GEU 0047: MeteorologyLecture 11: Air Masses and Fronts

• the air with similar properties of temperature and moisture which covers a large area• Semi-permanent structures in the atmosphere

Air Masses

• Moisture content– Dry Continental (c)– Moist Marine (m)

• Temperature– Cold Polar (P), Arctic (A)– Warm Tropical (T),

• Results in 5 basic types– cA Continental Arctic

– cP Continental Polar mP Marine Polar

– cT Continental Tropical mT Marine Tropical–

Air Mass Classifications

Global air mass source regions

Air Mass Source Regions

Source Region Profiles

• Arctic Air Mass– Bitterly Cold and Usually Very Dry– Develops over the snow or ice covered regions of continents– Similar to polar air masses but usually develops in regions

north of 60N where there is little or no insolation during the winter.

Arctic Air (cA)

• Continental Polar Air Mass– Cold, Dry– Develops over the interior of

• North America -- Central Canada -- Siberia

Continental Polar (cP)

• Marine Polar Air Mass– Cold, Moist– Develops over the cold ocean waters of the North

Pacific and North Atlantic

• Usually associated with conversion of cP siberian air to mP which is moistened and warmed from the Japanese “Gulf Stream Current”

• Moves across the N. Pacific to affect N. California and the Pacific NW

• Responsible for fueling “Nor-easters”

Marine Polar (mP)

• Continental Tropical (cT)– Hot, Dry– Develops over the deserts of Mexico and

the Southwestern United States– Very unstable because of heat and

convection, but cloudless because of lack of moisture.

• Marine Tropical (mT)– Warm, Humid– Develops over the tropical and subtropical

oceans and the Gulf of Mexico

Tropical (T)

mT

cP

Example Air Masses

• Air masses do not always remain over their source regions.

• If the air moves over a region that is different from where it originated, the air mass will be modified, or changed, by the land/ocean surface that the air is travelling over.

• Changes include: warming, cooling, adding or reducing moisture content

Air Mass Modification

cP

WarmerLand

The cP air mass willbe warmed by thewarmer land that itpasses over.

Air Mass Modification

Air Mass Invasion

Air Mass Invasion

Air Mass Invasion

Cold air mass moves over a warmer surface.

cP

Warm, Moist

The lower layers arewarmed and maybecome unstable.

Changes in Stability

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Altitude(m)

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Sounding Before Modification:Conditionally Unstable

After modification:Absolutely Unstable

Cold air mass moving over a warmer surface.

Dry AdiabatMoist AdiabatEnvironment

Stability Changes

Warm air mass moves over a colder surface.

mT

Warm, Moist

The lower layers arecooled and tend to

stablize.

Cold

Stability Changes

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Sounding Before Modification:Conditionally Unstable

After modification:Absolutely stable

Warm air mass moves over a colder surface.

Dry AdiabatMoist AdiabatEnvironment

Stability Changes

Frontal Boundary Symbols

Fronts

• Front– A narrow transition zone between air

masses of differing densities.

– The density differences usually arise from temperature differences.

– Density differences may be a result of humidity differences (summer).

• A front is the boundary or transition zone between different air masses.

Fronts

• The transition zone is usually narrow (15 to 200 km in width).

• Frontal Boundaries may be over 1000 km long.

• On maps, we draw the frontal boundary on the warm side of the transition zone.

Air Mass Invasion

• Cold Front– A boundary that moves in such a way

that the colder (more dense) air advances and displaces the warmer (less dense) air.

– The largest temperature differences are normally associated with cold fronts.

Idealized Cold Front

Cross-sectional Characteristics

Cold Air

Warm Air

Strong Lifting near thefrontal boundary can produce convective clouds.

Idealized Cold Front

Idealized Cold Front

• Precipitation:– Located on either side of the front.

– Convective, showery in nature

– If warm air is stable• Nimbostratus, Altostratus

– If warm air is unstable• Cumulus, Cumulonimbus, Thunderstorms

Idealized Cold Front

Weather conditions associated with a cold front

Weather Phenomenon

Prior to the Passing of the Front

Contact with the Front

After the Passing of the Front

Temperature Warm Cooling suddenly Cold and getting colder

Atmospheric Pressure

Decreasing steadilyLeveling off then increasing

Increasing steadily

Winds South to southeast Variable and gusty West to northwest

Precipitation ShowersHeavy rain or snow, hail sometimes

Showers then clearing

CloudsCirrus and cirrostratus changing later to cumulus and cumulonimbus

Cumulus and cumulonimbus

Cumulus

• Warm Front– A boundary that moves in such a way that

the colder (more dense) air retreats and is replaced by the warmer (less dense) air.

– The warm fronts tend to have weaker temperature gradients.

Idealized Warm Front

Idealized Warm Front

Weather conditions associated with a warm front

Weather Phenomenon

Prior to the Passing of the Front

Contact with the Front

After the Passing of the Front

Temperature Cool Warming suddenlyWarmer then leveling off

Atmospheric Pressure

Decreasing steadily Leveling offSlight rise followed by a decrease

Winds South to southeast Variable South to southwest

PrecipitationShowers, snow, sleet or drizzle

Light drizzle None

CloudsCirrus, cirrostratus, altostratus, nimbostratus, and then stratus

Stratus, sometimes cumulonimbus

Clearing with scattered stratus, sometimes scattered cumulonimbus

• Cold fronts typically move faster than warm fronts.

• Cold fronts can catch up and “overtake” a warm front.

• Depending on the different air masses, there are two types of occlusions:– Cold type occlusion– Warm type occlusion

Occluded Front

Cold Air

Cool Air

Warm Air

Cold-type Occlusionwhen the air behind the front is colder than the air ahead of the front.

Cold type occlusions are common on the east coast of continents and generally form when the continental polar or arctic air collides with maritime subtropic air.

Cold AirCool Air

Warm Air

Warm-type OcclusionWhen the air behind the front is warmer than the air ahead of the front

Warm type occlusions are common on the west coast of continents and generally form when maritime polar air collides with continental polar or arctic air.