MET4300

Lecture 33: Lightning (CH21)

LightningFrequency• About100lightningstrikeseverysecondovertheworld

• OvercontinentalUS,about20millioncloud-to-groundflashesannually.

• Nearlyhalfofallflashesstrikethegroundatmorethanonelocationsimultaneously;so,onaverage,about30millionlocationsarestruckeachyear.

LightningDistributions:Averageannualnumberoflightningflashesperkm^2worldwide

Moreflashesoverland,especiallycentralAfrica,centralSouthAmerica,southeastAsia,northernAustralia,andthesoutheastUS

Distributionoflightningstrikesperkm^2peryearintheUSfrom1989to1998

• HighestflashdensityinFL• HigheralongtheGulfandsouthAtlanticcoasts,andthemidwest• Decreasesnorthwardandwestward

LightningImpacts• Causemanyforestfiresin

remoteareas:12,000lightning-relatedforestfiresinUS.

• Damagebuildingsandstartstructurefires

• Kill26peopleayear(US),andinjure151peopleonaveragearound2015(80%male,50%deathsoccurredinopenareas:ballparks,undertrees,nearoronwater,ongolfcourses,onornearheavyequipment,incampgrounds,talkingoncordedtelephones,etc.)

• Floridahasthemostfatalities• Annualdamage$50million

mostlyduetolightninginducedfires.

Lightning-started fire in the bitterroot national forest

Lightning-started fires in Alaska (July 2004)

LightningStrokes(+)andAssociatedFires(reddots)intheNorthwestUSonaSingleDayinSummer

• ObservedbytheNationalLightingDetectionNetwork(NLDN)

• NLDNprovidesimportantinformationfortheUSforestservice

USLightningFatalities

Second most fatal after floods

USLightningFatalityRate

The rank of fatality rate (# of fatalities divided by # of lightning flashes)

LightningFacts• Lightning isanelectricaldischargeintheatmosphere,

aformofstaticelectricitysimilartothesparkcreatedbyrubbingyourshoesonacarpetandthentouchingametalobject.

• Alightningstrokeis~5kmlongand2-3cmindiameter

• HighVoltage:1-3millionvolts/meter• HighCurrent:15-30KAmperes• HighTemperature:30,000oC (5timesashotasthe

surfaceofthesun)• Energyradiatingfromthelightningheatstheairand

generatesthesoundwaveswehearasboomingthunder

• 100flashespersecondworldwide• Aflashmaybecomposedof1-30strokes;3-5istypical• Energyoutput(1-10billionjoules):

Totalenergyofaboltoflightingcouldsupply~0.5–5.5monthofpowerinahouseholdElectricalenergyonly:6-hofpowerinahousehold

Howdoeslightningforms?• 3typesoflightning:

– In-cloud-lightning:withinclouds– Cloud-to-cloudlightning:betweenclouds– Cloud-to-groundlightning:betweenacloudandtheground

– >80%ofallflashesareeitherin-cloudorcloud-to-cloud

• Hereweonlyconsidertheformationmechanismofcloud-to-groundlightning,whichcausesthemostdamagefromalllightning.

• Tounderstandlightningandhowitforms,weneedtoexaminethecloudatamolecularlevel.

Review:ElectricityandCharge

• Considertwotypesofchargedparticlesincloudmolecules:protons–carrypositivecharge;electrons– carrynegativecharge

• Anatomisneutralwhenitcontainsthesamenumberofprotonsandelectrons

• Anon-neutralatomiscalledion.• Anelectronicfieldispresentinanyregionexposedtocharges• Acharge,suchaspositiveornegativeion,whenplacedinan

electronicfield,experiencesattractiveorrepulsiveforce:samesignchargesrepelandoppositesignchargesattract

• Themagnitudeoftheforceofattraction:Volts• Strengthofanelectricfield:volts/meter• Currentflowseasilythroughconductors(metalandwater)and

poorlythroughinsulators(plasticandair)• Becauseairisanexcellentinsulator,averystrongelectricfieldmust

existbeforechargescanmovefreelythroughtheatmosphere.

Earth’sFairWeatherField(withoutclouds):~100volts/meter

• Anexcess#ofpositivelychargedionintheatmosphere• An excess # of negativelychargedionsontheearth’ssurface• Thisisaconsequenceoftheactionofthunderstorms,which

depositelectronsontheEarth’ssurfaceandremovethemfromtheatmosphere.

MaintainedbyThunderstorms

Distributionofchargeinathunderstormandonthegroundpriortoalightningstroke

• Upperpartispositivelycharged.• Lowerpartofthestormisnegativelycharged.• Thegroundbeneathhas“Imagecharge”:asthestrongnegativecharge

developsatthebaseofthestorm,thenegativechargesontheEarth’ssurfaceunderfairweatherconditionarerepelledawayfromthestorm’sbase,leavingapositivelychargedregioncalledimagecharge.

