Chap. 9 Air masses and fronts

Air mass •  Definition: Large body of air with relatively uniform

temperature and “moisture” (e.g., dewpoint temperature) characteristics

•  Formation – air remains over a source region for a sufficiently long time to acquire specific characteristics that depend on the surface and other atmospheric processes –  Low wind conditions –  Uniform surface properties (e.g., snow, water) –  Anticyclonic conditions (High Pressure) –  Semi-permanent low pressure regions

•  Source regions – examples to two pure air masses –  Continental polar air masses (from Canada) –  Maritime tropical air mass (from Gulf of Mexico)

Source regions The areas where air

masses form are called source regions.

More on definitions •  Main descriptors:

–  Arctic – bitter cold –  Polar – cold (not as cold as arctic) –  Tropical – warm to hot

•  Secondary descriptors –  Continental – origin over a land mass –  Maritime – origin over large body of water; air has

greater water vapor content •  Classification (based on T and Td)

–  cP (polar continental) –  A (Arctic) –  mT (tropical maritime) –  cT (tropical continental) –  mP (polar maritime)

Air mass structure

•  Horizontal extent: ~1000 km •  Vertical extent (depth): lower troposphere,

2-5 km AGL

Fronts

•  Defined as a boundary between two air masses.

•  The boundary zone can be wide or narrow, depending on the type of front – Warm fronts are typically wide, 10-100 km – Cold fronts can be very narrow down to < 1

km in width (1-10 km is typical) – Example: A cold front occurs when the cold

air advances and displaces (lifts upwards) the warmer air ahead of it

Focus 8.1: Frontal symbols Study these.

Frontal types

“Frontolysis”

Also: Arctic front (basically a strong cold front)

“Frontgenesis”

“Frontolysis” “Frontgenesis”

Fronts n A weather front is a boundary separating two air massesn Types: cold front, warm front, occluded front, stationary

front, dry line, squall line

What is an atmospheric front? •  A front is a transition zone between two air masses of different

densities.

•  The density contrast results from: –  Difference in temperature; –  Difference in humidity.

•  The frontal zone (surface) is the upward extension of the front.

•  Sometimes the frontal zones can be very sharp.

•  The intensity of the weather along the front depends on the contrast of the air mass properties.

•  The type of front depends on both the direction in which the air mass is moving and the characteristics of the air mass.

Types of Fronts •  Cold front: cold, dry stable air is

replacing warm, moist unstable air. •  Warm front: warm, moist unstable

air is replacing cold dry stable air.

•  Stationary front: boundary between the two air masses is not moving.

•  The symbols on a map are in the direction of the air mass motion.

•  Occluded front: when a cold front catches up with a warm front

Weather Map •  Shown: surface-pressure systems, air masses, fronts,

isobars, winds and air flow (large arrows) •  Green-shaded area: precipitation

Fig. 8.3. Example of a cold front moving southeastward across the central US. The cold air is lifting warm moist conditionally unstable air, creating a ling of thunderstorms along the front. This cold front would have a structure similar to that in Fig. 8.2A. Note that the cold air north of the warm front has a component toward the north, so that the cold air is retreating northward with time. Gray shading denotes cloud cover.

What are the Signs of a Passing Front?

•  Signs – Sharp temperature changes over a relatively

short distance. – Changes in the air’s moisture content (as

indicated by changes in the dew point). – Shifts in wind direction – Pressure and pressure changes. – Clouds and precipitation patterns.

•  The location of the front is not always very obvious! Even meteorologists sometimes disagree…

Cold Fronts •  A cold front is a mass of cold air advancing towards warm air. •  Typically associated with heavy precipitation, rain or snow, combined

with rapid temperature drops. •  Since friction decreases with height, winds move faster at higher

altitude. Then the surface of cold front becomes more steeper through time, leading to a narrow belt of precipitation.

•  Moving speed 0-30mph

Fig. 8.2 Cross section through three cold fronts. In A, the air ahead of the front is conditionally unstable and forms thunderstorms (or a squall line) when lifted by the front. In B, stable, moist air is lifted along the frontal surface. In C, dry air is lifted (but not to the lifting condensation level), so clouds do not form. In all cases, the cold air is advancing into warm air.

