Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hailstone. Unlike graupel, which is made of rime, and ice pellets, which are smaller and translucent, hailstones – on Earth – consist mostly of water ice and measure between 5 and 200 millimetres (0.20 and 7.9 in) in diameter. The METAR reporting code for hail 5 mm (0.20 in) or greater is GR, while smaller hailstones and graupel are coded GS. Hail is possible within most thunderstorms as it is produced by cumulonimbi, and within 2 nautical miles (3.7 km) of the parent storm. Hail formation requires environments of strong, upward motion of air with the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level. In the mid-latitudes, hail forms near the interiors of continents, while in the tropics, it tends to be confined to high elevations.
There are methods available to detect hail-producing thunderstorms using weather satellites and weather radar imagery. Hailstones generally fall at higher speeds as they grow in size, though complicating factors such as melting, friction with air, wind, and interaction with rain and other hailstones can slow their descent through Earth's atmosphere. Severe weather warnings are issued for hail when the stones reach a damaging size, as it can cause serious damage to human-made structures and, most commonly, farmers' crops.