>> P.6
EstimatingthedistancefromathunderstormAcommonruleofthumbforroughlyestimatingthedistancefromathunderstormistocountthenumberofsecondsbetweenaflashoflightningandthenextclapofthunder.Dividethisnumberby3togetthenumberofkilometres.HowfastDoeslightningapproach?Ifyoudonothearthunder30secondsafteryousawalightningflashyouareonthesafesideasthethunderstormisfarenoughaway.Ifyouhearthunder15secondsafteralightningflash,itisonlyabout5kmaway.Youareathighriskifyouhearthunderlessthan5secondsafteralightningflash.Alightningflashtravelsatabout300,000km/sandisapproximately900,000timesfasterthantherelativelyslowsoundwavewhichhasaspeedof"only"about330m/s.Thisiswhyyouseetheflashquiteawhilebeforeyouhearthethunder.WorldLightningMapLightningisnotuniformlydistributedacrosstheEarthLightningmapbyNASAWorldLightningMap:ThemapaboveshowstheaverageyearlycountsoflightningflashespersquarekilometerbasedondatacollectedbyNASA’sLightningImagingSensorontheTropicalRainfallMeasuringMissionsatellitebetween1995and2002.Placeswherelessthanoneflashoccurred(onaverage)eachyeararegrayorlightpurple.Theplaceswiththelargestnumberoflightningstrikesaredeepred,gradingtoblack.6
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