Hail

Hail is a form of precipitation that consists of balls or lumps of ice. It normally occurs during a thunderstorm or the passage of a cold front. If hail reaches the ground during a thunderstorm, it is then known as a hailstorm.
Hailstones begin as very small ice pellets which water droplets attach themselves to. The water then freezes, as the wind throw the pellets further into the sky, increasing the size of the pellets. 
The more this happens, the bigger the hailstone gets.
Larger hailstones indicate powerful updraft and downdraft winds in a thunderstorm. Large hailstones are therefore associated with severe hailstorms. 
Hailstone can vary in size, bigger ones are rarer than smaller ones. Hailstones bigger than 4.5inches(11.43cm) are especially rare.