Hail Hail is a form of precipitation that is formed under certain conditions. Hail can cause serious danger to people and property. Hail is rain packed into round or irregularly shaped pieces of ice called hailstones. Hailstones can be as small as a pea or as big as a grapefruit, sometimes even bigger than that. Most hailstones are about one inch in diameter. The bigger hailstones usually have bumps on them where they have grown more. The most layers ever found on a hailstone was twenty- five. The only place that hail falls from is the upper part of Cumulonimbus clouds. Hailstones fall only when the ground temperature is below freezing (World Book Encyclope