How Does Hail Form?

Hail is precipitation in the form of large balls or lumps of ice. There was a storm that produced hail early last week. 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, growing larger with each collision.  Hailstones can be as large as oranges and grapefruits.

When a hailstone is cut 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 freeze on the ice immediately. Instead, the liquid water spreads over the hailstones and slowly freezes. Because the water freezes slowly, air bubbles can escape, resulting in 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 as it collides with the hailstone. This quick freezing leads to air bubbles “frozen” 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.

Category: Phenomena
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