Usually a drop as you approach it Always reset your altimeter

Frontal Passage Detection

Cold Fronts

Hugs the ground as it

moves due to gravity

Forces warmer less

dense air aloft

Movement is usually in an

easterly direction

Cumulus Clouds

Turbulence

Showery Precipitati

on

Gusty Winds

Good Visibility

Speed Will Dictate the Weather

Fast Moving Cold Fronts

Moved along by intense high pressure system

Surface friction causes a steep frontal slope

Wide difference between temp & moisture between the two masses

Squall lines will precede if the air is moist and unstable. Usually 50 to 300 miles ahead.

Fast Moving Cold Front

Slow Moving Cold Front

• Much shallower frontal slop

Occurs when warm air overtakes cooler air Move at much slower rates Frontal slope is very gradual

Warm Fronts

Generalizations

• Stratus clouds• Smooth air• Steady precipitation• Poor visibility

Occurs when two air masses are equally balanced

A mix of both air masses may be present for several days

Stationary Fronts

Frontal Occlusions

• Occurs when a fast moving cold front catches up to a slow moving warm front

• The difference in temperature determines the front that is produced (warm or cold front occlusion)

Weather Hazards

Thunderstorms

• 3 conditions necessary– Unstable lapse rate– High moisture content– Some form of lifting action

3 stages Cumulus or building stage

Continuous updrafts Mature stage

Rain at the surface Dissipating stage

Downdrafts Anvil top

Thunderstorms

Types of T-Storms

• Air Mass– Convection on hot summer days

• Frontal– Collision of 2 AM’s

• Orographic– Mountainous areas

• Converging Air flow from 2 active storms• Squall lines

Turbulence Lightning Microburst Hail

Hazards

Do’s & Don’ts

DO NOT• Take off or land in the

face of a storm• Fly under a storm• Fly in a cloud mass

without airborne radar• Trust the outward

appearance of a cloud

DO• Avoid by 20 miles• Clear the tops by 1000’

for each knot wind• Circumnavigate the area

for 6/10th coverage• Avoid lightning areas• Regard tops of 35K or

higher as hazadous

Tighten your seatbelt & secure loose items Plan & hold your course for minimum time

through the storm Plan to enter below the freezing level Pitot heat & carb heat on Set power to establish Va Turn up flight deck lighting Turn off autopilot

If You Can’t Avoid

During Penetration

• DO keep you eyes on your instruments• DO maintain maneuvering speed• DO Allow the aircraft to “Ride the Waves”• DO Maintain Course for a minimum time

through the storm

Can take place at any altitude Can be caused by wind shear, convection

currents, & obstructions to airflow Often found near the jet stream

CAT

Wake Turbulence

• Caused by wing tip vortices• Cup in a cup

Icing

• Rime Ice– Tiny rain droplets or drizzle– Milky in color

• Clear Ice– Large water droplets– Cumulous clouds– Above the freezing level

Restrictions to Visibility

Fog

• Gets its name by how its formed– Radiation or ground– Advection– Upslope– Steam

Haze Airborne dust particles

Smoke Combustion materials

Smog Smoke & Fog mixed

Haze, Smoke, Smog

Cooling from aloft tends to make an air mass more…

1 2

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1. Stable

2. Unstable

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When an air mass is warmed from below, its stability is…

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1. Increased2. Decreased

Transition zone between two different air masses…

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1 2 3

1. Frontal zone

2. Trade wind

3. Trough

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This will always change when flying across a front…

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1. Wind Direction

2. Type of precipitation

3. Stability of the air mass

Cold fronts in the US generally move to the…

1. East-Northeast

2. East-Southeast

3. West-Southwest

4. South

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What type of front generally produces the most violent flying

weather?

1. Warm2. Cold3. Fast-moving cold front4. Stationary

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What type of cloud is associated with fast-moving cold fronts?

1. Cirrus

2. Altostratus

3. Altocumulus

4. Cumulonimbus

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