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2/11/17 Chapter14 Cold Waves Cold Waves • Aninflux ofunusually cold airfromthearctic thatintrudes into thelower andmiddle latitudes • Canfreezevegetationandcausestructural damage • Generally affectmuchlarger areasthan blizzards, icestorms, etc. EffectsofCold Waves • NWSkeepsstatistics ondeathsrelated tocold weather outbreaks – Onaverage, (3,30,300)deathsareattributedto coldwaves • CDC estimatesthat(60, 600,6000)deaths are attributed tohypothermia – Maynotnecessarilyberelatedtoacoldoutbreak – Onlyincludesdeathsdirectlyrelatedtocold – Disproportionatelyoccuramongstelderlymales 1 2/11/17 Cold Waveoutbreaks • Whereareeconomic lossesgreatest? • Whatismeantbya“Cold Wave”? Cold WaveIntensityMeasure • Temperatures during cold wavesarenot the mostmeaningful measureofacold waves intensity. Whatis? LowestTemperaturesonRecordby State 2 2/11/17 Cold WaveDevelopment • Core developsasastrong(high-pressure/lowpressure) systemathighlatitudes • Aircools neartheground byconductive heat losstotheground • Airdensity increases, temperature decreases, whathappens topressure? • Whattypeofairmass develops? ArcticAir • Temperatures intheArctic Regions can sometimes drop below -40C.Whatproblems doesthis cause? DevelopmentofPolarAirmasses • Whatistypicallycolder, Arctic Oceanregions nearthepole orland locations (Siberia, Northern Canada, Alaska)?Why? 3 2/11/17 PolarAirmass Depths • Generally shallow • Characterized bystrongtemperature inversions • Canleadtotemperatures atthesurfacethat are(5,10,15)Ccolder than severalhundred metersaloft • Howdoes topography playarole inin enhancing temperature inversions? Airmass Intensification • Whatother factors canintensify theairmass? Airmass Intensification 4 2/11/17 Cold WaveOutbreaks • Cold airoutbreaks require acoldairmass to formathighlatitudes andmovesouth • Whatmakesthemmove(twofactors)? • Themorequickly itmovestomiddle latitudes, thelesstimeithastomodify Equatorward Motion • Occur whenjetstream windshaveasoutherly component – Upperairpatterncontributestobothconvergence aloftandsouthernmovementoftheairmass • Factorofridge intensification overeastern North Pacific RidgeIntensification • AiroverNorth PacificWatersis (warmer/colder) thanairoverland atsame latitudes • Aleutian lowtransports warmairnorthward • Northward flowofmaritime airleadsto warming ofthelower troposphere. Whatdoes thisdo tothejetstream? 5 2/11/17 RidgeIntensification • Second factorisflowassociated with strong cyclones originating ontheeastsideofthe Rockiesor alongtheeastern Seaboard • Cyclones transport cold airsouthward ontheir (eastern/western) side • Enhances southward component ofjet stream flowon troughs western side,enhancing upstream ridge Cold WavePattern Cold WaveMovement 6 2/11/17 Subsidence Effects • Howdoes subsidence modify theairmass? • Cancold airmasses passovertherockies and headwest? • Whatother variables canaffectcold wave outbreaks? Cold OutbreakSummary • Whatfivefactors influence theoutbreak ofa cold wave? ExtremeCold inEuropeandAsia • Siberia – Whatenhancestheextreme coldinSiberia? 7 2/11/17 NorthAtlanticOscillation • Typicalflowiswesttoeast • Flowisconsequence oftheIcelandic lowand highpressure near theAzores • Both pressure centers weakeningsubstantially leadstotheNAO • Allows coldSiberian airtomove southwestward NorthAtlanticOscillation NorthAtlanticOscillation • Negativephasewhenpressures andwinds are allweakerthan normal • Positive whenallarestronger 8 2/11/17 EuropeanCold Outbreaks Wind Chill Factor • Accounts forthe effectoftemperature and wind ontherateatwhich exposed skinwill cool • Solid andliquid surfaces loseheatmore rapidly atagivenwind speedaswinds increase • Object losesheatduetoconduction • Evaporation fromskinalsoincreases aswind speedincreases Wind Chill Index • Estimateofperceived temperature basedon wind speedandactual temperature • Scientists first begandevelopment in1940’s, where at? • Measuredtime required forcooling and freezing ofknown volumesofwaterstarting at varioustemperatures • Latestrefinement madein2001 9 2/11/17 Wind Chill IndexModifications • 2001adjustments included factorsaccounting for: – Advancesinscience,techandmodelingofheat loss – New formulabasedonmodelofhumanface – Assumesnighttime,doesn’taccountforsunshine warmth Wind Chill Index Advisory vs Watch • Whatisthedifference between aWind Chill AdvisoryandaWind Chill Warning? 10 2/11/17 Cold WaveWarning • Issuedbasedonforecastofprolonged temperatures wellbelow normal Cold WavesandClimateChange • Whathazardsareassociated with coldwave outbreaks? • Whatwill climate changemeanforcoldwave outbreaks? 11