What is an Air Mass?An air mass is a large body of air with similar temperature and moisture concentrationsHorizontal onlyAir masses form over flat, homogeneous regions of the earth’s surfaceThe air acquires the characteristics of the underlying surfaceBlue shaded area below is an Arctic air massCenters of air masses are surface high pressure systemsAir mass boundaries are along surface troughs of low pressureThese are called frontsPredominantly found at the surfaceDo not confuse with upper-air troughs (found on 300/500 mb maps)Air Mass ClassificationClassified based on moisture and temperatureTwo-letter classificationFirst letter refers to moisture, second letter refers to temperatureMoisture:Continental (c): Dry (forms over land)Maritime (m): Moist (forms over water)Temperature:Tropical (T): Warm/hotPolar (P): Cool/coldArctic (A): FrigidcP = Continental PolarDry and coldcT = Continental TropicalDry and hotmP = Maritime PolarMoist and coolmT = Maritime TropicalMoist and warmcA = Continental ArcticDry and very coldAir Mass Source RegionsCounter-clockwise flow around low pressure systems (cyclones) moves air massesAir masses are modified by the surfaces over which they moveFrontsFronts are boundaries between air masses of different densitiesWarm air less dense than cold airMoist air less dense than dry airDo NOT confuse fronts and air massesFronts are generally strongest at the surface and weaken with heightTemperature gradients are found on the cold side of frontsFrontal Symbology—symbols point in direction that front is movingFront Motion: What is “ahead of” and “behind” the frontTriangles and semi-circles show the direction the front is movingFronts are identified by the wind direction (relative to the front) in the COLD AIR!Cold front: Wind in cold air blowing toward frontStationary front: Wind in cold air blowing parallel to frontWarm front: Wind in cold air blowing away from frontRegion south of warm front and head of cold front is called warm sectorTemperatures are warm; within warm air massCold FrontsCold air is advancingWinds in the cold air blow toward the frontCold air is more dense and lifts the less dense warm airTypically have a steep vertical slopePrecipitation generally showeryBrief but intensePossible thunderstorms, sometimes strongAs front passes, temperatures and dewpoints decreaseWinds shift from S or SW to N, NW, or W with passageCold fronts generally extend southward or westward from a low pressure systemWarm FrontsWarm air is advancing (cold air is retreating)Winds in cold air blow away from frontLess dense warm air gradually overruns more dense cold airGradual vertical slopePrecipitation widespread, mainly north of frontIntensity is light to moderateMixture of precipitation types (snow, ice) in winterTemperature inversions common north of warm frontsAs front passes, temperatures and dewpoints increaseWinds shift from E to S or SW with passageGenerally extends southward or eastward from a low pressure systemOverrunningMixed precipitationSnow farthest into cold airRain closest to warm airSleet and freezing rain found in betweenSnow, sleet, freezing rain, rainStationary FrontsStationary fronts do not moveFlooding riskThe wind in the cold air blows parallel to the front (generally has an easterly component)Mixed precipitation north of front in winterOccluded FrontsAs a low pressure system strengthens, it wraps cold air around itselfWhen the cold front “catches up” to the warm front, the warm air is forced aloft and an occluded front formsGenerally associated with widespread rain or snowTemperatures may rise or drop slightly, but remain coldWind shifts from E/NE to N/NW/W with passageOccluded fronts always connect either a low with another low or a low with its warm and cold frontsTriple Point – Intersection of cold, warm, and occluded frontsDrylinesSeparate mT air (warm/moist from Gulf of Mexico, to east) from cT air (hot/dry from deserts, to west)Typically found in KS/OK/TX during spring and can trigger strong to severe thunderstormsHow to Find FrontsTemperature gradientLarge change in temperature with distanceTightly packed isothermsDewpoint (moisture) gradientTightly packed isodrosothermsWind shiftRemember how to determine winds from surface isobars to helpTrough of low pressure at the surfaceLines of clouds or precipitationAir Masses and Fronts 10/14/2012What is an Air Mass?-An air mass is a large body of air with similar temperature and moisture concentrationsoHorizontal only-Air masses form over flat, homogeneous regions of the earth’s surfaceoThe air acquires the characteristics of the underlying surface-Blue shaded area below is an Arctic air mass-Centers of air masses are surface high pressure systems-Air mass boundaries are along surface troughs of low pressureoThese are called frontsoPredominantly found at the surfaceoDo not confuse with upper-air troughs (found on 300/500 mb maps)Air Mass Classification-Classified based on moisture and temperatureoTwo-letter classification-First letter refers to moisture, second letter refers to temperature-Moisture:oContinental (c): Dry (forms over land)oMaritime (m): Moist (forms over water)-Temperature:oTropical (T): Warm/hotoPolar (P): Cool/coldoArctic (A): Frigid-cP = Continental PolarDry and cold-cT = Continental TropicalDry and hot-mP = Maritime PolarMoist and cool-mT = Maritime TropicalMoist and warm-cA = Continental ArcticDry and very coldAir Mass Source Regions-Counter-clockwise flow around low pressure systems (cyclones) moves air masses-Air masses are modified by the surfaces over which they moveFronts-Fronts are boundaries between air masses of different densitiesoWarm air less dense than cold airoMoist air less dense than dry airoDo NOT confuse fronts and air masses-Fronts are generally strongest at the surface and weaken with height-Temperature gradients are found on the cold side of fronts-Frontal Symbology—symbols point in direction that front is moving-Front Motion: What is “ahead of” and “behind” the frontoTriangles and semi-circles show the direction the front is moving-Fronts are identified by the wind direction (relative to the front) in the COLD AIR!oCold front: Wind in cold air blowing toward frontoStationary front: Wind in cold air blowing parallel to frontoWarm front: Wind in cold air blowing away from front-Region south of warm front and head of cold front is called warm