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CHAPTER 9
AIR MASSES AND FRONTS

What’s an air mass?
◦ A large body of air with similar temperature and
humidity characteristics

Where do these air masses come from?
(Source Regions)





P = polar
T = tropical
A = Arctic
m = maritime
c = continental

cP: cold, dry, stable
◦ Extreme case: cA - Cold air rushes down into
central U.S. from Canada: “arctic high”
◦ Impacts:
 Citrus crop damage in southeast
 Lake-effect snows near Great Lakes when cold air
moves over warmer water

mP: cool, moist, somewhat unstable
◦ Air from Pacific is lifted by mountains on west
coast, producing rain & snow

Hot and dry, stable aloft but unstable near surface
◦ Only really occurs in U.S. in summer in southwest
◦ Few clouds and minimal precipitation
◦ Impacts:
 drought if a cT air mass remains in place for a long time
July 2005 heat
wave

Warm, moist, unstable
◦ Flow northward from Gulf of Mexico provides fuel
for thunderstorms in the eastern U.S.
◦ Flow from Pacific into California
◦ Impacts:
 Severe weather in the central and eastern U.S.
 Flooding in California
The “Pineapple Express”
Maritime Tropical air ahead of a tropical cyclone
69
0000 UTC 16 Aug 2007
250 hPa h (dam), 700 hPa  (105 s1),
precipitable water (mm)
850–500 hPa mean wind (kt)
69
0000 UTC 17 Aug 2007
250 hPa h (dam), 700 hPa  (105 s1),
precipitable water (mm)
850–500 hPa mean wind (kt)
69
0000 UTC 18 Aug 2007
250 hPa h (dam), 700 hPa  (105 s1),
precipitable water (mm)
850–500 hPa mean wind (kt)
69
0000 UTC 19 Aug 2007
250 hPa h (dam), 700 hPa  (105 s1),
precipitable water (mm)
850–500 hPa mean wind (kt)
69
1200 UTC 19 Aug 2007
mP
cP
mT
Fronts
A Front -
is the boundary between air masses; normally
refers to where this interface intersects the
ground (in all cases except stationary fronts, the
symbols are placed pointing to the direction of
movement of the front
Warm Front
Cold Front
Stationary Front
Occluded Front
Fig. 9.16, p. 252

Sharp change in temperature
◦ Sometimes, though…



Sharp change in dew point
Shift in wind direction
“Kink” in isobars
http://www.hpc.ncep.noaa.gov/sfc/namussfcwbg.gif


“Dome” of dense cold air is replacing warm air
Leading edge of the cold front is steep – often
leads to strong upward motion
Field
Before passage
During
After
Wind
S or SW
Temperature
Warm
Sudden drop
Dropping
Pressure
Falling
Reaches
minimum, then
sharp rise
Rising
Clouds
Ci, Cs, then Cb
Tcu or Cb
Cu or Sc
Precip
Brief showers
Heavy showers,
severe weather
Clearing
Dew Point
High
Drops sharply
Lowering
Gusty, shifting W or NW, often
strong
Fig. 9.15, p. 251



Warm air replacing cool air
Relatively gentle slope – leads to broad area
of upward motion
Warm fronts usually move slower than cold
fronts
Field
Before passage
During
After
Wind
S or SE
Variable
S or SW
Temperature
Cool or cold
Steady rise
Warming
Pressure
Falling
Leveling off
Slight rise, then
fall
Clearing
Clouds
Precip
Dew Point
Ci, Cs, As, Ns,
Stratus
St, then fog
Light rain,
Drizzle or none
snow, sleet,
freezing rain**
Steady rise
Steady
None
Rising, then
steady


Name is self-explanatory: doesn’t move much
In some cases where air is moist on both sides,
stationary fronts can lead to flooding – rain
forms along front and persists for many days
Fig. 9.9, p. 246

Warm Fronts and Ice Storms
◦ Warm front with wave like shape in Southeast
◦ Shallow layer of cold air
 Known as “Cold Air Damming”
◦ Rain freezes
p. 260

Occluded Front
◦ Cold front “catches up to” and overtakes a warm
front
◦ Purple line with purple triangles and semi-circles
◦ Cold occlusion, warm occlusion
Fig. 9.28, p. 262
Fig. 9.29, p. 263
Fig. 9.30, p. 263

Upper-Air Fronts
◦ Front aloft
◦ Tropopause dips downward and folds under the
Polar jet
◦ Impacts surface weather
Fig. 9.31, p. 264

Dryline: Boundary between really hot, dry air and warm, moist
air (separates cT and mT)
◦ Sets up in southern Plains very regularly
◦ Severe storms often form on dryline

Pressure trough: There’s a “kink” in the isobars, but very little
temperature gradient