On June 22, 2003, chunks of ice the size of softballs rained down on Aurora, Neb. One, a jagged behemoth with a 7-inch (17.8-centimeter) diameter, entered the record books as the largest U.S. hailstone ever.

Although large in size, it didn't unseat the champion by weight, which fell in Coffeyville, Kan., in 1970, according to the National Center for Atmospheric Research (NCAR). That hailstone weighed more than a pound and a half (0.75 kg).

So how do balls of ice bigger than grapefruit form out of thin air? The answer is in the wind.

Hail forms in the upper reaches of thunder clouds . Super-cooled water comes into contact with an ice crystal or dust particle and freezes, forming the core of a hailstone. Upward-blowing winds called updrafts keep the tiny hailstone aloft. As it hovers in the cloud, the hailstone core acquires more layers of ice. The stronger the updrafts , the more layers the hailstone can accumulate before it falls to earth, and the larger it gets.

Some of the largest hailstones can also from when lots of little hailstones that start to melt and then freeze back together in the atmosphere.

Updrafts blowing at 20 mph (32 kph) are capable of producing pea-sized hailstones, while winds twice as strong can create quarter-sized hail with a 1-inch (2.5 centimeter) diameter.

And as for the softball-sized hailstones that tore apart gutters and left craters in Nebraska cornfields in 2003? Those formed in a tempest with updrafts of at least 100 miles (160 kilometers) per hour.

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