Chapter 10: Thunderstorms and Tornadoes Thunderstorms Thunderstorms Tornadoes Tornadoes Tornadic...

Chapter 10: Chapter 10: Thunderstorms Thunderstorms and Tornadoesand Tornadoes

ThunderstormsThunderstorms TornadoesTornadoes Tornadic thunderstormsTornadic thunderstorms Severe weather and doppler radarSevere weather and doppler radar

ThunderstormsThunderstorms


What do we already know about What do we already know about thunderstorms?thunderstorms? Can produce intense rain, hail, wind, Can produce intense rain, hail, wind,

lightninglightning Can make tornadoesCan make tornadoes Formed by rising air from:Formed by rising air from:

Uneven surface heatingUneven surface heating Uplift along a frontal boundaryUplift along a frontal boundary Topographical upliftTopographical uplift


Definitions:Definitions: Supercell thunderstormsSupercell thunderstorms – large updraft – large updraft

storms that can produce flash floods, storms that can produce flash floods, severe weather, tornadoessevere weather, tornadoes

Severe ThunderstormSevere Thunderstorm – has to have one – has to have one of following:of following: Hail greater than or equal to ¾ inchHail greater than or equal to ¾ inch A tornadoA tornado Wind gusts 50 mphWind gusts 50 mph

Ordinary Cell Ordinary Cell ThunderstormsThunderstorms

Ordinary cell thunderstormOrdinary cell thunderstorm Form in region of low wind shearForm in region of low wind shear Can form due to surface air convergenceCan form due to surface air convergence

Cumulus stageCumulus stage (growth stage) – due to rising (growth stage) – due to rising warm, humid air condensing into cumulus warm, humid air condensing into cumulus cloudcloud Latent heat release keeps cloud warm and Latent heat release keeps cloud warm and

unstableunstable Grows quickly to towering cumulusGrows quickly to towering cumulus Usually no precipitation because of updraftsUsually no precipitation because of updrafts No lightning or thunderNo lightning or thunder


Mature stageMature stage – marked by considerable – marked by considerable downdraftsdowndrafts Cloud particles grow larger, and begin to fallCloud particles grow larger, and begin to fall EntrainmentEntrainment - Dry air is sucked into cloud, - Dry air is sucked into cloud,

causing evaporation and coolingcausing evaporation and cooling Heavier and cooler air descends…Heavier and cooler air descends…downdraftsdowndrafts Formation of updraft and downdraft Formation of updraft and downdraft cellscells Most intense time of thunderstormMost intense time of thunderstorm Storm may grow as high as stratosphere (anvil)Storm may grow as high as stratosphere (anvil) Heavy rain, lightning, small hailHeavy rain, lightning, small hail


Mature stageMature stage Cloud top may Cloud top may overshootovershoot into into

stratospherestratosphere


Mature stageMature stage Cloud top may Cloud top may overshootovershoot into into

stratospherestratosphere Downdraft reaches ground and spreads Downdraft reaches ground and spreads

along surface as a gust frontalong surface as a gust front Sometimes rain may not reach ground, Sometimes rain may not reach ground,

but cold air doesbut cold air does


Dissipating stageDissipating stage – thunderstorm – thunderstorm weakensweakens Updrafts begin to weaken after 15 to 30 Updrafts begin to weaken after 15 to 30

min.min. Gust front moves too far from the Gust front moves too far from the

storm, so updrafts have to weakenstorm, so updrafts have to weaken Light precipitation fallsLight precipitation falls Only anvil remainsOnly anvil remains All three stages in less than an hourAll three stages in less than an hour

Fig. 10-1, p. 265

Stepped Art

Severe Thunderstorms Severe Thunderstorms and the Supercelland the Supercell

So ordinary cell storms weaken So ordinary cell storms weaken because the updraft weakensbecause the updraft weakens

What if the updraft doesn’t weaken What if the updraft doesn’t weaken quickly?quickly? If moderate wind shear pushes If moderate wind shear pushes

downdraft downwind, updraft is not cut downdraft downwind, updraft is not cut offoff

If downdraft then undercuts updraft, a If downdraft then undercuts updraft, a multicell stormmulticell storm forms forms


Multicell stormsMulticell storms Cells of varying age co-minglingCells of varying age co-mingling Top of cloud well into stratosphereTop of cloud well into stratosphere Updrafts allow hail to grow largeUpdrafts allow hail to grow large


SupercellsSupercells If wind shear is strong and changes If wind shear is strong and changes

direction with height, downdraft may direction with height, downdraft may not undercut updraftnot undercut updraft

