Dear Straight Dope: Why don't we have lightning when we have a snowstorm? Rainstorms produce it all the time, and we have precipitation falling from the sky in both situations, so why not lightning too? Lisa H., St. Paul, MN

Una replies:

It’s possible to have lightning during a snowstorm. I personally witnessed it just last November, when my ladyfriend and I were awakened several times by crashing thunder while several inches of snow was falling. The conditions generally needed for lightning-producing storms are low-level humidity, low-level air instability, and strong dynamic updrafts of air. These conditions are usually absent in winter, so thunderstorms then are rare. However, when the right conditions are present, snowy winter thunderstorms called “thundersnow” can form.

The mechanism by which lightning is produced is complex and imperfectly understood, but we know moisture is important in two respects. First, heat is released when water vapor in the air condenses into liquid drops, and this heat helps provide energy to the thunderstorm. Second, interactions between supercooled liquid water droplets and ice crystals in the upper atmosphere (about 15,000 to 25,000 feet above sea level) are needed to generate the electrical charges that accumulate until a lightning bolt occurs. Cold winter air typically doesn’t contain a lot of moisture and so isn’t conducive to thunderstorms.

Air instability is also important to thunderstorm formation. It commonly occurs when warm air near the surface of the Earth rises due to convection–in the classic case, on a sunny summer day when the ground gets hot and warms up the air immediately above it. As the hot air rises, cooler air descends to replace it. If conditions are right, strong updrafts can form that quickly move the warm, humid surface air up to the higher reaches of the troposphere, where the water vapor in the air cools and condenses to fall as rain (or ice, if it’s cold enough). These updrafts are a hallmark of thunderstorms–the strong upward motion of the air encourages the interactions between water droplets and ice crystals that can lead to lightning. In winter, cold surface air temperatures and reduced sunlight mean there’s less surface heating, less convection, and thus fewer opportunities for thunderstorms.

All that having been said, several weather scenarios are known to favor winter electrical storms. One type of thundersnow is caused by lake-effect weather conditions of the sort encountered around the Great Lakes and Utah’s Great Salt Lake. In this case relatively warm lake water takes the place of sun-warmed ground. When a cold front passes over the water, strong convection currents can start, sending the moist air near the surface up into the colder atmosphere above. Another type, which is more common, occurs when a warm front containing a large amount of moisture moves in between cool surface air and even colder air above, creating a region of strong convection currents that starts well above the ground. This type of thundersnow is reported fairly often in the central Great Plains (Kansas, Colorado, Nebraska, and Missouri) as well as parts of Oklahoma and the Texas Panhandle. A third type–I don’t claim this list is exhaustive–sometimes occurs in mountainous regions when warm, moist air from a warm front is forced up a mountainside into the colder air at higher elevations.

Thundersnow is uncommon–between 1961 and 1990 it was officially recorded only around 375 times, with Utah holding the record for the most events (36). Thundersnow occurs most frequently in late winter and early spring due to the presence of marked temperature gradients and strong winds in the upper atmosphere. It has been recorded most often by far in March followed by February, April, December, and November. It’s rare in January and even more so in October.

Anecdotal accounts often associate thundersnow with heavy snowfall, especially tight, intense bands of snow (you know, the ones that dump a foot on your lawn but leave your boss’s house untouched, so he expects you to show up at work the next morning). Research hasn’t borne that out–one study showed that over a 30-year period, thundersnow resulted in only light snowfall 52% of the time. Nonetheless, under certain conditions thundersnow storms can produce significant accumulation. Meteorologists hope to get a better handle on those conditions so they can more accurately predict when what looks at first like a small storm might suddenly dump an extra foot or two of snow.

In my single experience of thundersnow, we had eight inches of snow in some parts of our yard, while only a couple of miles to the west the snow was no more than 2-3 inches deep. A true scientist knows you can’t draw conclusions from a sample size of one. Still, it was kind of cool, except for a tree snapping in half in the front yard.

Exactly how thundersnow forms is a subject of continuing study. The University of Missouri-Columbia has probably the best collection of web-accessible research data, although most of it is at a technical level more suitable to meteorologists and other scientists. Patrick Market, a professor of atmospheric science at UM, recently received a $460,000 grant from the National Science Foundation for a five-year study of thundersnow that will include collecting data on when and where it occurs. On their “Research on Convective Snows” website (see below), Market and his associates have a form where you, the Teeming Millions, can assist in their research by reporting when and where you experienced thundersnow.

Sources:

National Oceanographic and Atmospheric Administration Lightning site: www.noaa.gov/lightning.html

University Missouri-Columbia "Research On Convective Snows" site: weather.missouri.edu/ROCS/ROCS.html

Market, P.S., 2003: Thundersnow, a presentation to the CIPS/NWS St. Louis Winter Weather Workshop, 8 November 2003

Halcomb, C.E., 2001: Case Studies on Midwestern Thundersnow Events

Smith, L. L, C. J., Melick, and P. S. Market, 2005: Examination of Thundersnow Cases in the United States Utilizing NLDN Data. 85th Annual Meeting, Amer. Meteor. Soc., San Diego, CA

Una

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