Comma clouds = low pressureCyclone = low pressureNH: ccw; SH: cwPolar front theory: used to explain life cycle of mid-latitude cycloneStationary frontFrontal waveOpen wave (mature)Partially occluded (cold front overruns warm)System most intense when first becoming occludedOccluded (warm air pushed aloft, warm air wedge at surface shrinks significantly)DissipationSurface low weakens when directly below upper-level lowTriple point: point of occlusion where cold/warm/occluded fronts come togetherCyclogenesis: development/strengthening of a mid-latitude cycloneLifetime of mid-latitude cyclone: a few days to a weekFormation of mid-latitude cyclone: Strong upper-air divergence neededFound ahead of a troughLife cycle of thunderstorm:CumulusMatureDissipatingSevere thunderstorms: Wind gusts => 50 knots OR hail diameter => ¾” OR tornadoNeeds strong vertical wind shearDownburst: strong localized downdraft beneath a severe t-stormWinds can reach over 50m/s; damages forestry and structure, hazardous to aviationMicroburst: downburst less than 4km wideSquall line: a line of thunderstormsMesoscale Convective Complex (MCC): organized mass/collection of t-storms that extends across large region that can be 1000x larger than size of individual stormCan occupy the size of an entire stateDense arch-shaped cloud often forms on gust frontLeading edge of cold air from downdraft feels like a mini-cold front with strong, gusty winds, sharp temperature drop, rapid rise in pressureHighest frequency of t-storms in FloridaHighest frequency of hail in Colorado, surrounding statesTornado: Rapidly rotating column or “funnel” of high wind that spiral around very small region of low pressure beneath a thunderstormCounterclockwise rotationDiameter: 300-600mSpeed of movement: 20-70 knotsLifetime: a few minutes to an hourAverage path length: 7kmTornadoes occur anytime you have severe thunderstorms (including hurricanes)75% of tornadoes occur in US; most frequent from March to JulyLarge-scale outbreak in spring; lowest frequency in winterTornadoes occur in all fifty states; highest frequency in tornado alley in central Great PlainsFavorable conditions for tornado to form:Surface map: Open-wave cyclone with well-defined cold/warm fronts; tornadoes typically form in the warm section before the cold front from supercell storms850mb (1500m) map: Warm, moist air from south between 850 and 700mb700mb map: Cold, dry air from W or SW500mb map: Trough to the west of surface lowCyclone needs to be in front of upper-level low (trough)Fujita Tornado Scale Tornado watches issued when tornadoes are likelyTornado warning issued when tornado has been spottedOnce storm is observed/has passed, Fujita scale is used to classify tornadoes according to their rotational speed based on damage done by the stormRanges from F0 (Gale) Tornado to F5 (Incredible) Tornado based on wind speed (F0: 40-72mph; F5: 261-318mph)F-scale has several weaknesses (based on damage caused by tornado, difficult to apply with no damage indicators, based on worst damage from storm even if just at one building, etc.)Enhanced Fujita Tornado Scale Set of wind estimates (not measurements) based on damageRanged EF0 (weakest)-EF5 (strongest)Tropical cyclone = hurricaneHurricane (Americas), Cyclone (Indian Ocean, east of Australia), Typhoon (east of Asia)Ingredients for tropical cyclone:Warm tropical or subtropical ocean/gulf water (80 F or greater)Do not form over equatorial water because Coriolis force is too weak (close to zero) near equator; instead, hurricanes typically develop between 5 and 20 degree latitudesWeak winds aloft, low vertical wind shearHigh humidity throughout troposphereConditionally unstable atmosphereKey triggers: ITCZ, tropical waves, stalled frontsUltimate energy source for hurricane: warm, moist waterHurricane structure:Eye—calm, clear center (avg 10-20km)Eye wall—outer edge of eye where clouds are deepest and rain is most intenseRain-free area—band a way outward from eyeSpiral rain band—band just outside of rain-free areaStages of hurricane: *very likely to be on examTropical disturbanceTropical depression (20 knots) (number given to storm, i.e. TD 8)Tropical storm (35 knots) (name given to storm, i.e. TS Katrina)Hurricane (64 knots or 74 mph)Tropical cyclones:Warm-core lowsWeakens with heightSinking air in center (eye)Winds strongest at surfaceIsobars denser, more circularSmallerNo fronts associated with itMidlatitude cyclones:Cold coreIntensifies with heightRising air at centerWinds strongest in jet streamIsobars coarser, less circularBiggerAssociated with cold and warm frontsHurricane scale: Saffir-SimpsonCategories 1 through 5 based on wind speed (Cat. 5 is strongest)1: 74-95 mph2: 96-110 mph3: 111-130 mph4: 131-155 mph5: >155