The Onion Hiding Inside a Hailstone
Slice a big hailstone in half and you will find something that looks like an onion. Thick, clear layers swap with thin, white, cloudy ones, ring after ring. For a long time, people thought each ring meant the stone had fallen and been tossed back up again. Newer research shows that is not necessarily true.
Hail starts as water droplets inside a tall storm cloud called a cumulonimbus. As the droplets rise past the freezing level, they turn into supercooled water, which stays liquid even though it is colder than freezing. The moment it touches a tiny particle, it freezes.
The growing stone then travels through parts of the cloud with different amounts of water. Where droplets are packed tight, it picks up a clear layer. Where there is mostly water vapor, it picks up white, cloudy ice. Fast freezing also traps tiny air bubbles, which turns ice white. So one single trip through a storm can build all those rings.
Storms That Hold Ice in the Air
Hail needs a storm with a very strong updraft. Inside these storms, air can rush upward at up to 180 km/h. That is enough to keep balls of ice floating while they grow. The stronger the updraft, the bigger the stones it can hold up, which is why powerful storms such as supercells make the largest hail.
A hailstone keeps rising until the air can no longer carry its weight. That can take at least 30 minutes, in storms whose tops usually reach more than 10 km high. As water freezes onto the stone, it releases latent heat, which can keep the outside wet and sticky. A sticky stone can grab smaller hailstones and grow into an odd, lumpy shape.
Once it falls, size decides the speed. A 1 cm stone hits the ground at about 9 m/s, or 20 mph. An 8 cm stone arrives at around 48 m/s, about 110 mph. That top falling speed is its terminal velocity.
Hail Alley and the Equator Champion
You might guess the tropics get the most hail, since they have far more thunderstorms. They don't. The air there stays warm too high up, so hail in the tropics forms mainly on high ground. Hail is most common in the middle of continents at mid-latitudes. It also loves mountains, which push winds upward through orographic lifting and make storm updrafts stronger.
In North America, the corner where Colorado, Nebraska and Wyoming meet is known as "Hail Alley". Cheyenne, Wyoming is the continent's most hail-prone city, with nine to ten hailstorms a season. In South America, central Argentina sees some of the most frequent hailstorms in the world, with 10 to 30 storms a year.
The true champion sits close to the equator. Kericho, Kenya, high up at 2,200 metres, gets hail on about 50 days a year on average. In one year, it set the world record with 132 days of hail.
Record Stones and Four-Foot Drifts
The heaviest hailstone on record weighed 1.02 kg. It fell in Gopalganj District, Bangladesh, on 14 April 1986. The widest stone ever officially measured was 20 cm across and landed in Vivian, South Dakota, in July 2010. Giant chunks of ice that fall without a thunderstorm do not count. They are called megacryometeors, and the World Meteorological Organization does not treat them as hail.
Hail can also pile up like snow. Most hailstorms last from a few minutes up to 15 minutes, but a storm that stalls can bury one spot. On 5 June 2015, hail up to four feet deep fell on a single city block in Denver, Colorado, while the area around it stayed untouched. The stones ranged from bumble bee to ping pong ball size and kept falling for an hour and a half.
Tractors clearing the block filled more than 30 dump truck loads of hail. One meteorologist put it simply: "It's a meteorological thing."
Dented Cars, Ruined Crops and Hail Cannons
Hail is expensive. In 2023, hailstorms cost the United States $46 billion in damage to cars, roofs and crops, according to the Insurance Institute for Business & Home Safety. Wheat, corn, soybeans and tobacco are the crops most easily hurt. For aircraft, stones bigger than 13 mm can cause serious damage within seconds.
People have tried to fight back for centuries. In the Middle Ages, Europeans fired cannons to try to stop hail from ruining their crops, and modern hail cannons still exist. After World War II, the Soviet Union tried cloud seeding, firing silver iodide into clouds with rockets. It claimed crop damage dropped by 70 to 98%, but randomized trials in the West could not repeat those results.
Scientists are still learning. In spring 2025, a US project called ICECHIP began studying hail across the Great Plains and along the Rocky Mountains. It is the world's largest field campaign ever devoted to hail, with 100 scientists from four countries.
