Chapter 12 – Air masses and Fronts
c- continental
source island or water
m – maritime
P– polar
Source near poles or equator
T - tropical
Types of air masses invading continental US (see Fig 12.2)
[cA is colder than cP] A for arctic
1.
CP
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Cold, dry, stable air mass
Propelled southward by upper level winds
Little precipitation
Dense, cold air creates surface level high and sinking, stable air
H
H
Top view
Side View
Diverging air at the surface pulls air down from aloft.
-
stable air, clean water, cold
recall: subsidence inversion (p 171)
falling air mass, compresses and heats
top falls further than bottom and heats more
Get inversion
Warm
Cold
Cold on
ground warm
aloft
A summer cP is a milder air mass and the inversion is lacking because of surface heating from the hot
ground.
cP summer
Altitude
cP
winter
0° C
A cP flowing over a large, warm body of water causes condensation of moisture and clouds (fig 12.5)
Read Lake – Effect Snow, p 316
mP air approaching West Coast of US is cool (not cold), moist and unstable. The ocean has heated the
lower regions of the air mass causing a steep lapse rate and unstable air.
This air is lifted on encountering mountains giving rise to rain and snow
On eastside of Rockies the air mass has lost much of its moisture and forms Chinook winds.
Upper-level winds or high pressure regions can bring mT air to the southeastern US
Jet stream
H
Warm,
moist air
mT
If mT is lifted by a cold air mass storms will result.
If ground is warm, unstable conditions can cause thunderstorms.
CT air is hot and dry and enters the US from Mexico in summer. Fig 12.10 shows a CT heat wave from
June 1990 covering SW and Central US. Phoenix airport closed because of air density was to low for safe
lift.
Fronts – transition zone between 2 air masses of different density. Fig 12.11
Cold front – current weather map (surface)
• sharp change in temperature (gradient)
• change in dewpoint ( moisture content) H L
• shift in wind direction: S, SW → W, NW
• pressure change: falling → rising
• clouds and precipitation – cumulus and cumulonimbus warm, moist air forced upward
Warm Front: (slower than cold front)
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winds, S, SE → S, SW
temperature, cold → warm
pressure, falling → rise, then fall
clouds, stratus type
dew point, steady rise
Occluded Front: cold front overtakes a warm front.
Creates a boundary between very cold and air masses, and forces warm air up in between.
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•
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Winds, S, SE →W, NW
Temperature, cold → colder (cold occlusion)
cold → milder (warm occlusion)
Pressure, Falling → Rising