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Weather – October 2009, Vol. 64, No. 10 Figure 1. -to-ground from a lake-effect snowstorm downstream ofLake lightningfromFigure alake-effect Ontario, November 1.Cloud-to-ground 1985.(©JamesG.Ladue.) and R.James Vavrek dersnow’ (Figure 1). andlightning/thunderiscalled ‘thun- lightning andthunder. The combination of snowfallheavy canoccasionally produce During , however, snowstorms with , strong winds, andpossiblytornadoes. may beaccompanied by , heavy cumulonimbus . These thunder- duringthewarmseasonfromobserved Lightning andthunderare commonly 2 1 David M.Schultz ofthundersnow An overview Henry W.Henry Eggers School, Hammond, andFinnish ofHelsinki University Indiana, USA Finland Meteorological Institute, Helsinki, 1

2 to-ground (CG)to-ground lightningstrokes atrates of York, 2006 yieldingcloud- on12October , named ‘Aphid’, struckBuffalo, New thundersnow either. Anearly-seasonsnow- database. without electricity, according to the TORRO tion inLisburn,causing13500homesto be fell. Onelightningstrikedisabledasubsta- Ireland, 12–15centimetresNorthern ofsnow 2008inBelfast, of convection on3/4January another example, during onesix-hour period problems duringtheevening rushhour. In The wet snow froze, causingnumerous traffic winds, andatornado atCoombe Down, Bath. Britain, producing thundersnow, hail, gusty squall linepassedthrough muchofsouthern 2004,astrong cold front 28 January and events have On struckthe United Kingdom. The United States isnotimmuneto For example, two notablethundersnow thundersnow event inUtahduring2002, ing thewarmseason.For example, inone strokes thanthunderstorms withrain dur- storms generallyproduce fewer lightning uncommon addition,thundersnow event. In Thus, thundersnow isanexceptionally ning orthunder(Curran andPearson, 1971). snowfalls arereported associated withlight- dences with snowfall, andonly0.07%of inci- (October–May) report snowfall.the heavy area morningby wasparalyzed thenext between midnightand1a.m. The metro with aperiodoflightningandthunder centimetres ofwet snow fell overnight, whenmoreColumbia, than38 Missouri, few monthslater on1December 2006in of thestorm. Anotherevent occurred a 10+ strikesperminute duringtheheight In theUSA,only1.3%ofcool-seasonIn Thundersnow – October 2009, Vol. 64, No. 10 275 ., 2002). for most convective most convective for –1 et al – below-freezing temper- – below-freezing 2006). Such an unstable temper- storms to form (Schultz, 1999; Market form to storms et al., upward strong, favours profile ature ensuring an liftmotion to moist air, the into supply of moisture adequate of the process clouds and allowing of origin The electrification occur. to is typically these unstable profiles few in the lowest warm conditions surface the Earth’s kilometres above eight to of three air at levels and cold with associated generally kilometres, in the . disturbances air. is cold thundersnow for air Cold within clouds and near the atures air in these cold Without ground. the reach would layers, surface as rain. should have temperatures decreas- temperatures should have of more ing with height at a rate degCkm than 6 In what is called the mixed-phase region region In what is called the mixed-phase needed specificallyA fourth ingredient 4. produce storms Although most convective Although the exact light- mechanism for ning production a cloud is not fully within the formation scientists believe understood, to precursors of electrical that are charges the interaction between lightning involves types in clouds. of ice different in clouds at tempera- in clouds – the region °C – super- °C and –40 –10 between tures can co-exist. and ice liquid water cooled Lightning lightning, not all do. Next, examine the we not all do. lightning, electrification the for required conditions of clouds. – rap- 2006). Research 2006). Research et al., – abundant water vapour – abundant water – a mechanism to raise moist – a mechanism to Moisture clouds and pre- form in the air to can be moisture The cipitation. transported before long distances or can origi- precipitation falling as