Imagine balls of ice, some small, some the size of grapefruits, crashing to the ground in a storm. Sound like a nightmare? For people who experience these storms, it can be. Those balls of ice are hailstones, and they can damage cars, airplanes, and crops, and in some circumstances can even kill animals and people.

Hail forms from supercooled water. Supercooled water is something unique–it’s water that is below its normal freezing point of 0°C (32°F) and yet remains a liquid. Scientists aren’t exactly sure how this happens, but they do know that it occurs frequently in the upper atmosphere. Supercooled water will freeze when it comes in contact with something, like an ice crystal, dust particle, or raindrop. Hail is associated with high, vertical cumulonimbus clouds, the kind of clouds that produce severe thunderstorms.

Within a cumulonimbus cloud, ice particles develop from supercooled water. The particles fall toward the bottom of the cloud from the pull of gravity, but they are forced back up by powerful updrafts of air within the clouds. In the upper part of the cloud, they encounter more supercooled water, which freezes on the ice particles, adding another layer of ice to them. This happens repeatedly. In this way, the small bits of ice grow larger and larger, becoming balls of ice—hailstones. The hailstones finally become too heavy to be lofted back up to the top of the cloud and they fall to the earth.

Because of the way they form, hailstones have a layered structure, like an onion. Really large hailstones form in storm clouds with exceptionally strong updrafts.

The National Weather Service in the United States defines large hail as being at least 2.5 centimeters (one inch) in diameter. That’s about the width of a quarter. This size distinction is important because hailstones this size or larger can cause damage to property, crops, and structures. It is difficult to determine the speed hailstones fall from clouds. But one estimate, from the National Severe Storms Laboratory, is that a hailstone the size of a baseball will fall at about 161 kilometers (100 miles) per hour.

Hail does significant damage to homes and crops worldwide. One of the most devastating hailstorms in U.S. history hit near Phoenix, Arizona, in October 2010. Hailstones as big as 7.6 centimeters (three inches) were reported and $2.8 billion in damage was done. The largest hailstone ever recovered in the United States was produced by a storm in the U.S. state of South Dakota in June 2010. It was 20 centimeters (8 inches) in diameter and weighed about 0.9 kilograms (two pounds).