Air Masses and Fronts: Characteristics, Classification, and Impacts, Slides of Environmental Science

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Chapter 9 : Air MassChapter 9 : Air Mass •^ Air Masses •^ Fronts

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Fronts

•^

Fronts on Weather Maps

Air masses^ Contain uniform temperature andContain

uniform temperature and

humidity characteristics.

Fronts^ Boundaries between unlike airBoundaries

between unlike air

masses.

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Source RegionsSource Regions

•^

The areas of the globe where air masses from arecalled source regions.

•^

A source region must have certain temperatureand humidity properties that can remain fixed for asubstantial length of time to affect air massesabove it.

•^

Air mass source regions occur only in the high orlow latitudes; middle latitudes are too variable.

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Cold Air MassesCold Air Masses

January

July

-^

The centers of cold air masses are associated with high pressure on surface weather maps.

-^

In summer, when the oceans are cooler than the landmasses, large high-pressurecenters appear over North Atlantic (

Bermuda high

) and Pacific (

Pacific high

). ESS124ESS124Prof. JinProf. Jin-

-Yi YuYi Yu

-^

In winter, high-pressure centers form over the northern parts of Asian continent( Siberian high

)^ and North America.

Classification of Air MassesClassification of Air Masses

-^

Air masses are classified according to the temperature

d^

i t

h^

t^

i ti

f th i

i

and moisture characteristics of their source regions.

-^

Bases on moisture content: continental (dry) andmaritime (moist)

-^

Based on temperature: tropical (warm), polar (cold),

p^

p^

(^

), p

(^

arctic (extremely cold).

-^

Naming convention for air masses: A small letter (

c

-^

Naming convention for air masses: A small letter (

c ,

m

) indicates the moist content followed by a capital

letter (

T

,^

P

,^ A

) to represent temperature.

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Five Types of Air MassesFive Types of Air Masses

ypyp

-^

Theoretically, there should be 6types of air masses (2 moisturetypes of air masses (2 moisturetypes x 3 temperature types).

-^

But mA-type (maritime Arctic)But mA type (maritime Arctic)does not exist.

-^

cA: continental Arctic cP: continental PolarcT: continental TropicalcT: continental TropicalmP: maritime Polar

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mT: maritime Tropical

Modification ofModification of cP

cP Air Masses

Air Masses

-^

Migrations of cP air inducecolder drier conditions overcolder, drier conditions overaffected areas.A

P

i^

i^

t^

t^

d l

-^

As cP air migrates toward lowerlatitudes, it warms from beneath.

-^

As it warms, moisture capacityincreases while stabilitydecreasesdecreases.

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Continental Arctic (Continental Arctic (cA

cA) Air Masses

) Air Masses

•^

Continental Arctic (cA)

air represents extremely cold and dry

conditions as, due to its temperature, it contains very little watervapor.

-^

The boundary between cA and cP air is the shallow (~1-2 km)arctic front.

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•^

cA air masses can extend as far southward as the Canadian-United State.

Maritime Polar (Maritime Polar (mP

mP) Air Masses

) Air Masses

-^

Maritime polar air masses form overupper latitude oceanic regions and arepp

g

cool and moist.

-^

mP air masses form over high-latitudeocean as cP air masses move out from the interior of continents. (i.e., cP

Æ

mP).

-^

Oceans add heat and moisture into the dryand cold cP air masses.

-^

Along the west coast of the U.S., mP airaffects regions during winter and may bepresent before mid-latitude cyclonesd

t^

th

ti^

t

advect over the continent.

-^

Along the east coast, mP air typicallyaffects regions after cyclone passage asthe mP air wraps around the area of low

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the

mP air wraps around the area of low pressure.

Maritime Tropical (Maritime Tropical (mT

mT) Air Masses

) Air Masses

p

(

p

(

))

•^

Form over low latitude oceans and assuch are very warm humid andsuch are very warm, humid, andunstable.

-^

mT air masses from Atlantic and Gulff M

i^

i^

h^

i

of Mexico is the primary sourceregion for the eastern U.S.

-^

As air advects over the warm continent in summer the highhumidity and high heat occasionallycombine to dangerous levels.

-^

mT air masses have an enormousinfluence on the southwestern U.S,particularly in summer.

Advection of mT air also

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p^

y

Advection

of mT air also

promotes the so-called Arizonamonsoon.

Cold FrontsCold Fronts

-^

Cold fronts form when cold airdisplaces warm air.

p

-^

Indicative of heavy precipitationevents, rainfall or snow, combined with rapid temperature drops.

-^

Steep front slope, typically 1:100.

-^

Moving faster, up to 50 km/hr (30mph).

-^

Northwesterly winds behind a coldfront, and southwesterly in ahead of

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the front.

Cold Front WeatherCold Front Weather

• The type of precipitation that will occur

along a cold front depends on thecharacteristics of the warm air ahead ofcharacteristics of the warm air ahead ofthe front.

• If the warm air is moist and

diti

ll^

t bl

th

d^

t

conditionally unstable, thunderstormscan be triggered ahead of the coldfront; the thunderstorms will form in ali^

ll d

ll li

if^

ll

line called squall line; if supercellthunderstorms form, tornadoes, hail,damaging wind can occur.

• If the warm air is stable, the clouds that

form by lifting only produce light rain.

• If the warm air is dry and stable no

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-Yi YuYi Yu

If

the warm air is dry and stable, no clouds will form at all.

Warm FrontsWarm Fronts

-^

Created when warm air displaces colder air.

-^

Shallow horizontal stratus clouds and light precipitation.

-^

Frontal fogs may occur as falling raindrops evaporate inthe colder air near the surface. Sleet and freezing rain mayalso formed.

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-^

Half the slope of cold fronts, typically (1:200).

-^

Moving slower, about 20 km/hr (12 mph).

Warm Front WeatherWarm Front Weather

• Clouds are deepest just north of the

warm front boundary and progressivelybecomes thinner and higher toward the north.

• The intensity of the precipitation

depends on the stability of the warmdepends on the stability of the warmair.

• If the warm air is conditionally

t bl

th

d^

t^

d^

l

unstable, thunderstorms may developover the warm front.

• If the warm air is stable, the clouds will

be layered.

• If the temperature in the cold air is

below freezing snow ice pellets or

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-Yi YuYi Yu

below

freezing, snow, ice pellets, or

freezing rain may occur.