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11/10/2012
Air Masses
Air masses
Origin, modification, and weather
Definition
Classification
Definition and origin
Fronts
• Definition: large body of air having
homogeneous density properties
(temperature and moisture) horizontally
• Origin: source regions
• Separate unlike air masses (with different
density properties horizontally)
• Types
– Large surfaces of homogeneous properties
– Either all land (continental) or all water
(maritime)
– Cold
– Warm
– No front between regions of only contrasting
moisture
• Fronts are actually segments of the polar front
Cold and warm fronts
Lifting along fronts
Figure 9.11 (Lutgens and Tarbuck, 2003)
A cold front is in the cloud
region and aligned with it
Figure 9.19 (Lutgens and Tarbuck, 2010) and Figure 9.22 B (Lutgens and Tarbuck, 2013)
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Air mass modification
Air mass classification code
• Within source region
• Movement out of source region
– To regions with different surface temperature,
either
• Heated from below (destabilizes lower parts)
• Chilled from below (enhances stability of lower parts)
– To regions of different surface moisture properties
• Moving over open water or living vegetation adds water
vapor
• But moving from oceans to land does not remove water
vapor (condensation and precipitation processes are
needed)
Classification system: four letters
• First: lowercase, either
– c, continental or
– m, maritime
• Second: UPPERCASE,
– P, polar (poleward of polar front)
– T, tropical (equatorward of polar front)
– A, poleward of arctic front (winter only, and
usually not in the U.S.)
Stability characteristics
• Third letter (K or W)
– K means air mass is colder than underlying
surface
• Thus air heated from below
• Thus air becomes less stable
– W means air mass is warmer than
underlying surface
• Thus air chilled from below
• Thus air becomes more stable
Classification system: 3rd and
4th letters
• Third: UPPERCASE, either
– K, air mass colder than underlying surface,
or
– W, air mass warmer than underlying
surface
• Fourth: lowercase, either
– s, stable aloft (normally anticyclonic), or
– u, unstable aloft (normally cyclonic
Stability characteristics
• Most stable (based on 3rd and 4th letters)
– W (lower atmosphere warmer than surface)
– s, stability / subsidence aloft
– Example: cPWs, mTWs, etc.
• Most unstable (based on 3rd and 4th letters)
– K (lower atmosphere colder than surface)
– u, instability / rising aloft
– Example: cPKu, mTKu, etc.
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Classification system: major
temperature and humidity qualities
Generalized atmospheric flow
in the westerlies
• Temperature, either
– Cold (poleward of polar front)
– Warm (equatorward of polar front)
• Source region, either
– Continental
– Maritime
Figure 11.14 (Abbott, 2009)
Classification system: major
temperature and humidity qualities
• Cold (poleward of polar front)
– Continental
– Maritime
• Warm (equatorward of polar front)
– Continental
– Maritime
Seasonal contrasts
• Both polar and tropical air masses are
warmer in the summer and cooler in
winter
• Seasonal contrasts are much greater in
polar air masses
• Why?
Air mass classification code
Source regions
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Winter
Air Mass
Source
Regions
Figure 8.3 (Lutgens and Tarbuck, 2013)
Figure 8.3a (Lutgens and Tarbuck, 2010 and 2013)
Summer
From SE Florida
Figure 8.3b (Lutgens and Tarbuck, 2010 and 2013)
In the cT air mass source region
A maritime tropical (mT) air mass source region
In the cT air mass source
region (near Tucson, AZ)
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Southwestern Yukon Territory
Fig. 8.1 (Lutgens and Tarbuck, 2010)
In the cP air mass source region
In the cP air mass source region
Global air masses: July
Air mass
characteristics
Global air masses: January
Air mass
patterns in
North
America:
winter
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Air mass
patterns in
North
America:
summer
Notable “air mass” weather
Mostly either:
Cold and dry
Warm and humid
Typical
winter cP
air mass in
U.S.
Movement
of a
cold and dry
air mass
brings
winter weather
Note modification of
the air mass as it
moves southward over
a warmer surface
Figure 8.2 (Lutgens and Tarbuck, 2013)
Figure 8.3 (Christopherson, 2009)
“Sun dog”
About -30 C: steam fog over open water (Iowa)
Sun
Very cold cPKs air mass in January (near Greene, Iowa)
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A
warm and
humid
air mass
brings
sultry
weather
View from Bennington, VT, during hazy, hot, and humid weather
Adding heat and moisture
from below
Air mass modification
Moving out of source region
Modification of a cP air mass
Figure 8.7 (Lutgens and Tarbuck, 2013)
Movement
of a
cold and dry
air mass
brings
winter weather
Note modification of
the air mass as it
moves southward over
a warmer surface
Figure 8.2 (Lutgens and Tarbuck, 2013)
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Figure 8A (Lutgens and Tarbuck, 2013)
The “Siberian Express”
About -30 C: steam fog over open water (Iowa)
Figure 8.6 (Lutgens and Tarbuck, 2013)
Lake effect
snow: extreme
modification
Figure 8.5
(Lutgens and Tarbuck, 2013)
Fig..8D (Lutgens and Tarbuck, 2013)
Lake effect snow is due
to destabilization as very
cold air passes over open
water (which is relatively
much warmer)
Figure 8.6 B
(Lutgens and Tarbuck, 2010)
Lake effect
Lake
effect
Figure 8.5 (Christopherson, 2009)
Lake effect clouds near their eastern limits in central Michigan (CMU)
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Heating
from below
Post cold frontal modification
Figure 9.22 (Lutgens and Tarbuck, 2007)
Figure 8.4 (Lutgens and
Tarbuck, 2013)
Cold air passing over relatively
warm water is heated,
destabilized, and humidified
Page 231 (Lutgens and
Tarbuck, 2013)
Post cold frontal; cool air heated
from below by warmer ground
Removing moisture from air
masses
Evapotranspiration easily adds
moisture to an air mass, but only
precipitation can remove it
Summer cPKs air, Mt. Pleasant, Michigan
mT air in summer moves over heated land,
resulting in less stability
Annual
precipitation
in eastern U.S.
decreases
with
distance
from
Gulf of Mexico
Cumulus clouds, showers, and thunderstorms possible if
lifting mechanisms are present
Florida
Figure 8.9 (Lutgens and Tarbuck, 2013)
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“Monsoonal” thunderstorms in Arizona
Desert rainfall from mT air
mass (mostly July-Sept.)
A circulation change
mostly in the 2nd week
of July floods the
southwest with
relatively humid mT air
Figure 8.11B (Lutgens and Tarbuck, 2013)
View to NE from Tempe, Arizona; thunderstorms in mountain areas (July)
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