CHAPTER 9

AIR MASSES AND FRONTS

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

Temperature

Moisture

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 and 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”

http://www.atmos.albany.edu/student/cordeira/WEB/PWAT/global_loop.html

0000 UTC 16 Aug 2007

Maritime Tropical air ahead of a tropical cyclone

96

0000 UTC 17 Aug 2007

250 hPa h (dam), 700 hPa (105 s1),precipitable water (mm)

850–500 hPa mean wind (kt)

96

0000 UTC 18 Aug 2007

250 hPa h (dam), 700 hPa (105 s1),precipitable water (mm)

850–500 hPa mean wind (kt)

96

0000 UTC 19 Aug 2007

250 hPa h (dam), 700 hPa (105 s1),precipitable water (mm)

850–500 hPa mean wind (kt)

96

1200 UTC 19 Aug 2007

250 hPa h (dam), 700 hPa (105 s1),precipitable water (mm)

850–500 hPa mean wind (kt)

96

Rainfall Totals (in)

cP

mT

mP

FrontsFronts

Warm Front

Cold Front

Stationary Front

Occluded Front

A Front - is the boundary between air masses; normallyrefers to where this interface intersects the ground (in all cases except stationary fronts, the symbols are placed pointing to the direction of movement)

Fig. 9.16, p. 252

Other BoundariesOther Boundaries

Dryline

Dryline - Separation of warm, moist air (from Gulf) and warm, dry air (from SW United States)(common over west Texas in spring/summer)

Cloud cover and surface dew point

Sharp change in temperature◦ Sometimes, though…

Sharp change in dew point

Shift in wind direction “Kink” in isobars Weather/Clouds

http://www.hpc.ncep.noaa.gov/sfc/namussfcwbg.gif

**(Amanda) Fanning is one of my former students!

Air mass change on Wednesday, 7 March 2012

“Dome” of dense cold air is replacing warm air Steep leading edge – may have strong upward

motion Clouds and precip. usually ahead of fast-moving cold

front, lagging behind if slow-moving

Field Before passage

During After

Wind S or SW Gusty, shifting

W or NW, often

strong

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

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

Clouds Ci, Cs, As, Ns, St, then

fog

Stratus Clearing

Precip Light rain, snow, sleet,

freezing rain**

Drizzle or none

None

Dew Point Steady rise Steady 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

In 1-2 sentences, explain why there is no such thing as a “maritime Arctic” (mA) air mass.

BE SURE TO WRITE YOUR NAME ON YOUR PAPER

Have a great spring break and be safe!