Post on 05-Feb-2021
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
• ReadCH22DownburstsForNextTime
• Ex.21.1,21.2,21.4