Updraft remains strong for long time (> Updraft remains strong for long time (> 1 hour)1 hour)

One rotating vertical columnOne rotating vertical column Can create tornadoesCan create tornadoes Hail size of grapefruitHail size of grapefruit


At surface, open wave At surface, open wave with warm, humid air with warm, humid air ahead of cold frontahead of cold front

Overrunning warm air Overrunning warm air just above surfacejust above surface

Dry air at 700 mb levelDry air at 700 mb level Trough of low pressure Trough of low pressure

at 500 mbat 500 mb Divergence at 300 mbDivergence at 300 mb


Severe thunderstorms Severe thunderstorms form in light green area form in light green area because:because: Warm air below cold Warm air below cold

air is conditionally air is conditionally unstableunstable

Strong wind shear Strong wind shear creates severe creates severe thunderstormsthunderstorms

In morning, atmos is In morning, atmos is stable, no stable, no thunderstormsthunderstorms

Surface heating Surface heating creates thunderstormscreates thunderstorms

Squall Lines and Squall Lines and Mesoscale Convective Mesoscale Convective

ComplexesComplexes Squall lineSquall line – line of multicell t-storms – line of multicell t-storms

extending many kilometersextending many kilometers Mesoscale convective complexMesoscale convective complex- cluster of - cluster of

storms in one spotstorms in one spot

Squall Lines and Squall Lines and Mesoscale Convective Mesoscale Convective

ComplexesComplexes Squall lines Squall lines

often form ahead often form ahead of cold fronts of cold fronts (pre-frontal (pre-frontal squall-line squall-line thunderstorms)thunderstorms)

MCCs are long-MCCs are long-lasting and lasting and provide a lot of provide a lot of rain, and a ton of rain, and a ton of severe weathersevere weather Form during Form during

summersummer

Dryline ThunderstormsDryline Thunderstorms

DrylineDryline – narrow zone with a sharp – narrow zone with a sharp moisture contrast. Where do the moisture contrast. Where do the thunderstorms form?thunderstorms form?

Gust Fronts, Microbursts Gust Fronts, Microbursts and Derechoesand Derechoes

Gust frontGust front – leading edge of the thunderstorm – leading edge of the thunderstorm downdraftdowndraft Passage can feel like a cold front passagePassage can feel like a cold front passage

Shelf cloudShelf cloud – warm air rising above gust front – warm air rising above gust front creates cloudcreates cloud

Roll cloudRoll cloud – cloud just behind the gust front that – cloud just behind the gust front that spins horizontallyspins horizontally

Outflow boundaryOutflow boundary – a huge gust front formed by – a huge gust front formed by numerous thunderstorm gust frontnumerous thunderstorm gust front

DownburstsDownbursts – localized downdraft like water – localized downdraft like water from a tapfrom a tap MicroburstsMicrobursts – downbursts that extend 4 km or less – downbursts that extend 4 km or less

Gust Fronts, Microbursts Gust Fronts, Microbursts and Derechoesand Derechoes

Fig. 10-11, p. 271

MicroburstMicroburst Microbursts can Microbursts can

be dangerous to be dangerous to aircraftsaircrafts

Plane encounters Plane encounters uplift at (a), and uplift at (a), and pilot puts nose pilot puts nose downdown

Unexpected Unexpected downdraft downdraft crashes planecrashes plane

Has caused Has caused several airplane several airplane crashescrashes

Some airports Some airports have microburst have microburst detectiondetection

Bow Echo and DerechoBow Echo and Derecho Bow echoBow echo – a line – a line

of thunderstorms of thunderstorms often form a bow often form a bow echo on radarecho on radar

DerechoDerecho – winds – winds associated with associated with downdrafts that downdrafts that exceed 104 mph exceed 104 mph and can cause a and can cause a lot of damage…20 lot of damage…20 derechoes each derechoes each year in U.S.year in U.S.

Floods and Flash FloodsFloods and Flash Floods

Flash floodsFlash floods – floods that rise rapidly – floods that rise rapidly with little to no advance warningwith little to no advance warning

Thunderstorms Thunderstorms cause flash floods cause flash floods in two ways:in two ways:

Hovering above Hovering above one area for a one area for a long timelong time

TrainingTraining – – thunderstorms thunderstorms keep passing keep passing over same areaover same area

Distribution of Distribution of ThunderstormsThunderstorms

18 millions thunderstorms 18 millions thunderstorms worldwide annuallyworldwide annually

Need to have a Need to have a combination of combination of moisture and moisture and warmthwarmth

So do So do thunderstorms thunderstorms form in the form in the Arctic? ITCZ? Arctic? ITCZ? Gulf?Gulf?

