The same clouds that spawn thunderstorms and tornadoes also can produce hail. They are cumulonimbus clouds, the classic anvil-shaped forms that can reach from 30,000 feet altitude to 60,000 feet. That is very cold territory. They also house strong updrafts of air. And they can contain both frozen and liquid water vapor. The water vapor is super-cooled; it is at a temperature below freezing but still in a liquid state. These are the three conditions necessary for the formation of hail.
In order for super-cooled water vapor to form ice, it must come into contact with some sort of nucleus. This could be dust, pollen or even insects. The super-cooled vapor freezes instantly when it comes into contact with a solid object.
A hailstone is composed of several layers of ice, accumulated in its wild ride into the upper reaches of the cloud and back down. It starts as a small nugget of ice, sometimes as a snowflake. Then it is swept high on a strong updraft of air. As the ice particle travels through the cloud, it accumulates droplets of water that freeze to its surface. Then it begins to fall, either slowly or as fast as 90 mph. Before the ice stone can make it out of the cloud, it is again caught by a strong updraft.
Within the cloud, temperature varies. The developing hailstone may partially melt, leaving it with a thin coating of water. If it then encounters another small stone, the two will freeze together, creating a larger stone. As the newly enlarged stone is swept higher, the air becomes colder, and it refreezes.
This process, called riming, is repeated until the stone becomes large enough to fall through the updrafts. It then plummets to the ground. On the way to the ground, small hailstones of 5 mm or less may melt, becoming cold or even freezing rain. Larger stones up to 50 mm or more can do a great deal of damage if they strike people or vulnerable objects or structures.
Large hailstones form only if the updrafts are either titled or spiraling, and are very powerful. If they are vertical, the stone will simply be shot out the side of the cloud and then fall to the ground. However, if the updrafts carry the stone to the top of the cloud, they will shoot out but fall back into the cloud to accumulate more ice. This process can continue for hours and will produce very large stones.
The record for the largest hailstone ever to fall in the United States goes to one that fell in Aurora, Nebraska on June 22, 2003. Even after part of the stone was broken away when it struck the gutter of a house, it still measured 7” in diameter.
Most hailstones, thankfully, do not reach anywhere near that size. It’s a good thing because when they plunge to the earth they can reach speeds of up to 90 mph.
Crops are the major loss when it comes to hail. However, vehicles and glass may also be damaged. In the United States, death caused by hail is extremely rare. The last known was an infant in Fort Collins, CO in 1979. In other parts of the world, where people often live in flimsy structures, injuries and deaths are more common.
Hail storms usually occur during the summer because that is when conditions are right for producing thunderstorms. They are most common in the plains states in the United States. China, Russia, India and northern Italy also experience damaging hail.
The best thing to do when encountering a hail storm is to immediately seek shelter and stay away from windows. It’s an interesting phenomenon but is best appreciated after the storm has passed.
References and further information:
http://www.usatoday.com/weather/tg/whail/whail.htm
http://vortex.plymouth.edu/precip/precip2f.html
