AdvancedAdvanced SpotterSpotter TrainingTraining 20092009

LessonLesson 7:7: TornadoesTornadoes FromFrom LastLast TimeTime

WeWe discusseddiscussed thethe naturenature andand originorigin ofof wind.wind. WeWe exploredexplored howhow windwind causescauses damage.damage. WeWe discusseddiscussed thethe naturenature ofof gustgust frontsfronts andand downbursts.downbursts. WeWe exploredexplored thethe evolutionevolution ofof derechos.derechos. ThisThis TimeTime

ThisThis timetime wewe areare goinggoing toto discussdiscuss thethe mostmost severesevere weatherweather eventevent thatthat cancan happen,happen, thethe tornado.tornado. WeWe willwill firstfirst discussdiscuss whatwhat aa tornadotornado is.is. ThenThen wewe willwill discussdiscuss ideasideas aboutabout howhow tornadoestornadoes form.form. Finally,Finally, wewe willwill gogo overover wherewhere youyou areare likelylikely toto findfind tornadoes.tornadoes. HomeworkHomework ReviewReview

GoGo overover thethe homeworkhomework problemsproblems fromfrom lastlast time:time: DescribeDescribe howhow aa downdraftdowndraft producesproduces windwind atat thethe surface.surface. DescribeDescribe thethe effecteffect ofof evaporativeevaporative cooling.cooling. ExplainExplain howhow structuresstructures failfail underunder thethe actionaction ofof wind.wind. SpeculateSpeculate onon thethe risksrisks ofof flyingflying debrisdebris posedposed toto spottersspotters inin thethe field.field. HowHow wouldwould youyou avoidavoid suchsuch risks?risks? HomeworkHomework ReviewReview (continued)(continued)

SpeculateSpeculate onon thethe severesevere potentialpotential fromfrom aa gustgust front.front. SpeculateSpeculate onon thethe severesevere potentialpotential fromfrom drydry microbursts.microbursts. ExplainExplain howhow aa derechoderecho happens.happens. WhatWhat isis aa tornado?tornado?

AsAs statedstated inin LessonLesson 22 aa tornadotornado isis aa violentlyviolently rotatingrotating columncolumn ofof airair pendantpendant fromfrom aa thunderstormthunderstorm andand inin contactcontact withwith thethe surface.surface. TheThe firstfirst aspectaspect ofof thisthis definitiondefinition isis thethe partpart aboutabout violentlyviolently rotatingrotating air.air. WhatWhat doesdoes itit meanmean forfor airair toto rotate?rotate? HowHow doesdoes itit rotate?rotate? FirstFirst off,off, everyevery updraftupdraft rotates.rotates. WhatWhat isis aa tornado?tornado? (continued)(continued)

TakeTake youryour rightright handhand andand makemake aa fistfist soso thatthat youryour thumbthumb layslays acrossacross thethe toptop ofof youryour fistfist andand youryour fingersfingers areare alignedaligned verticallyvertically asas ifif youyou werewere holdingholding aa mug.mug. ExtendExtend youryour thumbthumb upward.upward. ExtendExtend youryour pointingpointing fingerfinger forward.forward. ExtendExtend youryour middlemiddle fingerfinger soso thatthat itit isis pointingpointing straightstraight toto youryour left.left. ThisThis isis aa representationrepresentation ofof whatwhat wewe call,call, inin physics,physics, thethe right-handright-hand rulerule .. WhatWhat isis aa tornado?tornado? (continued)(continued)

AssumeAssume thethe thumbthumb representsrepresents thethe directiondirection ofof thethe updraft.updraft. AssumeAssume thethe pointingpointing fingerfinger representsrepresents inflowinflow leadingleading intointo thethe updraft.updraft. TheThe middlemiddle fingerfinger pointspoints inin thethe directiondirection ofof thethe resultingresulting force;force; inin thethe casecase wewe havehave aa tendencytendency forfor risingrising airair withwith forwardforward velocityvelocity toto rotaterotate toto thethe leftleft (counterclockwise).(counterclockwise). WhatWhat isis aa tornado?tornado? (continued)(continued)

ThisThis isis whatwhat causescauses thethe updraftupdraft toto rotate.rotate. WhatWhat constitutesconstitutes violentviolent rotation?rotation? TheThe currentcurrent standardstandard isis thethe EnhancedEnhanced FujitaFujita scale,scale, butbut thisthis reliesrelies onon windwind estimatesestimates basedbased onlyonly onon thethe damagedamage done.done. LastLast timetime wewe examinedexamined howhow windswinds damagedamage structures.structures. ItIt takestakes timetime forfor windswinds toto penetratepenetrate aa structure.structure. WhatWhat isis aa tornado?tornado? (continued)(continued)