Screening Layer: the very thin layer of positive charge that appears along the sides of the clouds

HowthischargedistributioninT-stormscomeout?AmechanismcalledInterfaceCharging:• Apossiblemechanism forinitialcharging

inT-storms• Anelectricfieldexistbetweentwo

conductorswithdifferentmaterials• PanelA:Inacloud,graupel andhail

(grownbycollectionofsupercooledwater)areverydifferentwithtinyicecrystals(formed byvapordeposition). Achargepotential(voltage)existbetweenthetwotypesoficeparticles

• PanelB:When icecrystalscollidewithhailorgraupel, interfacechargingoccur:electronswilljump fromthesmallcrystaltothelargerparticle.

• PanelC:Icecrystals becomepositivelycharged,thelargerparticlesbecomenegativelycharged.

• Thepositivelychargedicecrystalsarebroughtupwardintoanvilbyupdrafts,whilethenegativelychargedheavierhail/graupel remainaround thefreezinglevelorbelowtowardtheEarthsurface.

AsecondmechanismInductionCharging:• Afeedbackmechanismwhichcan

acceleratethechargingprocess• PanelA:Inachargedthunderstorm

cloudwithpositivechargeaboveandnegativechargebeneath,strongelectricfieldsdevelop.Theelectronswithineachparticlewillmigratetowardupperpartofthecloud.

• PanelB:Nowwhenahailstonecollidewithicecrystalswhenthehailstoneisfalling,theelectronsontopoftheicecrystalswilljumptothebigparticle,leavingthehailstonebeingnegativelychargedandicecrystalsbeingpositivelycharged.

• PanelC:Icecrystalsmovingupandhailstonemovingdown,theelectricfieldisreinforced.

• Eventually,thestormchangestoapointwherelightningcanoccur.

Stagesofalightningstroke:A.SteppedLeader

• Acloud-to-groundstrokebeginswhentheelectricfieldinalocalareaofthecloudreachesabout3millionvolts/meter.

• Atthisfieldstrength,airnolongeractsasaninsulator,andelectronsbegintomovefreely.

• Theelectronssurgetowardthecloudbaseandtothegroundinaseriesofstepscalledsteppedleader.

• Eachstepisabout50-100mlong,withapauseofafewmillionthsofasecondbetweensteps.

• Negativechargessearchforthepathofleastresistancetotheground,jumpingdownwardthroughraindrops.

• Thechargescantakeseveralpathssimultaneously:forkedlookingoflightningstrokes.

Stagesofalightningstroke:B.ReturnStroke

• Asoneofthebranchesofthesteppedleaderapproachestheground,theelectricfieldbecomessogreatthatpositivechargesjumpupwardofftheobject(orground)tomeetthedescendingsteppedleader.Thisiscalledtravelingspark.

• Apowerfulreturnstrokedevelopswhenthetravelingsparkofpositivechargesurgesupwardtomeetthesteppedleader.

• Atthistime,thefullcurrentisreachedandtheflashoccurs.

• Thelightningflashisassociatedwiththeionization ofthemoleculesinthepathofthelightningstroke.

• Allthesteppedleader&returnstrokehappeninabout10microseconds.

• Allweseearethebranchesofthelightningwithforkedpath&brightreturnstroke nearground.

Branchingoccursinthesteppedleaderandinitialreturnstroke

Stagesofalightningstroke:C.DartLeader

• Thesameprocess,leader&returnstroke,willoccurinthesamechanneloneormoretimes.

• AsecondsurgeofnegativechargedescendsalongtheionizedpathofthepreviousstrokecreatingtheDartLeader.

Stagesofalightningstroke:D.Secondreturnstroke

• Asbefore,whenthedartleaderapproachestheearth,atravelingsparkwilljumpuptomeetit,initiatingthesecondreturnstroke.

• Theserepeatstrokescanoccurrapidly.

• Inrarecases,over20strokesoccurredwithinasingledischargeevent.

• Thewholeprocessrepeatsuntilthecloudisdischarged.Thisallhappenin10microseconds.Youreyescan’tseeanyofthisdetails.

AnotherViewRemember a Flash may comprise many Strokes

Othertypesofcloud-to-groundlightningstrokes:positivepolaritystrokes

• Betweenanvilandground• Positivechargestraveldownwardtotheground• Lesscommon,8%ofallcloud-to-groundlightningstrokearethistype• Moredangerousbecausetheyrequirestrongerelectricfieldsanddischarge

morecurrent.

Othertypesofcloud-to-groundlightningstrokes:boltfromtheblue

• Thebolt fromtheblueisatypeofcloud-to-ground lightning thatexitsfromthesideofathundercloud andcomestothegroundawayfromthethundercloud

• Theground strikecanbeover10kmfromthecloudboundary, arrivingattheground inanareawheretheskyisblue

• Normal(negative)polarity,beginsasin-clouddischarges,initiatedbetweenthemainnegativechargeandtheupperpositivechargewithin thecloud.

