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January-February 2011

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Weather Queries

Why was the “temperature” taken out of blizzard warnings, leaving only wind and snow?

Murray Brown
Long Beach, New York

The current official definition of “blizzard” from the U.S. National Weather Service (NWS) is “sustained wind or frequent gusts greater than or equal to 35 mph, accompanied by falling and/or blowing snow, frequently reducing visibility to less than ¼ mile for three hours or more.” Tom Filiaggi of the NWS looked back as far as 1992 and found the same criteria accompanied by the following note: “Although there is no set temperature requirement for blizzard conditions, the life-threatening nature of the low temperatures in combination with the other hazardous conditions of wind, snow, and poor visibility increases dramatically when the temperature falls below 20°F.” Thus, at least 18 years ago, there was no temperature criterion associated with blizzards.

In poking around the Web, I came upon this curious entry, apparently from the Columbia Encyclopedia: “Blizzard: winter storm characterized by high winds, low temperatures, and driving snow: according to the official definition given in 1958 by the U.S. Weather Bureau, the winds must exceed 35 mph and the temperature must be 20oF or lower” (http://www.answers.com/topic/blizzard). Thus, long ago, there may have been a temperature criterion, and this jibes with my boyhood memories. If so, I have been unable to find out when the temperature criterion was removed or why.

Canada has no temperature criterion, either. A blizzard warning is issued when winds of 40 kilometers per hour (25 mph) or greater are expected to cause widespread reductions in visibility to 400 meters (¼ mile) or less due to blowing snow, or blowing snow in combination with falling snow, for at least four hours. North of the tree line in Canada, where these conditions are more common, the duration must be at least six hours. Until recently, there were regional differences in the criteria for a blizzard and, in some places, temperature was one of the criteria.

Why don't we have lightning and thunder during snowstorms?

Bob Hetzel
Oak Park, Illinois

Thunder and lightning during snowstorms, often called thundersnow, do occur but only rarely. Especially rare is a good photograph of lightning emanating from a snowstorm, as shown in Figure 1, kindly supplied by Dave Arnold of Milan, New Mexico. Note the twin strikes to the ground.

Figure 1. Lightning follows a tortuous path to the ground on Black Mesa near Milan, New Mexico, at 4:09 p.m., February 28, 2010, as a thunder-snowstorm approaches.

Figure 1. Lightning follows a tortuous path to the ground on Black Mesa near Milan, New Mexico, at 4:09 p.m., February 28, 2010, as a thunder-snowstorm approaches.

Patrick Market and his two coauthors examined the climatology of thundersnow events in the contiguous United States in a December 2002 article in Weather and Forecasting (pp. 1290–1295). They looked at three-hourly reports from 204 stations across the contiguous United States from 1961 through 1990. They found just 229 reports of thunder occurring simultaneously with snow (freezing rain or sleet didn't count) in 30 years' time. From these reports, they deduced a total of 191 thundersnow events. For example, two consecutive reports of thundersnow at the same station (very rare when reports are three hours apart) or two or more adjacent stations reporting thundersnow at the same time constitute a single event.

Figure 2 shows the geographical distribution of these events. Thundersnow requires a source of moisture, an unstable atmosphere, a mechanism to lift the air in order to form a thunderstorm, and a lower troposphere almost entirely below freezing (the upper troposphere is always cold enough for snow). Thundersnow is quite rare south of 37°N latitude (the southern boundary of Utah, Colorado, and Kansas), mainly because it is too warm for snow most of the time except at higher elevations. The maximum in Utah has much to do with upslope flow across the mountains (a source of lift) and the Great Salt Lake, so salty that it hardly ever freezes, and thus it can add heat and moisture from below to cold air masses passing over it, thereby destabilizing the atmosphere. Thundersnow in the Great Plains is associated mostly with strong low-pressure systems that lift moist, unstable air, often carried northward from the Gulf of Mexico, over a cold front. If the cold air mass is entirely below freezing and has a steep slope near the forward edge, the lift can be sufficient to form elevated thunderstorms with snow. The Great Lakes are another source of thundersnow, particularly in the autumn and early winter, when very cold air sweeps across them before the surface water freezes. New England occasionally has thundersnow in connection with coastal storms driving moist unstable air over firmly entrenched cold air masses on land.

Figure 2. The number of thundersnow events at each station from 1961 through 1990 based upon observations at three-hour intervals. Contours are drawn for three, six, and nine occurrences in 30 years.

Figure 2. The number of thundersnow events at each station from 1961 through 1990 based upon observations at three-hour intervals. Contours are drawn for three, six, and nine occurrences in 30 years.

Hydrometeor is the generic name for any particle, liquid or solid, derived from the condensation of water vapor. Electrical charge builds in a growing cloud mainly when different kinds of hydrometeors collide and then separate. During collision, the hydrometeors acquire net positive or negative charges. If their fall speeds are different, they will move farther apart, and charge of one sign will tend to accumulate among particles that fall more slowly and of the other sign among particles that fall more quickly. Lightning occurs when the electrical field between the charge centers becomes great enough to trigger a spark.

Experts believe that the most common cause of lightning is the interaction of graupel and other ice particles in the presence of supercooled liquid water, which is very common in convective clouds. Graupel is the technical word for a snow pellet, a little white ball of ice with lots of air in it that grows from a snow crystal falling through a cloud of droplets at temperatures below freezing, hence the designation supercooled. The snow crystal falls with respect to supercooled droplets and collects them when collisions occur. After a while, the crystal collects so many droplets, which immediately freeze on its surface, that it becomes unrecognizable—an amorphous, lightweight ball of ice. When this graupel collides with unrimed crystals or crystal fragments, charge transfer occurs. The graupel acquires a charge of one sign and the crystals and fragments acquire a charge of the opposite sign. The sign of the charge on the graupel depends upon the temperature in the cloud and the amount of supercooled liquid water present. The graupel falls with respect to the ice particles in the updraft, and thus charge of one sign accumulates lower in the cloud and charge of the opposite sign accumulates higher in the cloud. Note that all of this occurs above the freezing level, where the temperature is below 0°C.

Though the process just described may be the most likely way in which charges are separated within summer thunderclouds, there's no assurance that the same process operates to produce thundersnow. That is still the subject of research.

Charge separation occurs primarily at cloud temperatures between −5 and −30°C, where supercooled water is lifted by strong updrafts. In summer, when the lower troposphere is quite warm, this region is roughly from 4,500 to 9,000 meters (15,000–29,500 feet) in altitude. In thundersnow, the lower boundary of the charging zone is at much lower altitudes, sometimes below 2,000 meters (6,500 feet). Almost all thunderstorms produce snow, but in summertime one has to be on a high mountain to experience it. All the solid precipitation except hail melts before reaching the ground at lower elevations. It is the unusual combination of an atmosphere near or below freezing at every level, unstable air, a moisture source, and a lifting mechanism that results in thundersnow at low elevations.

I have witnessed thundersnow only a few times in my life, and it is exciting. If you are directly beneath the thundercloud, snow often falls heavily, at a rate of 3 to 4 inches per hour. At night, the reflection of lightning off millions of snowflakes is both startling and beautiful.

Readers might be interested in a recent article on thundersnow by David M. Schultz and R. James Vavrek in Weather, a publication of the Royal Meteorological Society, United Kingdom, Vol. 64, No. 10, October, 2009, pp. 274–277.

Weatherwise Contributing Editor THOMAS W. SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at weatherqueries@gmail.com, or by mail in care of Weatherwise, Taylor & Francis, 325 Chestnut St. Suite 800, Philadelphia, PA 19106.

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