 The largest recorded hailstone in the US measured almost 19 inches in circumference and weighed nearly two pounds. Where did it come from and how in the world did it get that big? Thunderstorms are the main source of hailstone. Frozen droplets of water that form a chunk of ice that can range from pea to grapefruit size. Inside the turbulent storm cloud, a tiny piece of ice is blown into a super-cooled water droplet which freezes on its surface, forming a microscopic hailstone. The hailstone bounces up and down through the warmer and colder parts of the cloud, collecting and freezing water droplets as it moves. The more times the hailstone passes through the temperature zones, the more layers accumulate and the larger it gets until it becomes too heavy to stay airborne and falls to the ground.