When hail freezes over

photo of hail
Quarter-size hail fell during a July 11 storm in Champaign, Ill. Photo by Jennifer Smith.

URBANA, Ill. – Hail occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into balls of ice. Hail can damage aircraft, homes, and cars and can be deadly to livestock and people.

Hailstones grow by colliding with supercooled water drops, says Duane Friend, University of Illinois Extension energy and environment educator. "In clouds, water can actually stay in liquid form well below 32 degrees Fahrenheit." 

Supercooled water will freeze on contact with ice crystals, frozen raindrops, dust or some other nuclei. Thunderstorms that have a strong updraft keep lifting the hailstones to the top of the cloud where they encounter more supercooled water and continue to grow.

"The hail falls when the thunderstorm's updraft can no longer support the weight of the ice or the updraft weakens," Friend says. "The stronger the updraft, the larger the hailstone can grow."

Hailstones can have layers like an onion if they travel up and down in an updraft, or they may have few or no layers if they are “balanced” in an updraft. One can tell how many times a hailstone traveled to the top of the storm by counting the layers. Hailstones can begin to melt and then re-freeze together, forming large and very irregularly-shaped hail.

Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the earth. How it falls is dependent on what is going on inside the thunderstorm.

"Hailstones bump into other raindrops and other hailstones inside the thunderstorm, and this bumping slows down their fall," Friend says. "Drag and friction also slow their fall. If the winds are strong enough, they can even blow hail so that it falls at an angle."

According to the National Severe Storms Laboratory, though Florida has the most thunderstorms, Nebraska, Colorado, and Wyoming have the most hail storms. The area where these three states meet,“hail alley,” averages seven to nine hail days per year.

"The reason why this area gets so much hail is that the freezing levels (the area of the atmosphere at 32 degrees or less) in the high plains are much closer to the ground than sea level where hail has plenty of time to melt before reaching the ground," Friend says. Other parts of the world that have damaging hailstorms include China, Russia, India, and northern Italy.

Hail falls in paths known as hail swaths that range in size from a few acres to an area 10 miles wide and 100 miles long. Piles of hail in hail swaths have been so deep, a snow plow was required to remove them, and occasionally, hail drifts have been reported, Friend says.

Hail is usually pea-sized to marble-sized, but big thunderstorms can produce larger hail. The largest hailstone recovered in the U.S., with a diameter of 8 inches and a circumference of 18.62 inches, fell in South Dakota on June 23, 2010. It weighed nearly 2 pounds.

SOURCE: Duane Friend, Energy and Environment Educator, Illinois Extension
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