Balls of Ice
Every year hail causes hundreds of millions of dollars in damage to property and crops across the U.S. Although perhaps not as deadly or dramatic as tornadoes or lightning, hail is also a product of intense thunderstorms. In fact,the National Weather Service uses hail size as an official gauge of whether or not a storm is severe.
Exactly how hail is formed is still not completely understood. It is likely that hail is formed in slightly different ways in different sorts of thunderstorms. But in general, hailstones form as a complex process involving thunderstorm updrafts and the growth of precipitation particles.
Thunderstorms contain tremendous amounts of what is called "super cooled" water . . . that is, water droplets that are liquid at temperatures well below freezing. Ice crystals, dust particles or even unlucky insects that are at the right place in the thunderstorm can become quickly coated with these supercooled droplets as they freeze on contact. A developing hailstone that finds itself moving up or down in the right part of the thunderstorm updraft can contact millions or even billions of these droplets. Eventually some can grow to golf ball, baseball or even softball size before they finally become so large even the strongest updrafts can't support them.