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Posted at 11:00 AM ET, 06/14/2010

Thunderstorms: Fickle & capricious for sure

Damaging storms hit NW D.C., Bethesda 21 years ago

* Muggy Monday, then moderation: Full Forecast *

Just a little over 21 years after the famous Exxon Valdez oil spill in Prince William Sound, off the Gulf of Alaska, the nation continues to deal with its worst-ever oil spill in another gulf--this time the Gulf of Mexico.

While the earlier event was no doubt an ecological and environmental disaster (although not nearly as catastrophic as the current one), less than 3 months later the Washington area had its own mini-disaster--this one caused by Mother Nature.

It was on June 14, 1989 that one of the most severe thunderstorms to strike the metro area in many years hit a relatively small portion of Northwest Washington and Bethesda, Md. during the late afternoon rush hour. In a short time, wind gusts (probably downdraft microbursts) estimated at more than 80 miles per hour caused an estimated 28 million dollars in damage and the loss of power to 150,000 homes. The rest of our area, however, was virtually unscathed. (I was traveling westbound on that section of the beltway just after the storm wreaked its havoc but, aside from a few small branches on the roadway, noticed nothing out of the ordinary.)

Keep reading for more on summer thunderstorms...

This storm illustrates the extremely fickle nature of summertime squalls. But despite the difficulty in predicting them and the dangers associated with them, thunderstorms (which occur 30-40 days a year here) do have great value.

For one thing, they provide much of our rainfall during the hottest part of the growing season. And by separating atmospheric nitrogen and oxygen, lightning creates vast amounts of nitrogen compounds, or nitrates, which greatly benefit all types of vegetation.

In addition, it is said that lightning helps to restore the earth's electrical balance and help maintain the planet's heat budget. Last but certainly not least, experiments in the 1950s by Nobel Prize winner Harold Urey and Stanley Miller suggested that lightning may have created life itself by combining water, methane, ammonia, and hydrogen. Still other lightning benefits are thought to exist. Can you think of any?

Actually, though we tend to think of thunderstorms as warm season phenomena, they can occur at any time of year, if conditions are right.

Here in the Washington area, some of our worst snowstorms have been accompanied by thunder and lightning, sometimes for extended periods. These "thundersnows," as they're called, are occasionally embedded in an overall snow pattern and are a sign of extreme air mass mixing and convective currents aloft, almost as in summer. Under these conditions, a "heavy" snowfall rate of one inch per hour can turn into a "snowburst" of three to four inches per hour. (In the midst of our current heat and humidity, I hope these cool thoughts will bring your discomfort index down a little--even if you're an ardent snow-hater.)

Aside from "thundersnows," there are three main types of thunderstorms: air mass; cold front; and mountain, or orographic. (Thunderstorms, and even tornadoes, have also been known to form in and around intense forest fires.) The East Coast, including our area, is primarily subject to primarily the first two, discussed below.

Air mass thunderstorms are the typical convective type of storms that form during the late afternoon or evening in the midst of a summer heat wave (we've had a few of those scattered throughout the region over the last several days). Although they bring brief relief, they don't signal a change in the weather. If anything, once they end, air mass storms sometimes leave behind more oppressive conditions than before, due to the evaporation of standing water.

Cold frontal thunderstorms are the most dramatic and usually, but not always, the most violent of all electrical storms and are the ones feared most by pilots. This is because such storms can grow to tremendous heights (10 miles or more) and sometimes form into a nearly continuous line hundreds of miles long. When this happens, aircraft can fly neither over nor around them and must divert to distant locations. (A pilot would almost never risk flying THROUGH even a modest thunderstorm, since extreme updrafts and downdrafts can pose great danger to the integrity of the aircraft.)

Once the cold front causing these types of storms passes, the weather usually does change to a drier, if not cooler, interlude. Strangely enough, the "squall lines" sometimes associated with cold fronts generally precede the actual front by one hundred miles or more. Why this happens is not well understood but it does illustrate, once again, that Mother Nature, as always, plays by her own rules.

By  |  11:00 AM ET, 06/14/2010

Categories:  Thunderstorms, Thunderstorms, Thunderstorms

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