Cold Fronts •  Cold front- a front in which cold air is

replacing warm air at the surface. •  Notice the difference in

–  Temperature –  Dew point –  Wind direction –  Pressure

•  Associated with low pressure centers (low pressure troughs): follow the dashed line

•  The pressure is minimum as the front passes (first decreases as the front approaches and then increases behind the front)

Cold Front: cloud and precipitation patterns •  The warm, moist air ahead of

the front is forced upward and condenses –  Cirrus clouds well ahead of

the front –  Strong thunderstorms with

heavy showers and gusty winds along and ahead of the front: squall lines

–  Broad area of cumulus clouds immediately behind the front (although fast moving fronts may be mostly clear behind the front).

Warm Fronts •  Warm fronts are warm air moving towards cold air. •  This overrunning process produces large amounts of warm, moist air

over cooler, drier air. •  Shallow stratus clouds dominate and bring light precipitation to

affected regions. Stable regions above the warmer air create vertically limited clouds and light precipitation. Frontal fogs may occur as rain evaporates in the colder air near the surface.

•  Moving speed about 12 mph

Warm Fronts •  Warm front - in which warm air

replaces cooler air at the surface

•  Notice the difference in –  Temperature –  Dew point –  Wind direction –  Pressure

•  Notice the presence of precipitation well ahead of the front

Slope of warm Fronts

•  Friction decreases with height, so winds move faster at higher altitude

•  This causes the surface of the front to become less steep through time. Then clouds will be spread to a wider region.

Warm Fronts: cloud and precipitation patterns

•  Although they can trigger thunderstorms, warm fronts are more likely to be associated with large regions of stratus clouds and light to moderate continuous rain.

•  Warm fronts are usually preceded by cirrus first, then altostratus or altocumulus, then stratus and possibly fog.

•  At the warm front, gradual transition. •  Behind the warm front, skies are relatively clear.

Fig. 8.4. Cross section through two warm fronts. In A, the air south of the front is conditionally unstable and forms showers and thunderstorms as it rises over the cool air. In B, the rising moist air south of the front is stable, so the clouds are stratified and showers are absent (but rain bands may exist).

Warm air advances over the cool/cold air; the slope is generally shallower

Stationary Fronts •  Stationary fronts do not move. They do not advance.

They are two unlike air masses side by side. •  They may slowly migrate and warmer air is displaced

above colder.

From Environment Canada

Stationary Front •  Stationary front- a front which does not move or barely moves.

•  Stationary fronts behave like warm fronts, but are more quiescent.

•  Many times the winds on both sides of a stationary front are parallel to the front and have opposite direction.

•  Typically stationary fronts form when polar air masses are modified significantly so as to lose their character (e.g., cold fronts which stall).

•  Typically there is no strong precipitation associated with stationary fronts (why? – no big contrast in the air mass properties, no air uplifting and condensation).

Stationary Fronts •  Stationary fronts do not move. They do not advance.

They are two unlike air masses side by side. •  They may slowly migrate and warmer air is displaced

above colder.

From Environment Canada

Fig. 8.5. Maps showing two examples of stationary fronts and associated clouds (gray shading). Note that in both examples, the cold air on the north side of the front is moving parallel to the front. In A, clouds form on the warm side of the front and precipitation would fall in this region. In B, clouds form on the cold side of the front and any precipitation would fall out on the cold side.

Stationary fronts:

The boundary is stationary, not advancing

Opposing flows (at low levels) are nearly balanced in each side of the front

Fig. 8.6 Vertical cross sections through two stationary fronts. Cold air flow parallel to the front in each case. The warm air is conditionally unstable in each case. In panel A, which corresponds to the cross section shown in Fig. 8.5A, the warm air is lifted at the leading edge of cool air. In panel B, warm air flows over the cold air and forms less vigorous showers and/or thunderstorms.

