The Thunderstorm: Ultimate InstabilityThe Thunderstorm: Ultimate Instability

A convective rain or snow shower accompanied A convective rain or snow shower accompanied by thunderby thunder– What causes thunder?What causes thunder?– LightningLightning (spell it correctly): a giant spark between (spell it correctly): a giant spark between

two regions of opposite electrical charge. 50,000 two regions of opposite electrical charge. 50,000 deg F, hundreds of thousands of ampsdeg F, hundreds of thousands of amps

– Air is a poor electrical conductor (an insulator), Air is a poor electrical conductor (an insulator), therefore a huge charge difference builds up either therefore a huge charge difference builds up either in cloudin cloud (80% of all lightning) or between the cloud (80% of all lightning) or between the cloud and the ground (20%) (and the ground (20%) (cloud to groundcloud to ground))

LightningLightning I I FormationFormation: : PrecipitationPrecipitation TheoryTheory

– Collision of ice crystals and Collision of ice crystals and hailstones leaves each with opposite charge, hailstones leaves each with opposite charge, lighter ice crystals transport positive charge lighter ice crystals transport positive charge upward while heavier hail concentrates upward while heavier hail concentrates negative charge at lower levelsnegative charge at lower levels

– Probably more complicated than this, may need Probably more complicated than this, may need supercooled water in cloud in addition to ice, supercooled water in cloud in addition to ice, but it’s likely that ice is but it’s likely that ice is required to produce lightningrequired to produce lightning

– Hurricanes = little lightningHurricanes = little lightning– SpritesSprites

LightningLightning II II

Thunder: a warning that lightning is near, Thunder: a warning that lightning is near, explosive expansion of superheated airexplosive expansion of superheated air– Computing distance to the lightningComputing distance to the lightning

Speed of light: see lightning when it happensSpeed of light: see lightning when it happens Speed of sound: thunder is delayed 5 sec / mile Speed of sound: thunder is delayed 5 sec / mile

– All lightning produces thunderAll lightning produces thunder The rumbleThe rumble ““Heat lightning”Heat lightning”

Lightning SafetyLightning Safety Photo Courtesy of Dave CrowleyPhoto Courtesy of Dave Crowley

Don’tDon’t – Swim, boatSwim, boat– Stand in an open fieldStand in an open field– Stand under an especial-Stand under an especial-

ly tall or isolated treely tall or isolated tree– Use a wall phoneUse a wall phone– Use the showerUse the shower

DoDo– Go insideGo inside– Get into a non-Get into a non-

convertible carconvertible car– Get to lower elevationGet to lower elevation– Crouch on balls of feetCrouch on balls of feet– Perform CPR if someone Perform CPR if someone

is struckis struck

Thunderstorm TriggersThunderstorm Triggers

Start with an environment in unstable Start with an environment in unstable equilibriumequilibrium

Diurnal heating primes the atmosphereDiurnal heating primes the atmosphere– Localized thunderstorm triggersLocalized thunderstorm triggers

Sea breeze (Florida, Gulf Coast)Sea breeze (Florida, Gulf Coast) Elevated convection (eastern Rockies)Elevated convection (eastern Rockies)

– Large scale thunderstorm trigger (discussed later)Large scale thunderstorm trigger (discussed later) Convergence into low pressureConvergence into low pressure Fronts (all types, but cold preferred)Fronts (all types, but cold preferred)

Air Mass ThunderstormsAir Mass Thunderstorms Typical lifespan 30 minutes to an hourTypical lifespan 30 minutes to an hour Life CycleLife Cycle

– Cumulus StageCumulus Stage Instability and adequate moisture requiredInstability and adequate moisture required Building cumulus clouds dominated by updraftsBuilding cumulus clouds dominated by updrafts

Air Mass ThunderstormsAir Mass Thunderstorms Life CycleLife Cycle

– Mature StageMature Stage Top of cloud glaciates (freezes)Top of cloud glaciates (freezes)

– Precipitation through Bergeron-Findeisen ProcessPrecipitation through Bergeron-Findeisen Process– LightningLightning

Updraft continues and downdraft develops from falling Updraft continues and downdraft develops from falling precipitationprecipitation

Air Mass ThunderstormsAir Mass Thunderstorms Life CycleLife Cycle

– Dissipating StageDissipating Stage Downdraft dominatesDowndraft dominates Updraft ceases, convection collapsesUpdraft ceases, convection collapses Dry air Dry air entrainmententrainment

Thunderstorm DangersThunderstorm Dangers Flash flooding: more rain falls in a short time Flash flooding: more rain falls in a short time

than ground can absorbthan ground can absorb Hail: “The White Plague”Hail: “The White Plague” Microbursts: unusually strong downdraftsMicrobursts: unusually strong downdrafts Lightning, of courseLightning, of course Thunderstorm ComplexesThunderstorm Complexes

– Squall LinesSquall Lines– Mesoscale Convective SystemsMesoscale Convective Systems

Flash FloodsFlash Floods A big U.S. killer (~100 people/yr)A big U.S. killer (~100 people/yr) DO NOT DRIVE or walk on flooded streetsDO NOT DRIVE or walk on flooded streets Ingredients for flash floodingIngredients for flash flooding

– Weak upper-level windsWeak upper-level winds– OrographyOrography– TrainingTraining

HailHail

Ingredients for hailIngredients for hail– Strong updrafts to support growing chunks of iceStrong updrafts to support growing chunks of ice– Below-freezing air relatively close to groundBelow-freezing air relatively close to ground

FormationFormation– Ice crystals become coated with supercooled water Ice crystals become coated with supercooled water

blown upward within updraftblown upward within updraft– Hailstones grow as long as updraft can offset gravityHailstones grow as long as updraft can offset gravity– Fall to earth when updraft weakens or hail is thrown Fall to earth when updraft weakens or hail is thrown

out of updraftout of updraft

TheThe MicroburstMicroburst Huge threat to aviationHuge threat to aviation

– Rapid loss of liftRapid loss of lift– Occurs close to groundOccurs close to ground

Damaging “straight-line” windsDamaging “straight-line” winds

Thunderstorm ComplexesThunderstorm Complexes

Mesoscale Convective SystemsMesoscale Convective Systems– Long-lived bundle of thunderstormsLong-lived bundle of thunderstorms– Greatest threat is floodingGreatest threat is flooding– Beneficial to U.S. crops (usually)Beneficial to U.S. crops (usually)

Squall LinesSquall Lines– Organized band of thunderstorms, usually well Organized band of thunderstorms, usually well

ahead of a cold frontahead of a cold front– Greatest threat is damaging windsGreatest threat is damaging winds

Key FiguresKey Figures

9.5, 9.11, 9.14, 9.16, 9.19, 9.21, 9.32, 9.539.5, 9.11, 9.14, 9.16, 9.19, 9.21, 9.32, 9.53