transport Long-distance locally. nate in cases of thundersnow might occur mois- USA where the central over the Gulf of from originate may ture might In moisture contrast, Mexico. as when local sources, from come air cold into is evaporated moisture warm a relatively across blowing lake, such as a large body of water, as in the case of lake- or ocean, bay, events. effect thundersnow Lift clouds and precipitation. form air to a convective of lift produce Sources to include the rising of warm air storm or air rising up the side of a front over a hill or mountain. profile Unstable temperature with temperature idly decreasing vigorous height, which can favour motion in the . upward unstable in which clouds are layer The 1. 2. 3. Figure 2. Number of thundersnow occurrences during 1961–1990 (Market (Market during 1961–1990 occurrences 2. Number of thundersnow Figure thundersnow for A recipe storms, convective are storms Thundersnow snowfall prodigious generally producing (Crowe accumulations regions are favoured for thundersnow will will thundersnow for favoured are regions in this article. later be addressed scientists and forecasters have found that that found have scientists and forecasters of a in terms storms thinking of convective make a con- to recipe The is useful. recipe (Johnsingredients has three storm vective 1992). and Doswell, Most research on thundersnow climatolo- on thundersnow Most research The purpose of this articleThe sum- is to Observations thunder of lightning and gies and environments has been carried out has been carried out and environments gies of this the USA, which will be the focus over favoured two article. are Over the USA, there larg- The 2). (Figure thundersnow for regions USA north- the central extends from est area the Midwest and Great across eastward other The New England. to Lakes region these Why West. is the Intermountain region We know little about the global distribu- We because they events tion of thundersnow at their Even small areas. over occur to tend may events thundersnow most widespread, kilome- square hundred only a few cover has been observedThundersnow in tres. northernmany in the Northern countries including the USA, Canada, Hemisphere, , western Isles, the British Finland, the Sea of as over as well northern Europe, Japan and the North Sea. Climbers scaling Mt reported and other high peaks have Everest causing during their ascent, thundersnow can Thundersnow some anxious moments. and evacuation of the closure also lead to of skiers the protection ski for and areas personnel. Occurrence marize our current understanding about understanding about our current marize articleThis this unusual wintertime event. and when thundersnow describes where make needed to the ingredients happens, and new efforts at under- thundersnow, of the occurrence standing and forecasting thundersnow. with snowstorms have been documented been documented have with snowstorms Western century the nineteenth since in the and in China 1888) (Herschel, Hemisphere 1099 (Wangsince 1982). Chinese and Chu, storms thundersnow warriors believed attacks (Wang enemy to precursors were history this long 1982). Despite and Chu, of observations, little is known thun- about partly the rarity of such due to dersnow, snow- non-thundering to compared storms and partlystorms lack of direct the due to observations clouds producing inside the lightning light from Furthermore, lightning. likely more thunder are and sound from than in a compa- snow by be absorbed to rable situation during heavy rain (Fraser 1992), makingand Bohren, observations difficult. A greater more of thundersnow of thundersnow, of the threats appreciation new observing and a focused instruments, are programme and forecasting research our understanding of improve helping to is that sci- result The events. thundersnow the mysteries is only starting ence reveal to inside such storms. the National Lightning Detection Lightning the National Network lightning strokes than 50 CG more recorded In thun- warm- contrast, in one hour. thou- to hundreds produce can derstorms hour. per sands of strokes 276

