Department of Emergency Management

Thunder is the sound produced by explosive expansion of air heated by a lightning stroke. When lightning is close by, the thunder sounds like a sharp crack. More distant strokes produce growling and rumbling noises. Because the speed of light is about a million times that of sound, we see a lightning bolt before the sound of thunder reaches us. This makes it possible to estimate the distance (in miles) to a lightning stroke by counting the number of seconds between lightning and thunder and dividing by five.

Downdrafts along the leading edge of a thunderstorm form the gust front. It is usually marked by gusty cool winds that can produce damage. Strong localized downdrafts are called downbursts. These are intense concentrations of sinking air which fan out on striking the earth's surface, producing damaging "straight" winds, and in some cases blowing dust, particularly in the southwestern United States. Thunderstorms which have downbursts typically produce several in succession of varying strengths and sizes. Frequently, damage that is attributed to tornadoes is actually caused by the straight winds of a downburst. This is because downburst winds can be as great as those of strong tornadoes and may produce a "roaring" sound. At times, a thunderstorm may produce a combination of tornadoes and downbursts. The occurrence of downbursts makes it imperative that the threat of severe thunderstorms be taken as seriously as that of tornadoes.

Flash floods can result from the locally heavy rains associated with thunderstorms. In recent years, flash floods have taken an average of more than 150 lives a year and have been reported in almost every state.

In an average year, tornadoes in the United States claim about 100 lives and cause hundreds of millions of dollars in damage.

A severe thunderstorm may spawn a tornado, a violently rotating column of air which descends from a thunderstorm cloud system. On the average, tornadoes move about 30 miles an hour; however, some move very slowly, while others speed along at 60 miles an hour or more. The typical path of a tornado is about 50 yards wide and a few miles long, but some have cut a swath a mile wide and 300 miles long. The destructive rotating winds of a tornado can exceed 200 miles and hour.

Hailstones are precipitation in the form of lumps of ice that form during some thunderstorms. Hailstones range from pea size to the size of a grapefruit. They're usually round, but may also be conical, or irregular in shape, some with pointed projections. Hail is most devastating to crops but can also cause heavy damage to aircraft, automobiles, roofs, and windows.

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