An overview of thundersnow David M. Schultz1 For example, two notable thundersnow 10+ strikes per minute during the height 2 events have struck the United Kingdom. On of the storm. Another event occurred a and R. James Vavrek 28 January 2004, a strong cold front and few months later on 1 December 2006 in squall line passed through much of southern Columbia, Missouri, when more than 38 1 University of Helsinki and Finnish Britain, producing thundersnow, hail, gusty centimetres of wet snow fell overnight, Meteorological Institute, Helsinki, winds, and a tornado at Coombe Down, Bath. with a period of lightning and thunder Finland The wet snow froze, causing numerous traffic between midnight and 1 a.m. The metro 2Henry W. Eggers School, Hammond, problems during the evening rush hour. In area was paralyzed the next morning by Indiana, USA another example, during one six-hour period the heavy snowfall. of convection on 3/4 January 2008 in Belfast, In the USA, only 1.3% of cool-season Weather – October 2009, Vol. 64, No. 10 64, No. – OctoberVol. 2009, Weather Lightning and thunder are commonly Northern Ireland, 12–15 centimetres of snow (October–May) thunderstorms report inci- observed during the warm season from fell. One lightning strike disabled a substa- dences with snowfall, and only 0.07% of cumulonimbus clouds. These thunder- tion in Lisburn, causing 13 500 homes to be reported snowfalls are associated with light- storms may be accompanied by heavy rain, without electricity, according to the TORRO ning or thunder (Curran and Pearson, 1971). hail, strong winds, and possibly tornadoes. database. Thus, thundersnow is an exceptionally During winter, however, snowstorms with The United States is not immune to uncommon event. In addition, thundersnow heavy snowfall can occasionally produce thundersnow either. An early-season snow- storms generally produce fewer lightning lightning and thunder. The combination of storm, named ‘Aphid’, struck Buffalo, New strokes than thunderstorms with rain dur- snow and lightning/thunder is called ‘thun- York, on 12 October 2006 yielding cloud- ing the warm season. For example, in one dersnow’ (Figure 1). to-ground (CG) lightning strokes at rates of thundersnow event in Utah during 2002, 274 Figure 1. Cloud-to-ground lightning from a lake-effect snowstorm downstream of Lake Ontario, November 1985. (© James G. Ladue.) the National Lightning Detection Network recorded more than 50 CG lightning strokes in one hour. In contrast, warm-season thun- derstorms can produce hundreds to thou- sands of strokes per hour. Observations of lightning and thunder with snowstorms have been documented since the nineteenth century in the Western Hemisphere (Herschel, 1888) and in China Thundersnow since 1099 (Wang and Chu, 1982). Chinese warriors believed thundersnow storms were precursors to enemy attacks (Wang and Chu, 1982). Despite this long history of observations, little is known about thun- dersnow, partly due to the rarity of such 10 64, No. – OctoberVol. 2009, Weather storms compared to non-thundering snow- storms and partly due to the lack of direct observations inside the clouds producing lightning. Furthermore, light from lightning and sound from thunder are more likely to be absorbed by snow than in a compa- rable situation during heavy rain (Fraser and Bohren, 1992), making observations of thundersnow more difficult. A greater appreciation of the threats of thundersnow, Figure 2. Number of thundersnow occurrences during 1961–1990 (Market et al., 2002). new observing instruments, and a focused research and forecasting programme are helping to improve our understanding of regions are favoured for thundersnow will should have temperatures decreas- thundersnow events. The result is that sci- be addressed later in this article. ing with height at a rate of more ence is only starting to reveal the mysteries than 6 degCkm–1 for most convective inside such storms. A recipe for thundersnow storms to form (Schultz, 1999; Market The purpose of this article is to sum- et al., 2006). Such an unstable temper- marize our current understanding about Thundersnow storms are convective storms, ature profile favours strong, upward this unusual wintertime event. This article generally producing prodigious snowfall motion to lift moist air, ensuring an describes where and when thundersnow accumulations (Crowe et al., 2006). Research adequate supply of moisture into the happens, the ingredients needed to make scientists and forecasters have found that clouds and allowing the process of thundersnow, and new efforts at under- thinking of convective storms in terms of a electrification to occur. The origin of standing and forecasting the occurrence of recipe is useful. The recipe to make a con- these unstable profiles is typically thundersnow. vective storm has three ingredients (Johns warm conditions in the lowest few and Doswell, 1992). kilometres above the Earth’s surface Occurrence 1. Moisture – abundant water vapour and cold air at levels of three to eight We know little about the global distribu- in the air to form clouds and pre- kilometres, generally