Stone Power

Q. I’ve heard that if a penny is dropped from the Empire State Building it could kill someone. But what about hail? It’s often much larger and falls from much higher, so why do I never hear about any deaths caused by it?

A. Hail can cause human fatalities, but does not usually do so, according to the National Severe Storms Laboratory of the National Oceanic and Atmospheric Administration.

While one hail event in India in 1988 caused 246 deaths, this was truly exceptional. In the United States, most years see no deaths at all, though one or two are very rarely reported. It has been suggested that one reason for this is that Americans spend less time out in the open than people who live in regions like northern India where hail is a greater risk to human life.

Another important factor is that hail does not fall uninterrupted from the high reaches of the atmosphere, but is tossed up and down by the winds of a thunderstorm, bumping into raindrops and other hailstones, a process that slows the fall. The winds also frequently make the hailstones fall at an angle.

The friction with other precipitation deforms a hailstone from a perfect sphere, making its velocity hard to calculate when it does become heavy enough to fall to earth. One estimate is that a half-inch stone falls about 30 feet a second, while a three-inch stone falls nearly 160 feet a second.

C. Clairbone Ray, New York Times

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Full article and photo: http://www.nytimes.com/2010/10/19/science/19qna.html