IfIf youyou havehave aa veryvery fastfast movingmoving EF5EF5 tornadotornado itit mightmight notnot havehave timetime toto dodo EF5EF5 damagedamage alongalong itsits path.path. Similarly,Similarly, aa veryvery slow-movingslow-moving EF1EF1 scalescale tornadotornado maymay dodo extensiveextensive damagedamage sincesince potentialpotential targetstargets areare underunder thethe actionaction ofof thethe windwind forfor longerlonger periods.periods. So,So, wewe needneed toto settlesettle onon aa definitiondefinition forfor whatwhat violentviolent meansmeans inin thisthis context.context. WhatWhat isis aa tornado?tornado? (continued)(continued)

II proposepropose usingusing thethe severesevere windwind limitlimit asas thethe startstart ofof thethe tornadotornado scale.scale. WeakWeak Tornado:Tornado: 5050 knotsknots -- 100100 knots.knots. StrongStrong Tornado:Tornado: 101101 knotsknots -- 200200 knots.knots. ViolentViolent Tornado:Tornado: 201201 knotsknots +.+. WhileWhile thesethese areare arbitrary,arbitrary, soso areare allall ofof thethe otherother scales.scales. ThisThis hashas thethe advantageadvantage ofof beingbeing simple.simple. WhatWhat isis aa tornado?tornado? (continued)(continued)

TheThe ideaidea thatthat tornadoestornadoes areare pendentpendent fromfrom aa thunderstormthunderstorm isis likelylikely wrongwrong inin thethe faceface ofof currentcurrent data.data. ItIt seemsseems thatthat manymany tornadoestornadoes (perhaps(perhaps eveneven mostmost ofof them)them) formform fromfrom thethe groundground upup (see(see tornadogenesistornadogenesis below).below). IfIf thisthis isis true,true, thenthen thethe tornadotornado isis notnot pendantpendant fromfrom thethe thunderstorm.thunderstorm. ThisThis isis aa problem.problem. WhatWhat isis aa tornado?tornado? (continued)(continued)

TheThe thingthing thatthat connectsconnects thethe tornadotornado toto thethe thunderstormthunderstorm isis thethe updraftupdraft ofof thethe thunderstorm,thunderstorm, oror somesome otherother verticalvertical forcingforcing mechanism.mechanism. PerhapsPerhaps aa betterbetter definitiondefinition wouldwould bebe somethingsomething likelike this:this: "A"A tornadotornado isis aa violentlyviolently rotatingrotating columncolumn ofof airair inin contactcontact withwith thethe groundground andand drivendriven byby verticalvertical forcing."forcing." VerticalVertical ComponentComponent ofof thethe WindWind

TheThe verticalvertical forcingforcing ofof thethe updraftupdraft givesgives thethe tornadotornado aa verticalvertical componentcomponent toto itsits winds.winds. ThisThis isis oneone reasonreason whywhy vehiclesvehicles areare unsafeunsafe inin strongstrong oror violentviolent tornadoes.tornadoes. SinceSince thethe airair isis drivendriven upwardsupwards itit exertsexerts aa forceforce onon thethe undersideunderside ofof anyany object.object. AfterAfter somesome criticalcritical valuevalue isis reached,reached, thethe objectobject willwill becomebecome weightless.weightless. VerticalVertical ComponentComponent ofof thethe WindWind (continued)(continued)

AfterAfter thatthat thethe objectobject becomesbecomes debris,debris, whetherwhether itit isis aa playingplaying card,card, aa truck,truck, oror aa building.building. AA humanhuman beingbeing becomesbecomes weightlessweightless whenwhen verticalvertical windswinds reachreach aroundaround 9090 knots.knots. AA verticalvertical windwind speedspeed ofof 100100 knotsknots willwill makemake mostmost carscars weightless.weightless. AA verticalvertical windwind speedspeed ofof 110110 knotsknots willwill makemake mostmost minivansminivans weightless.weightless. VerticalVertical ComponentComponent ofof thethe WindWind (continued)(continued)

InIn 19991999 onon 33 MayMay thethe towntown ofof MooreMoore inin OklahomaOklahoma waswas hithit hardhard byby aa nearlynearly mile-widemile-wide violentviolent tornado.tornado. InIn aa damagedamage photographphotograph takentaken aa milemile fromfrom thethe tornadotornado damagedamage path,path, aa metalmetal foldingfolding chairchair cancan bebe seenseen neatlyneatly drivendriven throughthrough aa 4x4.4x4. ThisThis 4x44x4 waswas twelvetwelve feetfeet fromfrom wherewhere itit hadhad beenbeen located.located. VerticalVertical ComponentComponent ofof thethe WindWind (continued)(continued)

