Largest hailstone in American history. This hailstone landed in Aurora, Nebraska during a violent thunderstorm on 22nd June, 2003. The piece of ice was almost 18 centimetres in diameter with a circumference of 47. 5 centimetres. Hailstones are formed in storm clouds when water droplets remain fluid after their temperature has fallen below freezing point. This super-cooled water circulates in the updrafts within the cloud, forming ice on contact with dust particles in the air. The hailstones grow larger as they circulate until they are either too heavy to be supported by the cloud or are thrown out of the updraught area.