Weather – October 2009, Vol. 64, No. 10 Thundersnow in theUSAwhere thundersnow occurs. can better explainthegeographic locations occurs,nisms by which electrification we to produce thundersnow andthemecha- are elusive. occur, butfirmanswers to thesequestions eral hypotheses aboutwhy suchdifferences the cloudto theground. have Scientists sev- events, however, lower positive charge from CG lightningstrokes duringthundersnow charge from thecloudto theground. Many negative season thunderstorms transport tres for winter thunderstorms. ground, compared to oneto three kilome- storms: three to above ten kilometres the tend to behigherduringwarm-season ing occurs, thecharged areas ofthecloud and themixed-phase region iswhere charg- atmosphere is warmer duringthesummer charged regions inclouds. Becausethe cold-season storms isthealtitudeof significant difference between warm-and where exist.One farfewer observations charging differs winter storms, inelectrified storms, whether soscientistsdonotknow ing occurs withinwarm-seasonthunder- lightning. than intra-cloud lightning posesagreater threat to people forming CGand Earth, lightning. Clearly, CG fewer channelsmay form between cloud ing ‘intra-cloud’ lightning. Bycomparison, between pointswithinthecloud, produc- ductive, ionized tube. channelsform Most called a ‘channel’ con- andisanelectrically lightning stroke. The pathoflightningis happens, spark–the producing anelectrical becomes breakdown solarge thatdielectric tually may produce fieldthat anelectric become separated. cal motionswithinclouds, thesecharges ascend inupward verti- non-rimed particles fallthrough cloudsandsmaller particles charges.tion ofelectrical As larger rimed mechanismfor theproduc-are theprimary andothernon-rimedice particles studies have found thatcollisions between water to create andfield graupel. Laboratory cloud to allow abundantsupercooled liquid for supplyingmoistureis necessary to the through theunstabletemperature profile, parcelscal motion,associated withlifting ticle called ‘graupel’. Strong upward verti- ‘riming’) andproduce agranulated ice par- (aprocessfreeze called onice particles These supercooled liquid-water droplets two competing effects. First, theregion USA isattributed between to theinteraction region ofthundersnow across thecentral where andwhentheydo. Specifically, the explain why thundersnow events occur The four ingredients referred to above help Weather patterns Now thatwe have theingredients needed ofCGThe majority strokes duringwarm- Many questionsremain abouthow charg- The separationofthesecharges even- polar lows. 2004 event discussedearlier)andwithin along cold fronts (suchasthe28January commonly, thundersnow canalsooccur temperatures may beashigh8 °C. Less air between theshowers thatthesurface early May. may besostrong Insolation in ly insouthernBritainbetween March and November andFebruary, andlesscommon- coast ofScotland between around thenorth environment. Thundersnow tends to occur snow showers, likelyinapost-cold-frontal Europe occurs mainlyduringcold-season gested thatthundersnow intheseareas of The anonymous reviewer ofthispapersug- climatology inEurope hasbeenperformed. Europe,and northern althoughnoformal that for theBritishIsles, western Norway, USA appearsto bequite different from storm athigherelevations. at lower elevationsmightbeathundersnow rain-producing a face. Thus, not meltbefore reaching themountainsur- at higherelevations, however, snow does through warmer airbelow. themountains In high altitudesbutmeltswhiledescending the , precipitation begins assnow at elevations. thunderstorms, In even during upward, producing thunderstorms athigher air encountering themountainsisforced whereUSA isover Mountains theRocky long.kilometres elongated bandsofthundersnow tens of may produce resulting inhorizontally lift, (orographic alongthecoastal terrain lifting) resulting from orascent suchinstability moisture from evaporation.Circulations andabundantlow-levelducing instability over relatively warmeropenwater, pro- thesecases, 1999).In cold(Schultz, airblows Great LakesandtheGreat SaltLakeinUtah snow occurs isover anddownwind ofthe ant fronts moving through thisregion. with low-pressure systems andtheirattend- arisesfromThe theascent lift associated from thewarmwaters oftheGulfMexico. receive sufficientmoisture andinstability snow occurrence isfarenoughsouthto happens. Second, theregion ofthunder- for snow. southandsnow rarely Any further peratures duringthewinter thatare needed to receivenorth thebelow-freezing tem- of thundersnow occurrence isfarenough 1996, andanother manwasstruckin Vail, duringMarch Minnesota, in Minneapolis, was struckby lightningduring ablizzard Aman likely thattheywould bestruck. keeping more itless peopleinside, making The colder weather hastheadvantage of may bestruckby lightning inthewinter. during thewarmseason;however, people Nearly alllightninginjuriesanddeathsoccur andforecastingImpacts The thundersnow climatology for the Another locationfor thundersnow inthe Another region intheUSAwhere thunder- thundersnow. weather forecasts for theoccurrence of storms willhopefully leadto improved researchers develops duringthesewinter experience that Prof. Market’s team of results have notyet beenanalysed, the Although wholeadsthiseffort. Market cipitation type, according to Prof. Patrick dataonpre- high-quality storm, collecting researchers intercepted athundersnow of theprecipitation March 2007, type. In thuscreatingice crystals, detailedrecords ofsnowinstruments takepictures and mented weather balloons. Surface-based dersnow-producing cloudsusinginstru- the fieldto takemeasurements insidethun- sends teams ofscientistsandstudentsinto central USA.Part oftheresearch project programme for thundersnow events inthe istoof theproject develop aforecasting focusing onthundersnow. The emphasis hasbeen University ofMissouri–Columbia generally notahighpriority. ity, forecasting thundersnow occurrence is thundersnow. Furthermore, given theirrar- not developed into improved forecasts of weather over datacollected thepasthas ple any thundersnow events. The limited in winter storms over Utahdidnotsam- Trapp, 2002) to field measure theelectric has notbeencommon. Onestudy(Rustand better understandthundersnow occurrence insidecloudsto observations that collecting required hospitalization. wascriticallyinjuredwhereas and thefourth were treated andreleased atalocalhospital, teenagers whowere sledding. Three boys struck ahillinCaribou, Maine, injuringfour of23Februaryafternoon 2002,lightning Colorado, inAprilofthatsameyear. Onthe thundersnow. standing, andeventually better forecasts, of storms willhopefullyleadto better under- emphasis onunderstandingthese unusual ofclouds,the innerworkings andincreased better project, instrumentationto observe derstorms. anewresearch Nevertheless, compared to regular warm-seasonthun- about how thesestorms become electrified events meansthatwe have lessknowledge explained. ofthundersnowThe rarity Great LakesandGreat SaltLake)canbe West, central USAanddownwind ofthe thundersnow across theUSA(Intermountain likely to come together, favoured areas for tifying regions where theseingredients are the cloudandnearground level. Byiden- perature profile, andbelow-freezing airin moisture, anunstableatmospherictem- lift, ingredients neededfor thundersnow are snowfallated withheavy rates. The four convective storm thattends to beassoci- Thundersnow isan unusualcold-season Conclusion A five-year researchA five-year programme atthe ofthundersnowThe events rarity means Thundersnow Weather – October 2009, Vol. 64, No. 10 277

