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ESS5Prof. Jin-Yi Yu
Chapter 9: Air Masses and Fronts
What Characterize Air Masses?
What Define Fronts?
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Air masses
Contain uniform temperature andhumidity characteristics.
Fronts
Boundaries between unlike airmasses.
ESS5Prof. Jin-Yi Yu
Air Masses
Air masses have fairly uniform temperature and moisture content in horizontal direction (but not uniform invertical).
Air masses are characterized by their temperature and humidity properties.
The properties of air masses are determined by the the underlying surface properties where they originate.
Once formed, air masses migrate within the generalcirculation.
Upon movement, air masses displace residual air overlocations thus changing temperature and humiditycharacteristics.
Further, the air masses themselves moderate from surfaceinfluences.
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Source Regions
The areas of the globe where air masses from arecalled source regions.
A source region must have certain temperature
and humidity properties that can remain fixed fora substantial length of time to affect air massesabove it.
Air mass source regions occur only in the high or
low latitudes; middle latitudes are too variable.
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Classification of Air Masses
Air masses are classified according to the
temperature and moisture characteristics of theirsource regions.
Bases on moisture content: continental (dry) and
maritime (moist)
Based on temperature: tropical (warm), polar (cold),
arctic (extremely cold).
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.
ESS5Prof. Jin-Yi Yu
Five Types of Air Masses
Theoretically, there should be 6 types of air masses (2 moisturetypes x 3 temperature types).
But mA-type (maritime Arctic) does not exist.
cA: continental Arctic cP: continental PolarcT: continental TropicalmP: maritime PolarmT: maritime Tropical
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Continental Polar (cP) Air Mass
Continental Polar air masses form over large, high-
latitude land masses, such as northern Canada orSiberia.
cP air masses are cold and extremely dry.
Wintertime cooling over these land areas cause the
atmosphere to become very stable (even inversion).
The combination of dry and stable conditions ensure
that few if any clouds form over a cP source region.
Summer cP air masses are similar to winter cP, but
much less extreme and remain at higher latitudes.
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Modification of cP Air Masses
Migrations of cP air inducecolder, drier conditions overaffected areas.
As cP air migrates toward lowerlatitudes, it warms from beneath.
As it warms, moisture capacityincreases while stabilitydecreases.
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Fronts separate air massesand bring about changes intemperature and humidity asone air mass is replaced byanother.
There are four general typesof fronts associated with mid-latitude cyclones with thename reflective of theadvancing air mass.
Fronts
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Cold Fronts
Cold fronts form when cold airdisplaces warm air.
Indicative of heavy precipitationevents, rainfall or snow, combinedwith 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 ofthe front.
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Radar/Satellite Views of Cold Fronts
Radar Picture
Satellite Picture
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Warm Fronts
Created when warm air displaces colder air.
Shallow horizontal stratus clouds and light precipitation.
Frontal fogs, sleet, freezing rain may occur as fallingraindrops evaporate in the colder air near the surface.
Ha;f the slope of cold fronts, typically (1:200).
Moving slower, about 20 km/hr (12 mph).
ESS5Prof. Jin-Yi Yu
Radar/Satellite Views of Warm Fronts
Radar Picture
Satellite Picture
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Stationary Fronts
When two unlike air masses remain side byside, with neither encroaching upon the other,a stationary front exists.
Fronts may slowly migrate and warmer air isdisplaced above colder.
Fronts sloping over the cold air.
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Occluded Fronts
ย
Occlusion: the warm air is cut offfrom the surface by the meeting oftwo fronts.
ย
Usually, a fast-moving cold frontcatches a slow-moving warm front.
ย
A
cold-type occlusion
: eastern half
of the continent where a cold frontassociated with cP air meets awarm front with mP air ahead.
ย
A
warm-type occlusion
: western
edges of continents where the coldfront, associated with mP air,invades an area in which colder cPair is entrenched.
ESS5Prof. Jin-Yi Yu
Some occlusions form when the surface lowelongates and moves away from the junction ofthe cold and warm fronts
Some occlusions occur when the intersectionof the cold and warm fronts slides alongthe warm front
Two Other Ways to Produce Occluded Fronts
