Weather Whys

By Texas A&M University —

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 Dr. Dick 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 tens of minutes,
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, while in 1973 a
hailstone hit Cedoux, Saskatchewan, and measured 4 inches. But we know
that larger stones have fallen around the world.