SciTech

How Things Work: Inner machinations of hailstorms

Repeated thudding noises on your window might have you looking at small white pellets, commonly known as hail. Hail is a form of precipitation: Small pieces of ice fall from the sky and cause more than a billion dollars in damage each year, according to the National Weather Service Forecast Office.

During severe weather, a convective cell can form in the air. A convective cell refers to warm air rising and cool air sinking in the same space. As the warm air rises, it cools and eventually turns back into liquid form. If the air continues rising to where the temperature in the atmosphere is below freezing, water will supercool; when there is enough of this supercooled water, it begins accumulating as ice in clouds.

Strong updrafts, also present during severe weather, push up on the ice in the clouds, keeping them suspended. However, when the updraft stops, the ice falls down again. The process is repeated over and over as the ice begins collecting layers to form hailstones. According to weather.about.com, “If you cut a hailstone in half, you would see alternating concentric layers inside it.”

Larger hailstones form when the hailstone falls and melts as it encounters warmer temperatures only to be pulled by updrafts to refreeze. Multiple cycles of this process can result in extremely large hailstones. Hailstones as large as eight inches in diameter have been found in the central United States.

The ideal conditions for the formation of hailstones are found in tall clouds; these reach high into the atmosphere, allowing for a wide range of atmospheric temperatures in the cloud. The below-freezing temperature would be found higher, while warmer temperatures would be lower. As warm air rises within the cloud, it will freeze near the top. As it falls, it will melt and updrafts will push the ice back up the cloud to refreeze. Often, swirling updrafts like those found within a tornado are ideal for hail formation.

A hailstone needs a nucleus to form. An ideal nucleus forms when supercooled water droplets or clumps of snow gather together to form clusters. According to www.wisegeek.com, “If a bug, a piece of dirt, a seed, or another small particle gets blown up into the storm cloud, it creates another possible nucleus for a hailstone.” This nucleus serves as a site for ice to build up, but it can also melt in warmer temperatures and collide with other nuclei to form larger clusters.

If the weather is cold and windy enough, the clusters begin to add on layers of ice in a process called accretion, which is very similar to how a candle gains layers when it is dipped into wax. Opaque, white layers form on hailstones when air bubbles get trapped in the icy droplets. Clear layers are formed when large supercooled water droplets freeze upon contact with the emerging hailstone.

As the hailstones grow larger, they gain weight, which increases the chance of being pulled down. If the updrafts are not strong enough to counteract the pull of gravity, the hailstones will fall from the cloud.
Not every hailstone that falls from a cloud will make it to the ground. As the hailstones fall, they enter a warmer atmosphere. Therefore, hailstones can melt in the atmosphere, never reaching the ground as concrete objects; these hailstones become rain.

The outer layers of hailstones also melt when they come into contact with other precipitation in the air.

Hail showers are not limited to cold weather, since hail formation is dependent on the temperature in the upper atmosphere. On a very warm day, for example, the humid air can rise very quickly into a mass of cold air in the atmosphere and supercool, initiating hail formation.

With a hot and humid summer coming up, keep in mind that hail can still form. It is generally advisable to stay inside if hail begins falling, especially since the speed at which some fall can be harmful. Large hailstones can also cause damage to pets, outdoor furniture, and vehicles, so moving them inside a garage would be ideal protection.