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How is hail formed?

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Hail is a form of precipitation that consists of lumps or balls of ice. Hail occurs only in thunderstorms, and hailstorms occur most often in the summer months in the mid-latitudes.

In the United States most hailstorms occur over the inland states, especially on the eastern slope of the Rockies and in the Great Plains states. Many form in Wyoming in the month of May. Hailstorms normally last for less than half an hour, and their paths are usually only a few miles wide, but they can be extremely destructive. In the United States, hail annually ruins millions of dollars worth of crops.

Each distinct piece of hail is called a hailstone. The usual range of size is from less than one-quarter of an inch to three-quarters of an inch (0.6-1.9 cm) in diameter. In a few isolated instances very much larger hailstones, some more than 5 inches (12 cm) in diameter, have fallen. Hailstones are often irregular in shape, but conical and almost spherical specimens are common. Most have an onion-like structure with alternate layers of clear and opaque ice. Some are simply lumpy accumulations of several smaller hailstones.

There are two ideas on how hailstones are formed. One belief is that they are formed in clouds in which the water droplets are supercooled, or cooled below the normal freezing temperature without becoming solid. The hailstones start as supercooled raindrops, which freeze upon dust particles or snowflakes in turbulent air to the front of a rainstorm. They are lifted by air currents and coated with snow at a higher level. As they fall again, the stones collect a layer of rainwater that freezes when they are again lifted to colder levels. As they rise and fall, the stones acquire more coatings, until they get heavy enough to fall to earth. This explains the layered, onion-like structure of some hailstones.

Another theory is that hailstones make a steady descent, during which they encounter layers of air that contain different amounts of water. Accordingly, the stone's final size depends on the number of different layers through which it passes.

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