ItIt hadhad beenbeen oneone ofof twotwo verticalvertical supportssupports holdingholding upup aa balcony.balcony. TheThe foldingfolding chairchair waswas thrownthrown aa milemile fromfrom thethe tornadotornado withwith enoughenough forceforce toto knockknock aa 4x44x4 verticalvertical supportsupport outout fromfrom underunder aa balconybalcony andand drivedrive thethe legleg ofof thethe chairchair allall thethe wayway throughthrough it!it! ThusThus isis thethe powerpower ofof aa violentviolent tornado...tornado... StructureStructure ofof aa TornadoTornado

ToTo thethe leftleft wewe looklook downdown fromfrom aboveabove onon aa tornado.tornado. StructureStructure ofof aa TornadoTornado (continued)(continued)

InflowInflow jetsjets areare regionsregions wherewhere thethe circulationcirculation ofof thethe tornadotornado pullspulls thethe surroundingsurrounding airair intointo thethe vortex.vortex. InflowInflow jetsjets areare veryvery dangerousdangerous places,places, windswinds cancan bebe sufficientsufficient toto dragdrag objectsobjects intointo thethe tornadotornado whenwhen thesethese jetsjets areare atat groundground level.level. StructureStructure ofof aa TornadoTornado (continued)(continued)

InflowInflow jetsjets areare notnot limitedlimited toto thethe surface,surface, therethere havehave beenbeen somesome spectacularspectacular videosvideos ofof tornadoestornadoes withwith inflowinflow jetsjets higherhigher upup inin thethe vortex.vortex. StructureStructure ofof aa TornadoTornado (continued)(continued)

HereHere isis aa verticalvertical cross-sectioncross-section ofof aa tornado.tornado. ThereThere seemsseems toto bebe aa centralcentral downdraft,downdraft, asas indicatedindicated byby high-resolutionhigh-resolution radarradar images.images. StructureStructure ofof aa TornadoTornado (continued)(continued)

II believebelieve thisthis isis duedue toto airair beingbeing centrifugedcentrifuged outout ofof thethe vortexvortex atat thethe basebase ofof thethe vortexvortex column.column. AnyAny funnelfunnel cloudcloud thatthat developsdevelops formsforms insideinside thethe actualactual tornado.tornado. TheThe internalinternal downdraftdowndraft lowerslowers thethe pressurepressure sufficientlysufficiently toto produceproduce saturationsaturation insideinside thethe funnel.funnel. StructureStructure ofof aa TornadoTornado (continued)(continued)

IfIf therethere isis insufficientinsufficient moisturemoisture therethere willwill bebe nono visiblevisible funnelfunnel cloud.cloud. FirstFirst Discussion!Discussion!

DiscussDiscuss thethe multivortexmultivortex tornado,tornado, andand howhow thisthis mightmight comecome about.about. SectionSection 22

TornadogenesisTornadogenesis andand DissipationDissipation TornadogenesisTornadogenesis

TheThe processprocess ofof tornadotornado formationformation isis calledcalled tornadogenesistornadogenesis .. TheThe wayway aa tornadotornado formsforms isis stillstill aa mattermatter ofof bothboth intenseintense debatedebate andand research.research. TornadogenesisTornadogenesis (continued)(continued)

ThisThis processprocess seemsseems toto involveinvolve thethe followingfollowing elements:elements: AA sourcesource ofof verticalvertical forcing.forcing. ThisThis cancan bebe anan updraft,updraft, convergence,convergence, oror eveneven thethe verticalvertical motionmotion causedcaused byby rotatingrotating winds.winds. WheneverWhenever youyou havehave counterclockwisecounterclockwise winds,winds, thethe right-handright-hand rulerule requiresrequires verticalvertical motion.motion. TornadogenesisTornadogenesis (continued)(continued)

AA sourcesource ofof rotation:rotation: ThisThis cancan bebe causedcaused byby veeringveering windswinds withwith height,height, oror byby turbulenceturbulence causedcaused byby convergence,convergence, oror byby somesome otherother mechanismmechanism thatthat isis notnot currentlycurrently known.known. TornadogenesisTornadogenesis (continued)(continued)