• Afterneutralizingmuchoftheupperpositivechange, thedischargecontinues towardthepositivechargeatground.

Balllightning

• Balllightningisaluminous,small,glowingballofgas,typically10–40cm(4–16in)indiameter.Balllightningappearsveryrarelyandalwaysfollowsalightningstroke.

• Hasbeenseenbyabout5%ofthepeopleonEarth• Typicallybecomesbrightandthendisappearswithan

explosion.• Behavesstrangely.Itfloatsintheair,rangesincolorfromred,

orange,andyellowtoblue,andhasan“electrical”odor,indicatingthatozonemaybepresent.Itsometimesfloatsfromcloudtocloud,orcloudtoground,groundtocloud,orjusthorizontally.Itappearstobeattractedtoopenwindowsandtendstobeattractedtogroundedobjects.

• Itslifetimevariesfromafewsecondstoseveralminutes

LightninginHurricanes• Hurricane Hugo at

landfall on the South Carolina coast. Hugo produced only 16 lightning strikes in an 8-hour period as the storm devastated South Carolina’s coast.

• Updrafts in hurricanes are not as strong as those in T-storms

• Not enough supercooled water to produce hail & graupel, therefore lightning charges

Thunder• During lightning, theairchannelisheatedto54K°F(30K°C).• Theairexpandsexplosively, creatingashockwavethatevolvesrapidlyintocrashing

soundwaves.• Soundspeed is330m/s(5secondstotravelamile),whilelight speedisveryfast

(300millionm/s).Soyouseelightning first,thenhearthunder.• You’llheardifferentsoundsdepending onyourlocationfromthestroke.

• Thelowerthetemperature,theslowerthesound travels• ThesoundwavesarebendedupwardintheatmosphereduetoTdecreaseswithheight.• Higherfrequenciesbendmorerapidly,soyou’llhearthelower,deepersounds nearthestroke

atground.

OtherPhenomenaAssociatedwithElectricalDischargesintheAtmosphere

• HeatLightning:Producedbyathunderstormfaroffinthedistance.Thelightwillbescatteredbyairmoleculesordustparticlesorreflectedfromcloudsandappearasalightflashintheskyoverhead.

• Beadlightning:followingalightningstroke,lightningchannelssometimesbreakupverybrieflyintoaseriesofluminous“beads”.Can’tbeseenbytheeye,buthavebeenobservedwithhigh-speedcameras.

• Sheetlightning:lightningwithinorbehindacloud,illuminatingtheclouds’exterioruniformly,givinganappearanceofasheetoflightning.

• St.Elmo’sfire:Underneathathunderstorm,tallobjectsfromthegrounddeveloppositivechangesontheirtips.Iftoomuchchargeaccumulates,adischargeofsomesparkwilloccur.

RedSprites,BlueJets,andElvs

• Theyareopticalphenomenathatoccurbetweenthetopsofthunderstormsandthemesosphere

• Sprites:arered,largeweakluminousflashesthatoccursimultaneouslywithlightningstrokes.– Thebrightestregionsare65-75kmabovesurface.Afaintredglow

extendstoabove90km.– Bluetendrilsarebelowthebrightestregionsandextenddownwardto30

km.– Mostcommonlyfoundaboveanvil;alwaysoccurwithpositivepolarity

strokes.– Spritesdevelopaschargedparticles inthemesosphere&stratosphere

moveinresponsetorapidchargesintheelectricfieldtriggeredbyalightningdischargeinthetroposphere.

• Elves:aredisk-shapedregionsoflightthatlastlessthanathousandthofasecond.Theyoccurhighabovepositivepolaritylightningintheionosphereandarecenteredonthelightningstrkebelow.

• Jets:areblueandcannotbedetectedbytheeye.Theyextendupwardfromthecloudtopinnarrowconesandcanhaveupwardspeedof100km/sec.Usuallydevelopovertheregionofactiveconvectionwithinthuderstorms.

Sprites and elves are triggered by cloud-to-ground positive polarity strokes that occur in the anvil region of thunderstorms. Blue jets develop over the region of active convection where most lightning strikes occur.

LightningSafety• Signsoflightningstroke:Sizzlingsound

orhairstandingupindicatesastrongfield

• 5sgapbetweenflashandthunderimpliesa1mirange

• 30-30rule(conservative)– 30slagà 6mirange– Wait30min

• Takerefugeinacarorabuildingwithplumbingandwiring

• Notundertrees• Assumealowcrouchwithonlyfeetin

contactwiththeground• Avoidholdingelongatedmetalobjects,

likeriflesorgolfclubs;avoidopenwateroutdoors,avoidtakingshowerindoors.

• Detailsonlightninghazardsavailablefromwww.lightningsafety.com

Hair standing on end is a sign thatLightning is about to strike

InClassActivity

• ReadCH22Downbursts

ForNextTime

• Ex.21.1,21.2,21.4