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Weather Whys: Killer Hail Storms

Weather Whys: Killer Hail Storms

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Q: What causes hail to form?

A: Hail is caused when raindrops are lifted up into the atmosphere during a thunderstorm and then supercooled by temperatures below freezing, turning them into ice balls, says Richard Orville of Texas A&M University. "The faster the updraft on these balls of ice, the bigger they can grow," he explains. "On some hailstones, you can actually see rings inside them which show they were cycled through the thunderstorm more than once. What starts out as a tiny raindrop becomes a pea-size chunk of ice and some eventually get to be the size of baseballs or softballs. When these stones fall to earth, the damage can be devastating, even deadly. Large hailstones have been clocked at more than 90 miles per hour and there have been reports of humans and livestock being killed by large hailstones. Houses, buildings, cars and crops are usually the victims of hail, however. Hail usually forms over an area and leaves within a few minutes," Orville notes. "But there have been instances when hail can stay in the same area for a half hour or so, leaving several inches of ice on the ground."

Q: How large do hailstones get?

A: Hail is usually pea-sized to marble-sized, says Orville, but big thunderstorms can produce big hail. "Hail up to six inches in diameter has fallen in parts of the Midwest," he says. "A hailstone the size of a baseball weighs about one-third of a pound, and since it can travel up to 90 miles an hour from its source cloud, it can create a lot of damage. Entire crops have been known to be wiped out in a few minutes with large hailstones. In 1978, about 200 sheep were killed in Montana when baseball-size hail struck them. The largest hailstone ever recorded in the U.S. occurred in 1970 in Coffeyville, Kan., when a stone weighing 1.6 pounds and measuring 5.5 inches fell, but we know that larger stones have fallen around the world. In 1984, a hail storm hit Denver and lasted almost one hour and the result was knee-deep hailstones on the ground. This is why hail can be such a damaging weather force."

Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University.

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