ThereThere areare somesome otherother factorsfactors thatthat maymay playplay aa part:part: StrongStrong inflowinflow intointo thethe regionregion wherewhere thethe tornadotornado willwill form.form. ThisThis maymay carrycarry eddieseddies formedformed fromfrom convergenceconvergence intointo thethe updraftupdraft region.region. AA warmwarm RFDRFD hashas beenbeen linkedlinked toto increasedincreased vortexvortex productionproduction atat thethe surface.surface. VortexVortex lineslines areare producedproduced wheneverwhenever thethe atmosphereatmosphere spinsspins around.around. TornadogenesisTornadogenesis (continued)(continued)

ByBy collectingcollecting largelarge numbersnumbers ofof vortexvortex lineslines youyou getget aa strongerstronger vortexvortex (so(so longlong asas theythey areare allall pointingpointing thethe samesame way).way). TheThe RFDRFD cancan bringbring mid-levelmid-level vortexvortex lineslines (produced(produced byby aa mesocyclone,mesocyclone, forfor example)example) toto thethe ground,ground, thusthus increasingincreasing thethe numbernumber ofof vortexvortex lineslines atat thethe surface.surface. AnyAny RFDRFD cancan intersectintersect thethe inflowinflow windswinds atat aa sharpsharp angle,angle, producingproducing eddieseddies whichwhich cancan bebe drawndrawn intointo thethe updraftupdraft regionregion ofof thethe thunderstorm.thunderstorm. TornadogenesisTornadogenesis (continued)(continued)

ThereThere seemsseems toto bebe aa criticalcritical ratioratio betweenbetween thethe strengthstrength ofof thethe updraftupdraft andand thethe strengthstrength ofof thethe inflow,inflow, calledcalled thethe swirlswirl ratioratio .. WithinWithin thisthis ratioratio aa tornadotornado willwill form,form, otherwiseotherwise itit willwill not.not. AA tornadotornado willwill continuecontinue soso longlong asas therethere isis aa sourcesource ofof verticalvertical motionmotion andand inflow.inflow. IfIf strongstrong windswinds undercutundercut thethe vortex,vortex, thethe tornadotornado willwill dissipate.dissipate. TornadogenesisTornadogenesis (continued)(continued)

IfIf thethe thunderstormthunderstorm updraftupdraft weakens,weakens, oror thethe convergenceconvergence ceases,ceases, thethe tornadotornado willwill dissipate.dissipate. IfIf coldcold airair isis ingestedingested intointo thethe updraft,updraft, causingcausing thethe updraftupdraft toto weaken,weaken, thethe tornadotornado willwill dissipate.dissipate. TornadogenesisTornadogenesis (continued)(continued)

ItIt isis possiblepossible forfor thethe conditionsconditions thatthat producedproduced thethe tornadotornado toto movemove awayaway fromfrom thethe tornado.tornado. ThisThis maymay resultresult inin thethe developmentdevelopment ofof anotheranother tornadotornado furtherfurther away.away. ThisThis isis calledcalled cycliccyclic tornadogenesistornadogenesis .. TheThe firstfirst tornadotornado maymay stillstill existexist asas otherother tornadoestornadoes formform nearby.nearby. TornadogenesisTornadogenesis (continued)(continued)

ItIt isis alsoalso possiblepossible forfor multiplemultiple tornadoestornadoes toto developdevelop aroundaround aa veryvery strongstrong tornado.tornado. ThisThis isis notnot thethe samesame aa multiple-vortexmultiple-vortex tornado.tornado. ThisThis effecteffect isis sometimessometimes calledcalled aa tornadotornado cyclonecyclone ,, andand aa largelarge regionregion ofof thethe airair underunder thethe thunderstormthunderstorm willwill bebe seenseen toto bebe rotating.rotating. TornadogenesisTornadogenesis (continued)(continued)

ThisThis cancan becomebecome coupledcoupled withwith (or(or eveneven produce)produce) aa mesocyclone.mesocyclone. LargeLarge tornadoestornadoes undergoundergo aa processprocess wherewhere thethe flowflow insideinside thethe tornadotornado becomesbecomes disrupted,disrupted, butbut doesdoes notnot dissipate.dissipate. TheThe resultresult isis aa turbulentturbulent vortexvortex wherewhere smallersmaller sub-vorticessub-vortices areare producedproduced andand rapidlyrapidly dissipatedissipate withinwithin thethe largerlarger tornado.tornado. TornadogenesisTornadogenesis (continued)(continued)

ThisThis processprocess isis calledcalled vortexvortex breakdownbreakdown .. TornadogenesisTornadogenesis (continued)(continued)

SuchSuch aa tornadotornado isis calledcalled aa multivortexmultivortex tornadotornado (or(or aa multiplemultiple vortexvortex tornado)tornado) andand mightmight looklook likelike thisthis fromfrom thethe toptop down.down. SecondSecond Discussion!Discussion!

