How Hail Forms — Ice From a Summer Sky

The Paradox

Hail almost always falls during warm weather — in summer thunderstorms, not winter snowstorms. This seems counterintuitive, but hailstones need something winter clouds do not have: powerful updrafts.

The Formation Process

1. Raindrops are swept upward by strong updrafts inside a cumulonimbus cloud
2. At high altitude (often above 6,000m), temperatures are well below freezing — the drops freeze into ice pellets
3. The pellets fall, pick up more water, get swept up again, and freeze another layer
4. This cycle repeats, adding layers of ice like an onion
5. When the hailstone becomes too heavy for the updraft to support, it falls to the ground

Hailstone Size

Hail Records

The largest hailstone ever recorded in the US fell in Vivian, South Dakota in 2010: 20 cm in diameter and weighing nearly 900 grams.

Hail Damage

Large hail causes billions of dollars in damage annually — denting cars, shattering windshields, destroying crops, and damaging roofs. In extreme cases, hail can injure or kill people and animals.

Protection

• Move vehicles under cover when hail is forecast
• Stay indoors and away from windows during hailstorms
• If caught outside, protect your head and seek shelter immediately

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