ThunderstormsA thunderstorm is a tall, vertically developed cloud that produces lightning and thunderCan’t have thunder without lightning!Usually produce heavy precipitationThunderstorm clouds are called cumulonimbus cloudsSometimes informally called “thunderheads”Thunderstorm Ingredients: InstabilityAll thunderstorms require instabilityWarm and humid air near the surface and/or cold air aloftAir can rise on its own due to buoyancyThunderstorm Ingredients: TriggerAll thunderstorms require a triggerSomething to make the air riseWhat can make air rise?Fronts or drylinesSea breezes (or lake breezes)Outflow boundaries (gust fronts)MountainsTrigger: Front or DrylineTrigger: Sea Breeze (or Lake Breeze)Trigger: MountainsAtmosphere heats from ground upAir over mountain warmer than air around it and risesThunderstorm IngredientsSevere thunderstorms also require vertical wind shearRecall that wind shear is a change in wind speed or direction with heightHelps storms organizeSevere ThunderstormsA severe thunderstorm contains ANY of the following:Wind gusts at the surface greater than 50 knots (57.5 mph)Hail with diameter greater than 1.00 inch (2.54 cm)Roughly quarter-sizedTornadoTypes of ThunderstormsOrdinary (single-cell) thunderstormsUsually not severeForm in environments with weak vertical wind shearMulticell thunderstorms (MCSs)Thunderstorm complexesForm in environments with moderate vertical wind shearSupercell thunderstormsAlmost always severeProduce nearly all intense tornadoes and hail larger than golf-ball sizedForm in environments with strong vertical wind shearVertical wind shear most important in determining thunderstorm typeOrdinary ThunderstormsMost common type of thunderstormUsually not severeLast about an hour from formation to dissipationGenerally develop in regions of weak wind shearOften form in summer, not near frontsStages of development:1. Cumulus stageWarm air rises, expands, and cools. Water vapor condenses and forms a cloudThe rising air is called the updraftAir rises due to buoyancyCannot have a thunderstorm without an updraft!As cloud gets deeper, precipitation particles begin to formFair-weather cumulus clouds. Most amount to nothing, but some can grow into thunderstorms2. Mature stageUpdraft may eventually reach tropopauseTropopause is very stable (inversion) – Acts as lid on stormAir diverges outward when it reaches tropopause, forms anvil cloudTop of updraft may penetrate into stratosphereCalled overshooting topPrecipitation particles grow, become heavy, begin to fall into updraftFalling precipitation drags air downward with itSome rain evaporates, cooling air even moreWhen air is cooler than its surroundings, it sinksThis sinking air is called a downdraftPrecipitation is required to form a downdraft3. Dissipation stageRain falls into updraftCools updraft, drags rising air downwardUpdraft weakens, replaced by downdraftSurging outflow cuts off supply of warm, moist, unstable air needed to feed updraftUpdraft dissipatesThunderstorm dominated by downdrafts and precipitation and quickly dissipatesOutflowWhen cold downdraft reaches ground, it spreads outForms pool of cold air beneath stormKnown as a cold pool or outflowBoundary between cold outflow and warm inflow is called gust front or outflow boundaryThis is why temperature drops and winds increase just before a thunderstorm beginsAlso why it is cooler just after a thunderstormConvergence at outflow boundary may trigger new stormsCommon during mature stage of an ordinary thunderstormOrdinary Thunderstorms 11/28/2012ThunderstormsA thunderstorm is a tall, vertically developed cloud that produces lightning and thunder-Can’t have thunder without lightning!-Usually produce heavy precipitationThunderstorm clouds are called cumulonimbus clouds-Sometimes informally called “thunderheads”Thunderstorm Ingredients: InstabilityAll thunderstorms require instability-Warm and humid air near the surface and/or cold air aloft-Air can rise on its own due to buoyancyThunderstorm Ingredients: TriggerAll thunderstorms require a trigger-Something to make the air riseWhat can make air rise?-Fronts or drylines-Sea breezes (or lake breezes)-Outflow boundaries (gust fronts)-MountainsTrigger: Front or DrylineTrigger: Sea Breeze (or Lake Breeze)Trigger: MountainsAtmosphere heats from ground upAir over mountain warmer than air around it and risesThunderstorm IngredientsSevere thunderstorms also require vertical wind shear-Recall that wind shear is a change in wind speed or direction with height-Helps storms organizeSevere Thunderstorms-A severe thunderstorm contains ANY of the following:-Wind gusts at the surface greater than 50 knots (57.5 mph)-Hail with diameter greater than 1.00 inch (2.54 cm)oRoughly quarter-sized-TornadoTypes of ThunderstormsOrdinary (single-cell) thunderstorms-Usually not severe-Form in environments with weak vertical wind shearMulticell thunderstorms (MCSs)-Thunderstorm complexes-Form in environments with moderate vertical wind shearSupercell thunderstorms-Almost always severe-Produce nearly all intense tornadoes and hail larger than golf-ball sized-Form in environments with strong vertical wind shearVertical wind shear most important in determining thunderstorm typeOrdinary Thunderstorms-Most common type of thunderstorm-Usually not severe-Last about an hour from formation to dissipation-Generally develop in regions of weak wind shear-Often form in summer, not near fronts-Stages of development:o1. Cumulus stageWarm air rises, expands, and cools. Water vapor condenses and forms a cloudThe rising air is called the updraftAir rises due to buoyancyCannot have a thunderstorm without an updraft!As cloud gets deeper, precipitation particles begin to formFair-weather cumulus clouds. Most amount to nothing, but some can grow into thunderstormso2. Mature stageUpdraft may eventually reach tropopauseTropopause is very stable (inversion) – Acts as lid on stormAir diverges outward when it reaches tropopause, forms anvilcloudTop of updraft may penetrate into stratosphereCalled overshooting topPrecipitation particles grow, become heavy, begin to fall into updraftFalling precipitation drags air downward with itSome rain evaporates, cooling air even moreWhen air is cooler than its surroundings, it sinksThis sinking air is