How does hail form?

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How does hail form?

A large hailstone that fell in Harper, Kansas on May 14, 2004, from the National Weather Service – Wichita, Kansas

Hail is precipitation in the form of large balls or lumps of ice that grow in thunderstorms and other severe convective storms. Hailstones begin as small ice particles that grow primarily by accretion; to grow large, they require abundant water droplets. As the hailstone moves up and down through a storm, it collides with water droplets and grows steadily larger.

When you cut a hailstone in half, you can see rings of ice. Some rings are milky white; others are clear. This ringed structure suggests that a hailstone can grow by two different processes, wet growth and dry growth.

In wet growth, the hailstone is in a region of the storm where the air temperature is below freezing, but not super cold. When the hailstone collides with a drop of water, the water does not immediately freeze on the ice. Instead, the liquid water spreads over the hailstone and slowly freezes. During this slow freezing, air bubbles can escape, forming a layer of clear ice.

Dry growth of hailstones occurs when the air temperature is well below freezing. In these conditions a water droplet freezes immediately upon colliding with the hailstone. Air bubbles quickly freeze in place, leaving cloudy ice. Counting the layers of clear and milky white ice gives an indication of how many times the hailstone traveled to the top of the storm.

Hailstones can be as large as oranges and grapefruits. If hailstones collide and freeze together, they can form hailstones with very irregular shapes.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.