GEO 203 Notes – 11/16/10 (Lecture #16, continued)(Review of beginning of lecture from Thursday)Air Mass: extremely large body of air with similar temperature and moistureBest source region for air masses: large, flat areas with light winds where air can be stagnant long enough to take on the characteristics of the surface belowClassified according to “origin”Geographical characteristic: tropical, polar, arcticSurface properties: maritime, continental(Low winds in polar and tropical regions)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, SoutheastMaritime = moistPolar = coldCold Asian and polar air passing over the ocean south of the Aleutian low will pick up warmth and moisture, and reaches the Pacific Coast as cool, moist, and unstable air, bringing rain and snow.A strong anticyclone in eastern Canada creates northeasterly winds that may bring cold, unstable Atlantic mP air and storms into New England and the middle Atlantic States. These storms are known as northeasters.Fronts: the boundary between two air masses with very different propertiesIn all cases except secondary front, the symbols are placed pointing to the direction of movement of the interfaceWarm front: red line with semicirclesCold front: blue line with trianglesStationary front: combination of red/blue, semicircle/triangleOccluded front: purple line with triangle, semicircle, triangle, etc.Front identification:Change of temperatureChange of moisture characteristicChange of wind directionChange in pressure tendencyCharacteristic precipitation patternsDeciding type of front:From vantage point of the ground: warm air replaces cold air = warm frontCold air replaces warmer air = cold frontFront does not move = stationary frontOccluded front does not intersect ground; interface between air masses is aloftCold FrontCold air replacing warm airTypically moves from NW to SEAir behind front is colder and drier than air aheadFrontal surface rises steeply at ~1km: 50km for fast-moving cold front at 25 knotsSo, 50km behind front of front, altitude/frontal surface is 1kmWarm air rises along frontal surfaceCirrus clouds before surface front arrivesCumulus clouds at the frontMoves faster than warm frontWinds are S, SW ahead; windy at front; W, NW behind“Back door” cold front moves toward SW, affects east of Appalachian (esp. NE)See “Cold Front Weather Condition” chart (Lecture #16, slide #17)(remember: moisture doubles with 10-degree increase in dew point temperature)Warm FrontWarm air replacing cold airTypically moves from SW to NEAir behind front is warmer and moister than air aheadWarm air rides up cold airSlope (~1km of height = 300km distance) is not as steep as cold frontMoves slower (~10kts) than cold front (~15-25kts)Clouds well in advance of surface frontCirrocumulus, cirrostratus, and altostratus clouds far before front, changing to nimbostratus andstratus as surface front approachesPrecipitation is less intense, but more widespreadMore gradual transition in temperature, moisture, and pressure than transition across cold frontSee “Warm Front Weather Condition” chart (Lecture #16, slide #21)Just know differences between warm and cold frontStationary FrontFront that does not moveWarm air behind warm front and ahead of cold front; cold air behind cold front—fronts in opposite directionsNotable temperature change or shift in wind directionOccluded FrontDeveloping low pressure system typically has warm front preceding it and a fast moving cold frontCold front moves faster and may catch up to/overtake warm frontBoundary that separates the cool air already in place north of the warm front and the cold air behind the cold frontVery intense precipitationAir west of front is colder/drier than east of frontStrong wind direction shift across frontCold occluded front (behaves similar to cold front): air behind cold front is much colder than air ahead of warm frontWarm occluded front (behaves similar to warm front): air ahead of warm front is colder than air behind cold frontLake-effect snowOccurs most frequently in early winter (November-January)cP overtakes mT lake surface; warms from below, quickly becomes unstableAir with added moisture rises and forms cloudsIncreased friction when air hits shore, reducing wind speed and producing convergence, which further enhances