WATERFALL MIST PLUMES: Niagara Falls, USADepartment of Geology, University at Buffalo, NY 14260C. Fletcher, M. Bursik

IntroductionThemistgeneratedbyNiagaraFallsoftenpropagatesdownstreamintheNiagaraGorge,butperiodicallyrisesvertically,reachingheightsupto1km(Fig.1).The30millionwhovisitNiagaraFallseachyearhaveexperiencedmoremistydaysoverthepast15years,withensuingrainyconditionsandpoorviewingunfavorablefortourism,especiallyinNiagaraFalls,ONT.Watermixeswithairatthebaseofthefalls.Itishypothesizedthatthedirectionofflowofthemixtureisduetocontrastingwaterandambientairtemperatureswherewaterfallmistthatisconsiderablywarmerthantheairdevelopsabuoyantplumeandacooleraerosolizedmixturedrivesdenserdownstreamcurrents.

Fig.1:NiagaraFallsfromNSC,UniversityatBuffalo

BUOYANT PLUMEItwasexpectedthatrisingmistplumesgeneratedbythefallsareduetothermalbuoyancywhichisdrivenbythetemperaturedifferencebetweenwaterwarmerthanthesurroundingair(+ΔT),mixingwithittocreatealessdense,buoyantplume.Therisingofthisbuoyantplumewouldresultinambientupdrafts(+Vv)comingoutofthegorgeandspreadingawayfromthesource(-WD).

ThedatafromMarch2nd (Figs.5,8)meettheseparametersforplumerise:positiveΔT,andpredominantwindupdrafts/lateralmovementawayfromthefalls.Fig.3,recordedonthesamenight,showsevidenceofawarmmistplumerisingupandabovetheedgeofthewaterfall.

DOWNSTREAM DENSITY CURRENTItwastheorizedthatadensercurrentofmistcreatedbythefallsisduetomixingofcolderwaterwithwarmerair(-ΔT),causingcontractionandincreaseindensity.Thesinkingoftheresultingdensitycurrentwouldcausedowndrafts(-Vv)asmorefreshairispulleddowntowardthegorge/fallstofillspaceleftbysinking/settlingmist(+WD).

DatafromMarch27th (Figs.6,9)meettheconditionsfordensitycurrentpropagation:negativeΔT,anddominatingdowndraftsandlateralairflowtowardthefalls.Fig.2,recordedonanightwithsimilarΔT,showsacooler,mistydensitycurrentsinkingbelowtheedgeofthewaterfall.

Data

Atnightsofvaryingwater-airtemperaturedifference(ΔT),meteorologicaldata andIRfootagewastakenoftheAmericanFallsfromLunaIslandatthetopofthefallsorfromTheCrow’sNest atthebaseofthefalls.

THERMAL IMAGES & METEROLOGICAL DATA• IRvideofootagewastakentovisualize

temperaturesandmixingofairandwaterindifferentpartsofthefalls.(Figs.2,3,4)

• Ambienttemperature,humidity,&pressure,horizontalwinddirection,andbothverticalandhorizontalwindspeedwererecorded.

Methods

Conclusion

UP AND DOWNDRAFTS WITH TEMPERATUREInFig.5,positiveVv valueswerepredominantlypositive,reflectingupdraft-dominatedwinds.TheseresultfromΔT >+9.8°;aircolderthanwaterby9.8° ormore.InFig.6,negativeVv indicatedowndraft-dominatedwinds.ΔT<-11.5° correspondstoairconsistentlywarmerthanwater.InFigs.4&5,gustsofsustainedheightenedVv regardlessofdraftdirection,coincidedwithlargechangesinΔT.InFig.7,Vv valuesof0implynosignificantverticalwindcomponent.SlightlypositiveΔTvaluesshowairmarginallycoolerthanwater,varyingbylessthan0.25°.(Fig.4)

ResultsHORIZONTAL WIND MOVEMENT AND DIRECTION

InFig.8,relativewinddirectionsweremostlynegative,indicatingwindsblowingawayfromthefalls.VH ofthesewinds(max.~7m/s)weremuchfasterthanwindmovingtowardthefalls(max.~2.5m/s).

InFig.9,relativewinddirectionswereallpositive,denotingwindpredominantlyheadedforthefalls.Velocityspikesanddropscorrelatedtosharpchangeinwinddirection.

InFig.10,VH<0.5m/sattheirpeak.Theselowvelocitiescorrelatedwithhighlyvariablewinddirection.(Fig.4)

Basedontheresultsofthisproject,wecandeterminebettertimesforcertainactivitiesatNiagaraFalls.Whentheairtemperatureismuchwarmerthanthewater,thesinkingofthemistplumeisbetterforviewingthefallsclearlywhilealsostayingrelativelydryabovethegorge.(Fig.12)

Mistyandrainyconditions(Fig.13)arecausedbytheplumerisingabovethetopofthegorgewhenthewaterismuchwarmerthantheair.Thiswouldbemostlikelytoinearlymornings,duewater’srelativelyslowrateofheatlossovernight.

BecausemistcurtaingeometryatNiagaraFallsisdependentonlyonthedifferenceintemperatureratherthanonthevalueofthetemperaturesthemselves,thefrequencyofpredominantlymistydayscannotbeentirelyconstrainedtoseasonalvariability.

Thereasonfortheheightenedfrequencyinmistydaysatthefallsoverthelast15yearsisnotknown.

Implications

WIND DIR. (WD) & HORIZONTAL WIND VELOCITY (VH)WDvaluesarerelativetolocation:1isthedirectionofairflowtowardthefalls,-1indicatesthereverse.AtTheCrow’sNest,positiveWDvaluescorrespondtowindblowingeitheratthefallsordowntheNiagaraRiver.

Fig.8: WDandVH vs.TimeatLunaIsland:+WDshowairflowtowardthefalls.

Fig.9:WDandVH vs.TimeatLunaIsl.:-WDshowairflowisawayfromthefalls.

Fig.10:WDandVH vs.TimeatTheCrowsNest:LowVHcorrelatestovariableWD.

ΔT AND VERTICAL WIND VELOCITY (VV)ΔTisthetemperatureofthemistsubtractedbytheambientairtemperature(ΔT=Twater-Tair).Negative velocityvaluesaredowndrafts,positiveVV values denoteupdrafts.

Fig.5:ΔT& VV vs.TimeatLunaIsland:mostly+Vv valuesshowprevailingupdrafts.+ΔTshowaircoolerthanthewater.

Fig.6:ΔTandVV vs.TimeatLunaIsl.:-Vv valuesshowprevailingdowndrafts.-ΔTshowairalwayswarmerthanthewater.

Fig.7:ΔT&VV vs.TimeatTheCrow’sNest:Nosizeableup/downdraftsmeasured.+ΔT;airwasslightlycoolerthanwater.

Fig.3:IRimageoftheAmericanFallsatLunaIsland,whereTmist=34° andTair =~24°

Fig.2:IRimageoftheAmericanFallsatLunaIsland,whereTmist =33° andTair =~38°

Fig.4:IRimageoftheAmericanFallsatTheCrow’sNest,whereTmist =33° andTair =~34°

Fig.11:(TripAdvisor,2015,TableRockWelcomeCentre)PhotooftheHorseshoeFallsundermistyconditionsfromtheTableRockWelcomeCenter,ONT

Fig.12:(Lu,2015,GoogleMapsImagery:TableRockWelcomeCentre) PhotooftheHorseshoeFallsonanon-mistydayfromtheTableRockWelcomeCentre,ONT