Transcript Slide 1
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 (105 s1), precipitable water (mm) 850–500 hPa mean wind (kt) 69 0000 UTC 17 Aug 2007 250 hPa h (dam), 700 hPa (105 s1), precipitable water (mm) 850–500 hPa mean wind (kt) 69 0000 UTC 18 Aug 2007 250 hPa h (dam), 700 hPa (105 s1), precipitable water (mm) 850–500 hPa mean wind (kt) 69 0000 UTC 19 Aug 2007 250 hPa h (dam), 700 hPa (105 s1), 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