Asked by: Dave Ferris, London
Hail is produced during thunderstorms, when an updraught of warm, moist air carries tiny droplets of water upwards. The droplets freeze at high altitude, and the resulting ice crystals grow until they are too heavy to stay suspended.
For hail to form, a warm air mass has to meet a much colder one (this is what creates the updraught). The hail only forms in a narrow region where these two air masses meet, meaning that it falls over just a small strip of land.
These high-energy storms also tend to move quickly, so someone on the ground experiences the hail as a short burst. Finally, as they need a supply of warm, moist air to fuel them, hailstorms relatively quickly run out of energy and dissipate.
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