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How Does Hail Form?

Eastern Pennsylvania had a widespread damaging hail event. Have you ever wondered how these balls of ice form in warm weather?
Hail is the result of raindrops that are brought up into the cooler layers of the atmosphere by the rapid upward movement of air in strong thunderstorms. This “fountain” of air is known as an updraft. In these updrafts, water is lifted to a colder part of the clouds and freezes. This bit of ice bumps into supercooled drops (or water that is still liquid but below freezing). These collisions help to form hailstones. The hail may try to fall, pick up more water, but the updraft will recycle back up and through the process again to grow. In fact, if you slice open a hailstone you can count rings of growth similar to the rings of a tree. When hailstones become too heavy for the updraft air to hold, they fall toward the ground. Sizes vary depending on the size of the thunderstorm and the speed of their updrafts. Hailstones, on average, are about the size of peas or marbles. But they can get to the size of softballs, which can result from thunderstorms that have updraft speeds greater than 100 mph. Depending on their size and travel speed, hailstones can create more damage than just a few dings and dents. The largest hailstone on record in the U.S. fell in Vivian, South Dakota in 2010, and measured 8.0 inches in diameter. Pennsylvania’s largest was 5.5 inches, which fell near Meadville in 1950.
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