GEO 203 Notes – 11/11/10 (Lecture #15 and 16)Global wind system, air mass/fronts: quiz on TuesdayLow pressure systems over land, high pressure over oceans (in summer? Check)Pressure gradient drives wind from high to low, creating monsoonsThermal low developed over desert SW (on example map)Wind moves in clockwise direction in NHStrengthening Bermuda High affects US weather; Pacific High does not affect our weather a lotJet stream: very strong winds concentrated within narrow band in atmosphereLength: maybe thousands of km longWidth: few hundred metersDepth: few kilometersUsually in tropopause (10-15km)Jet streak: region of greatest wind within jet streamTypically, two are found in NH: polar front jet stream (around polar front) and subtropical jet stream (around subtropical region)US most affected by polar frontWhat produces jet streams: rapid changes in pressure in a relatively narrow region sets up a steep pressure gradient that intensifies the wind speed and causes the jet streamRemember: pressure decreases with heightTropopause height decreases with increasing latitude, so polar jet stream is at lower height than that of subtropical jet streamSummer: polar jet typically found over central CanadaWinter: jet stream typically found over northern/central USEl Niño: extensive ocean surface warming that begins along coast of Peru and EcuadorIncrease in ocean surface temperature = change in pressure/wind/precipitation patterns in tropical PacificImpacts felt far from tropical PacificOccurs at irregular intervals (once every 2-7 years; 9 events in the last 40 years)Upwelling: raising of cold waterWind (Pacific high) pushes surface water away from continent, replacing it with cold, deep ocean waterTrade wind reduces strength of Pacific high, decreases upwelling = warmer waterNormal conditions:High pressure over SE Pacific, lower pressure near IndonesiaEasterly trade winds along equatorRising air, heavy rain over W Pacific; sink motion, dry weather over E PacificEl Niño conditions:Pressure decreases over W Pacific, rises over E PacificChanges in pressure = weakened/reversed trade windsRising air, rain over W Pacific; sinking air, dry weather over E PacificWarmer than normal temperatures over eastern tropical PacificEl Niño: warmer than normal temperature over eastern tropical PacificLa Niña: colder than normal surface water moves over central and eastern PacificEl Niño winter:Wetter and cooler over CA, southern USWarmer and dryer over large part of North AmericaLa Niña winter:Colder in western USWetter over Pacific NW, MidwestWarmer, dryer in southern USHadley CellRises up from tropical region, sinks at 30-degree (subtropical) latitudeSubtropical jetJet stream developed from west to east along subtropical Trade windsCoriolis force turns winds to NE in NH; SE in SHWesterliesFound in mid-latitudePolar frontLocated around 60-degree latitudePolar jetForms along polar front(read about three-cell model before exam)Lecture #16: Air Masses and FrontsAir mass: extremely large body of air with similar temperature and moistureBest source regions for air masses: large flat areas with light winds where air can be stagnant long enough to take on characteristics of surface below itTropical regions are good to air masses to form (low pressure gradient force; uniform, warm surface)Air masses classified according to originGeographical characteristic: tropical, polar, or arcticSurface properties: maritime (ocean), continental (land)Air Mass Classification:cP: continental polar (cold, dry, stable)Midwest, EasterncT: continental tropical (hot, dry, stable aloft, unstable below); usually only in summerMexico, plains (Texas, Arizona, NM, etc.)[cA: continental arctic (extremely cold and dry); not as often used]Canada, extreme northern USmP: maritime polar (cool, moist, unstable)Pacific Northwest, NortheastmT: maritime tropical (warm, moist, usually unstable)Southwest,