What causes hailstones to form during storms?

Hailstones form during storms when updrafts carry raindrops into extremely cold regions of the atmosphere. The droplets freeze and are coated with additional layers of ice as they cycle through the storm cloud. When the hailstones become too heavy for the updrafts to support, they fall to the ground. The size of the hailstones depends on the strength of the updrafts and the duration of the freezing cycles.