Hail is formed in huge cumulonimbus clouds, commonly known as thunderheads. When the ground is heated during the day by the sun, the air close to the ground is heated as well. Hot air, being less dense and therefore lighter than cold air, rises and cools. As it cools, its capacity for holding moisture decreases. When the rising, warm air has cooled so much that it cannot retain all of its moisture, water vapor condenses, forming puffy-looking clouds. The condensing moisture releases heat of its own into the surrounding air, causing the air to rise faster and give up even more moisture.
|NCAR scientist Nancy Knight holds a hailstone that fell in Coffeyville, Kansas, in 1970. The largest hailstone ever documented, it weighs 0.75 kilograms (1.67 pounds), and spans 14.4 centimeters (5.67 inches).|
Cross sections of hailstones often reveal layers, much like those of an onion. These layers are caused by the different rates of accumulation and freezing of supercooled water, as the hailstone forms. When there is a great deal of supercooled liquid in the air through which the hailstone falls, water accumulates faster than it can freeze, so a coat of liquid forms. This becomes a layer of clear ice when it does freeze. When a hailstone falls through air with a smaller amount of liquid, the liquid freezes on contact with the hailstone, forming small air bubbles in the opaque layers. The more supercooled water a hailstone makes contact with, the larger and heavier the stone is likely to become. When the hailstone becomes so heavy that the updraft can no longer support it, it falls from the sky.
Hail falls along paths scientists call hail swaths. These vary from a few square acres to large belts 16 kilometers (10 miles) wide and 160 kilometers (100 miles) long. Swaths can leave hail piled so deep it has to be removed with a snow plow. In Orient, Iowa, in August 1980, hail drifts were reported to be 2 meters (6 feet) deep. On 11 July 1990, softball-sized hail in Denver, Colorado, caused $625 million in property damage, mostly to automobiles and roofs. Forty-seven people at an amusement park were seriously injured when a power failure trapped them on a Ferris wheel and they were battered by softball-sized hail.
Hail also does a great deal of damage to crops. U.S. costs run into hundreds of millions of dollars annually. While hailstones have been found weighing as much as 0.75 kilograms (1.67 pounds), even much smaller hail can destroy crops, slicing corn and other plants to ribbons in a matter of minutes. Farmers cope with the hail hazard by purchasing insurance. Illinois farmers lead the United States in crop-hail insurance, spending more than $600 million annually. However, U.S. hail is most common in the area where Colorado, Nebraska, and Wyoming meet, known as "Hail Alley." Parts of this region average between seven and nine hail days a year.
|Corn crop damage caused by hail.|
After World War II, scientists across the world experimented with cloud "seeding" as a means of reducing hail size. In Soviet Georgia, scientists fired silver iodide into thunderclouds from the ground. Such methods supposedly stimulated the formation of large numbers of small hailstones, which would melt before they reached the ground, but comparable experiments performed in Switzerland and the United States did not confirm Soviet theory.
While hail suppression continues to elude scientists, sophisticated radar has been developed that can detect the presence of hail before it falls to the ground. Eventually, warnings may be issued as much as 15 minutes before hail strikes, allowing pilots to avoid threatening air space, people to seek shelter, and property to be protected.