Wea. a few thousand feet 1959, doi:10.1029/ (1982) Unusual light- 2002. Initial 2002. bal- 29:

119–122. 1023–1031. 35: 1999. Lake-effect snow-

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Rust WD, Trapp RJ. Trapp Rust WD, of the electricloon soundings in field in the USA. nimbostratus clouds winter Lett. Res. Geophys. 2002GL015278 Schultz DM. in northernstorms New western Utah and with and without lightning. York Forecasting Wang P.-K, Chu J.-H. P.-K, Wang ning events in ancient Chinese literature. ancient Chinese literature. in ning events Weatherwise Observations ASEs can generate show When the ambient air is sub-saturated, for When the ambient air is sub-saturated, for there occur to precipitation anthropogenic tower the cooling the need for is probably typically one the ground, reach plume to downstream heights tower cooling five to and Hoult, 1971). Such plume (Overcamp wake the aerodynamic to deviation is related which can act to tower, the cooling around substantially pull the plume downwards, have been in North been have America. Near power in annual precipitation a 1% increase stations, can be expected up to (Huff, towers station cooling power from as local observations, well ASEs1972). As images (Koenig, satellite from can be inferred 2008). den Berg, Van 1981; hun- to of metres at hundreds snowfall of a power of kilometres downstream dreds and scales, the greater-length station. At a seeder- with natural snowfall, coinciding to suggested mechanism has been feeder at accumulations snowfall whereby operate, a factor up to by enhanced are the ground the falls through as the natural snow of two 1987). plume (Campistron, tower cooling and when towers, the cooling to Nearer present, is no natural precipitation there effects vary depending on whether the near-surface or not. air is foggy Non-foggy conditions Correspondence to: Prof. David M. Schultz, Prof. to: Correspondence Institute, Meteorological Finnish Aukio 1, 503, Erik Palménin Box P.O. Helsinki, FI-00101, Finland. E-mail: [email protected] 2009 Society, Meteorological @Royal DOI: 10.1002/wea.376

et al., Wea. Wea. 2002. 2006. 222–225. 1565–1571. 14:

