Hail is one of weather's strangest sights: chunks of ice crashing down on a warm summer afternoon, sometimes big enough to dent cars and shatter windshields. How does solid ice form and survive its fall when it's 90°F (32°C) at the ground? The answer lies miles overhead, inside the violent engine of a thunderstorm, where some of the most powerful winds in the atmosphere recycle raindrops through freezing air over and over until they're too heavy to hold up.
Here's exactly how a hailstone is built, why some storms produce golf-ball-sized stones while others drop harmless pea-sized bits, and how to know when hail is on the way.
Hail forms inside strong thunderstorms when a powerful upward gust of wind — the updraft — carries raindrops high into the part of the cloud that's well below freezing. There the drops freeze, get coated in more water and ice, and are tossed up and down repeatedly, growing a new layer each trip. When a stone finally becomes too heavy for the updraft to support, it falls to the ground as hail.
Every thunderstorm has a rising column of warm, moist air at its core. The stronger that updraft, the bigger the hail it can make — because the wind has to be fast enough to suspend a growing ball of ice in midair. In severe storms, updrafts can exceed 100 mph, strong enough to hold a baseball-sized stone aloft. These same monster updrafts power other violent weather, which is why hail so often travels with the storms that produce microbursts and tornadoes.
High in the storm, temperatures fall far below freezing — often colder than 5°F (−15°C). When the updraft lifts raindrops into this zone, they freeze into small ice pellets called embryos. That's the seed of every hailstone.
Once an ice embryo exists, it grows by colliding with supercooled water droplets — liquid water that's still below freezing because it has nothing to freeze onto. Each droplet that hits the stone freezes around it, adding a layer. The updraft keeps lofting the stone back up before it can fall out, so it makes pass after pass through the wet, cold region, gaining mass each time.
If you slice a large hailstone in half, you'll often see rings like an onion. Clear layers form when the stone passes through wetter air and the water freezes slowly into solid ice; cloudy, white layers form in drier, colder air where the water freezes instantly and traps tiny air bubbles. Those rings are a record of how many trips the stone made through the storm.
A hailstone keeps growing only as long as the updraft can hold it up. Eventually it becomes too heavy, or it drifts into the storm's downdraft, and gravity wins. The bigger the stone, the faster it falls — large hail can hit the ground at over 100 mph. The reason it doesn't simply melt on the way down is that it spends very little time in the warm lower air, and big stones have less surface area relative to their mass, so they survive the warm final mile largely intact. Smaller stones may partly melt, which is why a summer storm sometimes drops a slushy mix of ice and rain.
Hail size is usually described by comparing it to familiar objects, and the National Weather Service considers hail "severe" once it reaches quarter size — about one inch across.
| Size comparison | Diameter | What to expect |
|---|---|---|
| Pea | ~0.25 in (6 mm) | Harmless, common in ordinary storms |
| Quarter | ~1 in (2.5 cm) | Officially "severe" — can dent and chip |
| Golf ball | ~1.75 in (4.5 cm) | Damages roofs, cars, and siding |
| Baseball | ~2.75 in (7 cm) | Shatters windshields, dangerous to people |
| Softball+ | 4 in+ (10 cm+) | Rare, extreme damage, life-threatening |
The largest hailstone ever recorded in the United States fell in Vivian, South Dakota, in 2010 — 8 inches (20 cm) across and nearly 2 pounds. It takes an extraordinarily violent storm to build something that large and hold it aloft long enough.
Hail needs tall, energetic thunderstorms, so it's most common in late spring and summer when the atmosphere is most unstable. In the U.S., "Hail Alley" — where Colorado, Nebraska, and Wyoming meet — sees the most large hail, thanks to high elevation that puts the ground closer to the freezing level, so stones have less warm air to melt through on the way down. Hail tends to come from the same severe storms that prompt a severe thunderstorm warning, and a warning that mentions large hail is one worth taking seriously.
Hail is built by a thunderstorm's updraft, which hurls raindrops into freezing air and recycles the growing stones until they're too heavy to stay aloft. The stronger the storm, the bigger the hail — and the more damage it can do when it finally falls. So when a summer sky turns dark green and the wind picks up, those are the storms most capable of dropping ice on a warm afternoon, and a good reason to head indoors.
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