associated with tion of thundersnow events because they cipitation. The moisture can be disturbances in the jet stream. tend to occur over small areas. Even at their transported long distances before A fourth ingredient needed specifically most widespread, thundersnow events may falling as precipitation or can origi- for thundersnow is cold air. cover only a few hundred square kilome- nate locally. Long-distance transport 4. Cold air – below-freezing temper- tres. Thundersnow has been observed in might occur in cases of thundersnow atures within clouds and near the many northern countries in the Northern over the central USA where mois- ground. Without cold air in these Hemisphere, including the USA, Canada, ture may originate from the Gulf of layers, precipitation would reach the Finland, the British Isles, western Norway, Mexico. In contrast, moisture might surface as rain. northern Europe, as well as over the Sea of come from local sources, as when Although most convective storms produce Japan and the North Sea. Climbers scaling Mt moisture is evaporated into cold air lightning, not all do. Next, we examine the Everest and other high peaks have reported blowing across a relatively warm conditions required for the electrification thundersnow during their ascent, causing body of water, such as a large lake, of clouds. some anxious moments. Thundersnow can bay, or ocean, as in the case of lake- also lead to the closure and evacuation of effect thundersnow events. ski areas for the protection of skiers and 2. Lift – a mechanism to raise moist Lightning personnel. air to form clouds and precipitation. Although the exact mechanism for light- Most research on thundersnow climatolo- Sources of lift to produce a convective ning production within a cloud is not fully gies and environments has been carried out storm include the rising of warm air understood, scientists believe the formation over the USA, which will be the focus of this over a front or air rising up the side of of electrical charges that are precursors to article. Over the USA, there are two favoured a hill or mountain. lightning involves the interaction between regions for thundersnow (Figure 2). The larg- 3. Unstable temperature profile – rap- different types of ice in clouds. est area extends from the central USA north- idly decreasing temperature with In what is called the mixed-phase region eastward across the Midwest and Great height, which can favour vigorous in clouds – the region in clouds at tempera- Lakes region to New England. The other upward motion in the atmosphere. tures between –10 °C and –40 °C – super- region is the Intermountain West. Why these The layer in which clouds are unstable cooled liquid water and ice can co-exist. 275 These supercooled liquid-water droplets of thundersnow occurrence is far enough Colorado, in April of that same year. On the freeze on ice particles (a process called north to receive the below-freezing tem- afternoon of 23 February 2002, lightning ‘riming’) and produce a granulated ice par- peratures during the winter that are needed struck a hill in Caribou, Maine, injuring four ticle called ‘graupel’. Strong upward verti- for snow. Any further south and snow rarely teenagers who were sledding. Three boys cal motion, associated with lifting parcels happens. Second, the region of thunder- were treated and released at a local hospital, through the unstable temperature profile, snow occurrence is far enough south to whereas the fourth was critically injured and is necessary for supplying moisture to the receive sufficient moisture and instability required hospitalization. cloud to allow abundant supercooled liquid from the warm waters of the Gulf of Mexico. The rarity of thundersnow events means water to create graupel. Laboratory and field The lift arises from the ascent associated that collecting observations inside clouds to studies have found that collisions between with low-pressure systems and their attend- better understand thundersnow occurrence graupel and other non-rimed ice particles ant fronts moving through this region. has not been common. One study (Rust and are the primary mechanism for the produc- Another region in the USA where thunder- Trapp, 2002) to measure the electric field Thundersnow tion of electrical charges. As larger rimed snow occurs is over and downwind of the in winter storms over Utah did not sam- particles fall through clouds and smaller Great Lakes and the Great Salt Lake in Utah ple any thundersnow events. The limited non-rimed particles ascend in upward verti- (Schultz, 1999). In these cases, cold air blows weather data collected over the past has cal motions within clouds, these charges over relatively warmer open water, pro- not developed into improved forecasts of become separated. ducing instability and abundant low-level thundersnow. Furthermore, given their rar- The separation of these charges even- moisture from evaporation. Circulations ity, forecasting thundersnow occurrence is tually may produce an electric field that resulting from such instability or ascent generally not a high priority.