Hailstones on front porch. Hailstones are lumps of ice formed in storm clouds (cumulonimbus) when water droplets remain fluid after their temperature has fallen below the 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 around the cloud until they are either too heavy to be supported by the cloud or are thrown out of the updraught area. Large hailstones like these can damage property and injure people and livestock when they fall.