Hail is a type of graupel (a form of precipitation) composed of balls or irregular lumps of ice. It occurs when supercooled water droplets (remaining in a liquid state despite being below the freezing point, 0°C/32°F) in a storm cloud collide with some solid object, such as a dust particle or an already-forming hailstone. 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, when the updraft is no longer strong enough to support its weight.
Hail often forms in strong thunderstorms, often along 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 gravity. The longer the stones bounce around, the larger they grow. For the same reason, larger hail can occur in warmer regions in the world due to stronger updrafts. These strong, severe, or even supercell thunderstorms can also occur in summer, even without a cold front.
Hailstones, while 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, skylights, and glass-roofed structures. Pea or golfball-size hailstones are not uncommon in severe storms. Rarely, massive hailstones have been known cause concussions or to kill people by causing head trauma.
The first image above shows an aggregate hailstone. It is a large hailstone with smaller stones visible. The ruler shows the size of this remarkable hailstone. The diameter is approximately 6 inches or 15cm, the size of a grapefruit. In the picture to the right, large hail collects on streets and grass during a severe thunderstorm. Larger stones appear to be nearly 2 to 3 inches (50 to 75mm) in diameter.