1Chapter 9: Weather Patterns Mid Latitude Cyclones: extratropical cyclones, Nor’eastersRegion southern FL to AlaskaLifecycle and associated weatherRegional InfluenceGenerates cyclone at the surfacePolar Front Theory: -Norwegian Cyclone Model-WWI-describes birth, growth and decay of cyclones2Midlatitude Cyclone: -primary weather producers -low pressure systems, 1000 km dia.-counterclockwise circulation toward center-warm and cold fronts-upward flow initiates precipitationFronts: -boundary surfaces that separate air masses of different densities temperaturemoisture-15-200km wide-line on the weather map3Fronts: -surface slope is gradual-warm air overlies cold air-air masses move at different speeds-one air mass will advance-clashing produces weatherFronts: -warm air is always forced aloft-overrunning: warm air gliding on top of cold air45 Types of Fronts: -warm-Cold-Stationary-Occluded-DrylineWarm Front: -warm air mass is advancing, displaces colder air at the surface -red line with half circles-gradual slope (1:200)-speed = 25-35 km/hr20015Warm Front: -adiabatic cooling -cloud sequence-gradual slope & slow advance: widespread, light precip. long duration-precipitation precedes front-E to SW wind shift2001Warm Front: -precipitation and temperature profile6Cold Front: -cold air mass is advancing, displaces warmer air at the surface-cold air more dense forces warm air aloft -blue line with triangles-steeper frontal boundary (1:100)-speed = 35-50 km/hr1001Cold Front: -precipitation follows the passage of the front-steeper frontal boundary & speed: more violent weather-altocumulus and cumulonimbus10017Cold Front: -same lifting of warm air, only quicker over a shorter distance-heavy downpours, short duration, narrow band of precipitation-passage of front temperature drops, polar air, clear skies-SW to NW wind shift1001Stationary Front: -little to no horizontal movement across the front by either air mass-lateral motion -overrunning, light precipitation8Occluded Front: -rapid moving cold front overtakes a warm front-warm air driven aloft-precipitation from wedging-strong temperature gradients-intense weatherOccluded Front: -cold type occluded frontadvancing air is colder than air mass it is overtakingcommon east of the Rockies (cP overtakes mP)9Occluded Front: -warm type occluded frontadvancing air is warmer than air mass it is overtakingPacific coast (mP overtakes cP)Drylines:-fronts based on moisture content-not necessarily a difference in temperature-dry air forces moist air aloft-cT (southwest US) displaces mT (Gulf) spring and summer-severe T-storms from Texas to Nebraska10Life of a Midlatitiude Cyclone (2-10 days)6 basic stagesFront developsWave developsCyclonic circulation establishedOcclusion beginsOccluded front developedCyclone dissipatesCyclogenesis = cyclone formationFront developsStationary frontcP on the North (easterlies)mT on the South (westerlies)11Wave developswavelength (O 100 km)wave steepensLife of a Midlatitiude CycloneCyclonic circulation establishedWarm air invades north (warm front)Cold air advances south (cold front)Low pressure at the crest12Occlusion: beginning of the endCold front advances past the warm frontStrong temperature gradients, storm intensifiesLife of a Midlatitiude CycloneOccluded front developedblizzards, strong windsenergy is being exhaustedwithin a few days warm front driven aloftLife of a Midlatitiude Cyclone13Cyclone dissipatescold air mass surrounds the low at the surfacehorizontal temperature gradient eliminatedLife of a Midlatitiude Cyclone14Cyclone generally move from west to eastSteered by the general westerly circulationRight side of the storm passes firstIdealized Weather of a Midlatitiude CycloneA) Cirrus cloudsFront ~1200 km awayWarm front advances, cloud base lowers (cirrostratus, altostratus, stratus)Idealized Weather of a Midlatitiude Cyclone15B) Nibostratus cloudsLight precipitation, gets heavier as front advancesTemperatures increaseWinds shift from an easterly direction to a southerly directionIdealized Weather of a Midlatitiude CycloneC) mT Air massWarm, moistClear skies Southerly windsIdealized Weather of a Midlatitiude Cyclone16D) Cumulonimbus CloudsHeavy rainsViolent weather as cold front approachesIdealized Weather of a Midlatitiude CycloneE) Temperatures DropcP Air massDescending airClear skiesLow precipitationWind shifts from southerly to westerly.Idealized Weather of a Midlatitiude Cyclone17F-G)Occluded front regionTemperature remains coolPrecipitation beneath the frontType of precipitation depends on the lower temperature profileOF moves slower than the warm or cold frontsSystem rotatesIdealized Weather of a Midlatitiude CycloneVeering: winds rotate/shift in a clockwise directionSouth of the storm Skies will clear as you move into the mT region or cP region18Backing: winds rotate/shift in a counterclockwise directionNorth of the storm, pass through the occlusion Cold with precipitation19Wave develops1. Topographic irregularities (Mnts)2. Temperature contrasts (land/sea)3. Ocean current influence (hurricanes)Polar Front Theory developed from surface observations20Conditions aloftSurface cyclones are preceded by intensification of airflow aloftZonal airflow (W-E) little cyclonic activityLongitudinal airflow (N-S) increase cyclonic activitySurface cyclone: centered below the jet streamdownwind of a upper level trough21Cyclones and anticyclones are typically found togetherSurface divergence under an anticyclones feeds surface convergence under the cycloneDivergence aloft must be greater than convergence at the surfaceunder a cycloneCyclonic & Anticyclonic CirculationRegions of Cyclogenesis- Topographic irregularities (Mnts)- Temperature contrasts (land/sea)- Ocean current influence (hurricanes)22In general east to northeast trackMost of the north Pacific storms that influence the west coast do not make it over the Rockies in tact (redevelop)Storm Tracks: Patterns of MovementStorm Tracks: Patterns of Movement23Modern View: The Conveyor Belt Model3 intersecting air streams (belts)2 belts originate at the surface and ascend1 belt originates aloft and descendsWarm Conveyor Belt: mT air moves toward the middle of cylone, north over mP or cP airAscends to middle troposphere (JS) joins the general westerly flowPrimary producer of precipitationTroposphere12km = avg. thickness16km = tropics09km = poles24Cold Conveyor Belt: Originates at surface ahead of the warm frontFlows westerly around the center and ascends,