J. Geophys. Res. J. Geophys. 120: 1971. Proximity Proximity 1971. 1992. Severe local 1992. Severe

1992. Is virga 1992. Is virga Wea. Forecasting Forecasting Wea. swizzle. 1290–1295. 1888. Lightning in snow- is perhaps a more unusual is perhaps a more American Meteorological American Meteorological 1 17:

Seventh Conference on Severe on Severe Seventh Conference Mon. Wea. Rev. Mon. Wea. Q. J. R. Meteorol. Soc. J. R. Meteorol. Q. D19208, doi:10.1029/2006JD007061 588-612. ? weather Curran JT, Pearson AD. Pearson JT, Curran snow. with thunderstorms soundings for Preprints, Storms. Local pp 118–119. Society: MO. Kansas City, CF. Bohren AB, Fraser the reaching before rain that evaporates ground? storms. storms. Herschel AS. Johns RH, Doswell CA. Johns RH, Doswell forecasting. storms 7: Ebert CE, Halcomb PS, R. Market events of thundersnow A climatology States. United the contiguous over Forecasting in soundings of thundersnow Proximity States. United the central Market PS, Oravetz AM, Gaede D, AM, Gaede D, Oravetz PS, Market AR, Melick C, Bookbinder Lupo E, R, Redburn R. Thomas Smith LL, 111: During freezing , it is reasonably com- it is reasonably fog, During freezing The possibilityThe of ASEs in the UK is investi- Because of the similarity in the circumstances Because of the similarity in the circumstances and predictions from forecasting models. models. forecasting and predictions from reports important snow since are Finally, used as a (e.g. records climatological for the indicator), change threshold is important. origin snow’s liquid fog airborne supercooled mon for contact into with sub-zero come to droplets surfaces and solidify on the surfaces form to essen- fog falling from droplets Large rime. (or snow- but snowflakes , tially form falling grains) concerned, we suggest describing drizzle-like we concerned, as fog from snowfall 1976; Koenig, 1981); almost all reported ASEs1976; Koenig, 1 Anthropogenic snow snow Anthropogenic phenomena by directly caused or influenced Precipitation human activity many has been reported for with early reports Blumdecades, of drizzle by (1962). ASEs Culkowski by (1948) and snowfall effluents typically to been attributed have and either industrial plants (Charlton from stations (Kramer or power 1984) Park, occurrence, at least in southern Britain. occurrence, further based on the phenome-gated here, reported previously from nology developed events.

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in the UK:urban examples of , which showed a sat- , which showed Geophys. Res. Lett. Res. Geophys. 2006. An investiga- Anthropogenic snowfall events snowfall Anthropogenic Weather L24812, doi:10.1029/2006GL028214 Crowe C, Market P, Pettegrew B, Melick B, Pettegrew P, C, Market Crowe J. C, Podzimek and deep snow tion of thundersnow accumulations. Studying such snowfall from fog is is fog from Studying such snowfall References important local because of the associated it pro- hazard changes and the potential might not have the snow Since duces. might arise from a hazard been forecast, but – perhaps of roads; gritting inadequate worryingly is such an more – as snowfall an objective it provides event, obvious be widely may basis on which the forecast a Even ‘wrong’. been have to perceived the local sur- alters of snow light covering microclimato- resulting The albedo. face further to contribute changes could logical the local weather between differences ellite picture of scattered snow patches over over patches snow picture of scattered ellite a following immediately Countries the Low (van anticyclonic weather period of foggy was illustrated snowfall The 2008). den Berg, of urban and indus- typically downstream raising the possibility that the trial areas, events snowfall anthropogenic were events artificially and with fog associated (ASEs), particles. injected University of Reading, UK University of Reading, or con- frontal to is not confined Snowfall was evident This systems. weather vective a striking image in the August from weather 2008 issue of Curtis R. Wood Wood R. Curtis and R. Giles Harrison Acknowledgements that improved the comments appreciate We Mansell and David Ted this article Drs by Storms Severe NOAA/National the Rust from the Market from Patrick Laboratory and Prof. follow- The of Missouri–Columbia.University their expertiseing individuals also provided L. Wendy this article: improve and time to also We Vavrek. J. and Jennifer Grebbien who provid- reviewer thank an anonymous about thundersnow ed valuable comments support Partial Schultz comes for in Europe. Vaisala Oyj. from