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

 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”

0000 UTC 16 Aug 2007 Maritime Tropical air ahead of a tropical cyclone 9 6

0000 UTC 17 Aug hPa h (dam), 700 hPa  (10  5 s  1 ), precipitable water (mm) 850–500 hPa mean wind (kt) 9 6

0000 UTC 18 Aug hPa h (dam), 700 hPa  (10  5 s  1 ), precipitable water (mm) 850–500 hPa mean wind (kt) 9 6

0000 UTC 19 Aug hPa h (dam), 700 hPa  (10  5 s  1 ), precipitable water (mm) 850–500 hPa mean wind (kt) 9 6

1200 UTC 19 Aug hPa h (dam), 700 hPa  (10  5 s  1 ), precipitable water (mm) 850–500 hPa mean wind (kt) 9 6

cP mT mP

Fronts Warm Front Cold Front Stationary Front Occluded Front 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

Fig. 9.16, p. 252

 Sharp change in temperature ◦ Sometimes, though…  Sharp change in dew point  Shift in wind direction  “Kink” in isobars

 “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 DuringAfterWind S or SW Gusty, shifting W or NW, often strong TemperatureWarm Sudden drop Dropping PressureFalling 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 DuringAfterWind S or SE Variable S or SW Temperature Cool or cold Steady rise Warming PressureFalling Leveling off Slight rise, then fall Clouds Ci, Cs, As, Ns, St, then fog StratusClearing 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

 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

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