Lightning and ThunderLightning and Thunder

LightningLightning – simply a discharge of – simply a discharge of electricity, usually in mature electricity, usually in mature thunderstormsthunderstorms Heats air up to 54,000F (5x hotter that Heats air up to 54,000F (5x hotter that

surface of sunsurface of sun ThunderThunder – the explosive expansion of – the explosive expansion of

the hot air creates a sound wave the hot air creates a sound wave that travels in all directionthat travels in all direction• According to the Guinness Book of World Records,According to the Guinness Book of World Records,

Roy Sullivan holds the world’s record for being struckRoy Sullivan holds the world’s record for being struckby lightning seven times between 1942 and his deathby lightning seven times between 1942 and his deathin 1983. in 1983.


Light travels faster than sound, so Light travels faster than sound, so we see the lightning before we hear we see the lightning before we hear the thunderthe thunder Sound takes 5 seconds to travel 1 mileSound takes 5 seconds to travel 1 mile So if we hear thunder 5 seconds after So if we hear thunder 5 seconds after

we see the lightning, the stroke was 1 we see the lightning, the stroke was 1 mile awaymile away

Electrification of CloudsElectrification of Clouds

Must have charging of Must have charging of the cloud to have the cloud to have lightninglightning

Electrification of clouds Electrification of clouds not fully understoodnot fully understood

Theory 1: Supercooled Theory 1: Supercooled droplets fall through droplets fall through cloud and collide with cloud and collide with warm hailstone. Latent warm hailstone. Latent heat is released (warm or heat is released (warm or cold?)cold?)


Net transfer of positive ions Net transfer of positive ions from warm to cold objectfrom warm to cold object

Falling hailstone is Falling hailstone is negatively charged, falls to negatively charged, falls to bottom of cloudbottom of cloud

Light positively charge Light positively charge particle is lifted to top of particle is lifted to top of cloudcloud

Small area of positively Small area of positively charge particles near charge particles near melting levelmelting level


Theory 2: When precip Theory 2: When precip forms, it has a neg in forms, it has a neg in the upper portion and the upper portion and pos charge in the lowerpos charge in the lower

As droplets collide, the As droplets collide, the large droplets become large droplets become neg and fallneg and fall

The small droplets are The small droplets are lifted by updrafts and lifted by updrafts and riserise

The Lightning StrokeThe Lightning Stroke

Basics of lightningBasics of lightning Opposite charges attractOpposite charges attract Positive charges on Positive charges on

ground follow negative ground follow negative charges at base of cloudcharges at base of cloud

Electrical current will Electrical current will not flow because air is not flow because air is good insulatorgood insulator

Charge must be large (< Charge must be large (< 1million volts per meter) 1million volts per meter) to create a lightning boltto create a lightning bolt


Cloud-to-ground lightningCloud-to-ground lightning Discharge of electrons from cloud to Discharge of electrons from cloud to

ground as a ground as a stepped leaderstepped leader (many (many times)times)

Positive charges race back up from Positive charges race back up from elevated object as a elevated object as a return strokereturn stroke


Cloud-to-ground lightningCloud-to-ground lightning Many electrons flow to ground and a Many electrons flow to ground and a

stronger return stoke follows (this is stronger return stoke follows (this is what you see)what you see)

1/10,000 of a second so it looks like one 1/10,000 of a second so it looks like one continuous flashcontinuous flash


Dart leaderDart leader – subsequent initial – subsequent initial stroke that follows same path as stroke that follows same path as initial stepped leaderinitial stepped leader Causes the multiple flash of the Causes the multiple flash of the

lightninglightning

Types of LightningTypes of Lightning

Forked lightningForked lightning – – dart leader takes dart leader takes different path than different path than stepped leaderstepped leader

Ribbon lightningRibbon lightning – – wind blows charges wind blows charges into ribbon-like into ribbon-like lightninglightning

Dry lightningDry lightning – – lightning that lightning that occurs in a dry occurs in a dry thunderstormthunderstorm

Types of LightningTypes of Lightning

Heat lightningHeat lightning – – lightning that is lightning that is seen by not heard seen by not heard (can be orange)(can be orange)

St. Elmo’s FireSt. Elmo’s Fire – – luminous green or luminous green or blue halo around blue halo around the top of pointed the top of pointed objects (antennas, objects (antennas, masts of ships). masts of ships). Lightning may occur Lightning may occur after this is seen.after this is seen.

