Strong rising currents of air within a thunderstorm, called updrafts, carry water droplets to a height where freezing occurs. These ice particles continue to grow in size, finally becoming too heavy to be supported by the updraft and fall to the ground as hail. Sometimes hailstones fall out of weaker portions of the updraft, then intercept the stronger updrafts again on their way down. If this happens, the hailstones can make multiple trips through the depth of the storm. In those cases the hailstone grows concentric rings - each ring representing a new trip through the storm. Almost all of the larger hailstones (1 inch or greater in diameter) form in multiple trips. When larger hailstones finally fall to earth, they can travel at speeds faster than 100 mph.

The "spikes" that are frequently observed on the outside edges of large hailstones result when large stones encounter smaller stones and the two stick together, or "aggregate."

This photograph is of a severe thunderstorm developing over Fort Collins, Colorado on the evening of July 31, 1979. Hailstones from this storm grew to over 4 inches in diameter and were responsible for the second recorded death in United States history from hail.

Definitions and Introduction | Facts About Hail | How Does Hail Develop?