Shelley Woodrow, age 16, of Cleveland, Ohio, for his question:
WHEN DOES HAIL FORM?
Hail is made of falling lumps of water that have frozen. Hail can only form when the atmosphere contains a large amount of moisture. Hail is then formed when frozen raindrops pass through a layer of cloud where the temperature is below freezing, or below 32 degrees Fahrenheit.
The cloud droplets usually do not freeze at temperatures above zero degrees Fahrenheit.
Hailstones grow as they bump into supercooled raindrops or the cloud droplets, either or both of which then freeze onto the hailstone. Hailstones can from time to time be built up to measure four or five inches in diameter, and they can weigh up to a pound.
There have been hailstones weighing as much as a pound and a half and measuring more than five inches in diameter. Some of these large hailstones can reach a speed of 100 miles an hour as they fall to earth.
Hail can be found in several different shapes. A hailstone can be a flat disk, a cone or a sphere with points. Usually the hailstone is made up of alternating layers of opaque and clear ice.
There are times when small hailstones are carried upward into supercooled clouds by strong wind currents. They then fall, only to be carried upward again by another current. This rising and falling may happen a number of times. Eventually enough layers of ice are built up around the hailstones that they fall to the ground.
Hail has done a great deal of damage in the past. Primary damage is to farm crops. In a strong hailstorm, trees may be stripped completely of their leaves, windows broken and automobile tops and metal roofs punctured. During very strong storms, hail can kill animals and injure people.
Attempts have been made to control the formation of hailstones. Certain salts have been dropped by aircraft into cumulo nimbus clouds, in which hail is formed. The "seed" is intended to cause rain or only small hailstones.
Scientists don't completely understand the formation and other characteristics of hail. New theories must be evaluated regularly. Some scientists, for example, believe that the electricity in thunderstorms may be caused by electrified hail.
Winter isn't the usual season for hail, since this season usually brings either rain or snow. Hailstorms are usually associated with the warmer weather of spring or early summer, as well as fall, to a lesser degree.
Thunder and strong winds are usually associated with hailstorms. Such storms most often occur in the interior of continents. They are most common in the middle latitudes, or the latitudes that are midway between the poles and the Equator.
True hailstorms happen only at the beginning of thunderstorms and never when the ground temperature is below freezing