Hail is a form of precipitation composed of balls or irregular lumps of ice. It occurs when "supercooled" water particles (remaining in a liquid state despite being below 0 degrees Celsius) in a storm cloud collide with some solid object, such as a dust particle or a forming hail stone. The water then freezes around the object. Depending on the wind patterns within the cloud, the hailstone may continue to circulate for some time, increasing in size. Eventually, the hailstone falls to the ground.
Hail can also form in a cold front[?] where the layer of air on top is much colder than that on the bottom. The smaller hailstones can bounce up and down between the warm and cold layers due to updrafts[?] and gravitational force. The longer the stones bounced around, the bigger they grow. For the same reason, bigger hail can occur in warmer regions in the world due to stronger updrafts.
Hailstones, whilst most commonly only a few millimetres in diameter, can sometimes grow to several centimetres or occasionally even bigger. Such large hailstones can do serious damage, notably to automobiles and glass-roofed structures. Golf ball size hailstones are not uncommon in severe conditions.
The first image above shows an aggregate hailstone. It is a large hailstone with smaller stones visible. The ruler shows radius of this remarkable hail stone. The diameter is approximately 6 inches (15 cm) - the size of a grapefruit. In the picture to the right, large hail collects on streets and grass during severe thunderstorm. Larger stones appear to be nearly 2 to 3 inches (50 to 75 mm) in diameter.
The world record hailstone fell in Coffeyville, Kansas on September 3, 1970. It weighed 770 g.