SpeculateSpeculate onon howhow thisthis informationinformation cancan telltell youyou wherewhere aa tornadotornado isis likelylikely toto occur.occur. SectionSection 33

LocatingLocating TornadoesTornadoes LocatingLocating TornadoesTornadoes

TheThe mostmost likelylikely regionregion ofof aa thunderstormthunderstorm forfor tornadotornado formationformation andand developmentdevelopment isis inin thethe updraftupdraft region,region, butbut onlyonly ifif therethere isis aa goodgood sourcesource ofof warmwarm andand moistmoist inflow.inflow. TornadoesTornadoes thatthat formform herehere areare thethe mostmost likelylikely toto bebe long-lastinglong-lasting andand violent.violent. LocatingLocating TornadoesTornadoes (continued)(continued)

AnotherAnother placeplace wherewhere tornadoestornadoes occur,occur, thoughthough lessless frequently,frequently, areare inin thethe convergenceconvergence zoneszones wherewhere outflowoutflow boundariesboundaries areare intersecting.intersecting. ThisThis cancan occuroccur inin stormstorm trainstrains wherewhere thethe outflowoutflow fromfrom thethe dissipatingdissipating leadlead cellcell encountersencounters outflowoutflow fromfrom thethe maturingmaturing cellcell andand aa briefbrief strongstrong updraftupdraft formsforms betweenbetween them.them. LocatingLocating TornadoesTornadoes (continued)(continued)

TornadoesTornadoes thatthat formform inin convergenceconvergence areasareas tendtend toto bebe disorganizeddisorganized andand weak,weak, butbut theythey cancan occuroccur inin largelarge families,families, andand occasionallyoccasionally theythey cancan developdevelop strongstrong tornadoes.tornadoes. AnotherAnother eventevent thatthat occursoccurs occasionallyoccasionally isis aa weakweak tornadotornado willwill formform onon thethe outflowoutflow andand bebe drawndrawn intointo thethe inflowinflow regionregion andand willwill strengthenstrengthen there.there. LocatingLocating TornadoesTornadoes (continued)(continued)

AnotherAnother typetype ofof vortexvortex occursoccurs onon thethe outflow;outflow; thisthis typetype failsfails toto reachreach thethe cloudcloud level,level, oror soso itit isis thoughtthought (we(we dodo notnot reallyreally knowknow forfor sure).sure). ThisThis typetype ofof vortexvortex isis calledcalled aa gustnadogustnado .. TheThe onlyonly partpart ofof thethe gustnadogustnado thatthat isis visiblevisible isis thethe debrisdebris cloud,cloud, soso itit isis notnot reallyreally possiblepossible toto telltell howhow highhigh upup thethe vortexvortex extends.extends. LocatingLocating TornadoesTornadoes (continued)(continued)

TheseThese vorticesvortices occasionallyoccasionally reachreach thethe strengthstrength ofof aa weakweak tornado.tornado. ItIt isis possiblepossible thatthat aa gustnadogustnado cancan bebe drawndrawn intointo thethe updraftupdraft andand becomebecome aa truetrue tornado.tornado. FinalFinal Discussion!Discussion!

DiscussDiscuss thethe destructivedestructive potentialpotential ofof tornadoes,tornadoes, andand theirtheir ramificationsramifications forfor spotters.spotters. HomeworkHomework DueDue NextNext WeekWeek

DescribeDescribe whatwhat aa tornadotornado is.is. DetermineDetermine threethree criteriacriteria forfor whetherwhether somethingsomething isis aa tornado.tornado. DescribeDescribe twotwo waysways thatthat tornadoestornadoes cancan form.form. WhatWhat isis aa multivortexmultivortex tornado?tornado? HowHow isis thethe EnhancedEnhanced FujitaFujita scalescale limited?limited? DrawDraw aa diagramdiagram ofof eacheach typetype ofof thunderstormthunderstorm thatthat wewe havehave discussed.discussed. SpeculateSpeculate aboutabout wherewhere toto looklook forfor tornadoestornadoes forfor each.each. HomeworkHomework DueDue NextNext WeekWeek (continued)(continued)

WhyWhy isis aa gustnadogustnado notnot aa tornado.tornado. IsIs thisthis aa realisticrealistic distinction?distinction?