Hail: The Hidden Danger in Severe Thunderstorms

While most people might associate destructive wind and tornadoes with thunderstorms, another common hazard is damaging hail. Unlike its counterpart, sleet, hail can dig deep into a homeowner’s wallet.

Thunderstorms contain supercooled droplets, which are very small drops in the clouds that remain in liquid form despite temperatures being below the freezing point. Storms also contain updrafts and downdrafts, which are critical to the development of large chunks of ice.

A sustainable updraft forms when surface temperatures are warm and the air aloft is very cold. This causes air to rise rapidly and condense as water droplets onto dust and other particles in the air, then continue to rise above the freezing level in a cloud and form a layer of ice. Think of the updraft as a river that needs a constant supply of water to flow. Two triggers, such as a cold front and large geographic barrier like a mountain, can keep the air rising for an extended period to maintain the updraft.

Within the cloud, the updraft can cross paths with the downdraft at various points, causing this now frozen water droplet to go on a roller coaster ride through the cloud. During it's journey, the frozen droplet adds water that again freezes as it cycles back into the updraft and the process continues until the chunk of ice is too heavy to withstand the force of the updraft. At that point, it falls as hail, but at speeds of 25 to 40 mph for severe thunderstorms (those that produce hail 1.0 to 1.75 inches in diameter). According to the Insurance Information Institute, hail-related losses from 2000 to 2019 averaged between $8 to $14-billion per year.

Hail often has layers like an onion because of its wet and dry growth in the cloud. In one part of its journey, a hailstone moves into a layer where the air temperature is below freezing but not super cold. When the hailstone collides with water droplets in this zone of the cloud, water slowly freezes on the hailstone and air bubbles can escape more readily, resulting in a layer of clear ice.

Usually much higher in the clouds, about 25,000 to 30,000 feet above the Earth’s surface, the air temperature is far below freezing and the supercooled small droplets in the cloud immediately freeze onto the hailstone and the air bubbles don’t have a chance to escape. This results in a cloudy film of ice.

The largest hailstones, softball sized and larger, fall from a thunderstorm that produces an updraft of 103 mph! The way to generate an updraft of this magnitude is for an individual storm to grow in magnitude but not connect with other storms, such as in the development of a squall line. The more storms merge together or connect, the less chance to develop a vigorous and sustainable updraft that could produce very large hail. However, other hazards develop when storms join together, such as damaging wind and flooding rain.

Hail is not to be confused with sleet. Sleet consists of smaller ice pellets that are initially snowflakes when they fall from the cloud. After encountering a shallow layer of warm air about 10,000 feet up in the atmosphere, the snowflakes melt and become raindrops. The raindrops then encounter a large sub-freezing layer from about 5,000 feet to the Earth’s surface, so they have enough time to freeze and become small ice pellets.

While Florida is the lightning capital of the U.S., the freezing level is so high in the atmosphere that hail usually melts before it reaches the ground. Conversely, New Mexico, Colorado and Wyoming have the most frequent hailstorms because the high elevations act as a source for a sustainable updraft in summer thunderstorms and the freezing level is much closer to the surface. The U.S.’s largest hailstone on record fell in Vivian, S.D., on July 23, 2010. It measured 8 inches in diameter and weighed 1.9375 pounds!

The next time you witness the side-profile view of a severe thunderstorm way in the distance, just think of the many processes occurring with the updrafts and downdrafts to foster hail underneath the ominous cumulonimbus cloud.
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Story Image: Hailstones are seen. (Courtesy of NOAA’s National Severe Storms Laboratory)