Lightning Detection and Lightning Detection and SuppressionSuppression

Lightning direction-finderLightning direction-finder

Tornado Life CyclesTornado Life Cycles

TornadoTornado – rotating column of air – rotating column of air blowing around a small low pressure blowing around a small low pressure that reaches the groundthat reaches the ground

Tornado Life CyclesTornado Life Cycles Funnel cloudFunnel cloud

Tornado that hasn’t Tornado that hasn’t reached the groundreached the ground

Dust-whirl stageDust-whirl stage Swirling dust at the Swirling dust at the

ground marks the ground marks the tornadoes circulationtornadoes circulation

Mature stageMature stage Funnel at greatest Funnel at greatest

width, most intense width, most intense damage. Often verticaldamage. Often vertical

Decay stageDecay stage Funnel shrinks, damage Funnel shrinks, damage

becomes less, tornado becomes less, tornado becomes stretchedbecomes stretched

Tornado OutbreaksTornado Outbreaks

Tornado familiesTornado families – tornadoes – tornadoes spawned by the same thunderstormsspawned by the same thunderstorms

Tornado outbreaksTornado outbreaks – many tornadoes – many tornadoes that form over same regionthat form over same region

• Much ground-breaking research on tornadoes wasMuch ground-breaking research on tornadoes wasconducted by Professor Ted Fujita of the Universityconducted by Professor Ted Fujita of the Universityof Chicago. The “F-scale” of tornado intensity wasof Chicago. The “F-scale” of tornado intensity wasnamed after him. named after him.

Tornado OccurrenceTornado Occurrence Tornado alleyTornado alley – part of the Central Plains from – part of the Central Plains from

Texas through NebraskaTexas through Nebraska Time of dayTime of day – most often in the afternoon – most often in the afternoon Times of yearTimes of year – most often in Spring, lease often – most often in Spring, lease often

in Winterin Winter

Tornado WindsTornado Winds

Multi-vortex Multi-vortex tornadoestornadoes A single tornado A single tornado

with multiple with multiple rotating columns rotating columns within itwithin it

Suction vorticesSuction vortices The small rotating The small rotating

columns within columns within multi-vortex multi-vortex tornadoestornadoes

Seeking ShelterSeeking Shelter

Tornado watchTornado watch – tornadoes are likely – tornadoes are likely to develop within the next few hoursto develop within the next few hours

Tornado warningTornado warning – issued once a – issued once a tornado is spottedtornado is spotted

• It’s always a good idea to know what to do if a It’s always a good idea to know what to do if a tornado watch or warning is issued for your area. tornado watch or warning is issued for your area.

The Fujita ScaleThe Fujita Scale

Tornado classification based on Tornado classification based on damagedamage

Supercell TornadoesSupercell Tornadoes MesocyclonesMesocyclones – Rising, rotating column – Rising, rotating column

on the south side of a supercell. Acts to on the south side of a supercell. Acts to increase the updraftincrease the updraft

Bounded weak echo regionBounded weak echo region – area inside t- – area inside t-storm where radar does not pick up storm where radar does not pick up precipprecip

Hook echoHook echo – Radar representation of a – Radar representation of a tornado (looks like a hook)tornado (looks like a hook)

Rotating cloudsRotating clouds – First sign tornado is – First sign tornado is about to formabout to form

Wall cloudWall cloud – rotating clouds that have – rotating clouds that have lowered below the base of a supercelllowered below the base of a supercell

• A rotating wall cloud is an unforgettable sight - justA rotating wall cloud is an unforgettable sight - justask a successful storm chaser. ask a successful storm chaser.

Fig. 10-35, p. 290

Nonsupercell TornadoesNonsupercell Tornadoes

GustnadoesGustnadoes – tornadoes that flow – tornadoes that flow along a gust frontalong a gust front

LandspoutsLandspouts – similar to water spouts – similar to water spouts and form due to developing cumulus and form due to developing cumulus congestuscongestus

Severe Weather and Severe Weather and Doppler RadarDoppler Radar

Doppler shiftDoppler shift – Change in frequency – Change in frequency in sound waves for moving objectsin sound waves for moving objects

Tornado vortex signatureTornado vortex signature – rapidly – rapidly changing wind direction in changing wind direction in mesocyclonemesocyclone

NEXRADNEXRAD – Network of 150 radar – Network of 150 radar stations in U.S.stations in U.S.

WaterspoutWaterspout – Tornado that forms over – Tornado that forms over waterwater

Chapter 10: Thunderstorms and Tornadoes Thunderstorms Thunderstorms Tornadoes Tornadoes Tornadic...

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Transcript of Chapter 10: Thunderstorms and Tornadoes Thunderstorms Thunderstorms Tornadoes Tornadoes Tornadic...