Fig. 8.7. Development of an occluded front. See text caption

Occluded Fronts •  Occluded fronts occur when two fronts meet, the warm air

mass between them is displaced aloft. •  This typically occurs when a cold front meets a warm front as

it circulates the low pressure center of a mid-latitude cyclone. •  The cold and warm fronts curve naturally poleward into the

point of occlusion, which is also known as the triple point.

Formation of Occluded

Fronts

Occluded fronts. •  Cold fronts move faster than warm fronts.

They can catch up and overtake their related warm front. When they do, an occluded front is formed.

•  Cold occlusion: very cold air behind, not so cold air ahead of, the warm front

•  The upper warm front follows the surface occluded front

Cold occlusion

Different types of occluded fronts •  A cold-type occlusion usually

occurs in the eastern half of the continent where a cold front associated with continental/Polar air meets a warm front with maritime/Polar air ahead.

•  A warm-type occlusion is typical

of the western edges of continents where the cold front, associated with maritime/Polar air, migrates to an area that is occupied by continental/Polar air.

Warm Occlusion •  Very cold air ahead of, not so cold air

behind, the warm front •  The cooler air from the cold front cannot

lift the very cold air ahead, rides “piggyback”

•  The warm front aloft precedes the surface occluded front

Fig. 9.7 (cont.)

This of each sequence in terms of a variation in north-south location at a given time, or a temporal evolution at a (nearly) fixed location. For the latter, time would advance upwards in each column above.

Drylines •  Drylines are boundaries between lighter humid air and denser

dry air. •  Air masses with similar temperatures but strong humidity

gradients will act as fronts. •  They frequently occur throughout the Great Plains, and are a

favored location for thunderstorm development.

Dry lines

Fig. 9.8. A dry line is defined as a boundary separating warm dry air to the west, from cooler but moister air to the east. The dry line often serves as a focusing mechanism for severe storms over the Great Plains, especially over Texas, Oklahoma, and Kansas.

Dry lines are rarely analyzed over Alabama.

Dry Line

•  Found just east of the Rocky Mountains –  Formed from warm, dry

air on leeward side

Dry Line

Fig. 9.9. Surface conditions and 500 mb flow patterns associated with an upper-level cold front.

Such fronts often have a trailing cold front at the surface.

Upper level fronts can initiate thunderstorms, but forecasting them is difficult since the frontal location cannot be determined from surface data.

Many unknowns

Text, p. 146

Upper-level fronts (cold fronts)

Fig. 8.10. Analysis of sea-level pressure and surface temperature at 1200 UTC, 11 Dec 2000. Note the effects of cold air damming on the east side of the Rocky Mountains, and also along the east side of the Appalachian Mountains over NC, SC, and GA.

Frontolysis & Frontogenesis

•  Frontolysis – What kind of temperature gradient is

associated with frontolysis? •  Frontogenesis

– What kind of temperature gradient is associated with frontogenesis?

Cold Air Damming

•  Cold air is trapped topographically – Usually along

eastern slopes – Why?

•  Circulation around the high pressure

•  Mesoscale event

Cold Air Damming in US •  Cold air is confined along the eastern

coastal plain of Appalachian Mountains – Usually occurs during winter and early spring –  Ice storm

How does Cold Air Damming Occur?

•  High pressure over New England – What is the circulation around the high? – Brings in cold or warm air over New England? – Northeast wind over eastern third of US

•  Midlatitude cyclone advancing eastward through the Midwest – What kind of pressure is at the heart of

midlatitude cyclone-high or low pressure? – Brings in cold, dry air or warm, moist air over

Appalachians?

Cold Air Damming

H

L

What about upslope along eastern slopes of Appalachian?

•  Some upslope does occur – The very cold air begins to rise

•  Cools even more •  Becomes more dense

•  Air is so dense is “slides” along the side of the mountains

Cold Air Damming

•  Warm, moist air overruns cold, dry, dense air at the surface

Cold Air Damming

•  Effects on weather – Extensive cloud cover – Cold temperatures – Freezing precipitation – Enhanced precipitation

Cold Air Damming & Rocky Mountains?

•  Colorado Rockies elevation is almost double of Appalachian Mountains

•  Moisture source is much further away –  Air becomes “modified”