Mike Vague, Age 11, Of Shreveport, La., for his question:
Do they know how hail is formed?
Our grandparents claimed that everybody talks about the weather, but nobody does anything about it. Then came the age of science, and people began probing into the ancient mysteries of nature. They took up the challenge to do, or try to do, something about the weather.
Someday, perhaps, we shall be ab1e to control the weathery precipitation that falls from the clouds. But this cannot be done until we know how nature transforms the misty clouds into rain and snow, s1eet and hail. Then maybe we can change the key factors in the process arid order the weather we want. At this time we are still groping for the facts. The experts, for example, recently gave up one idea on the formation of hail and adopted a new one.
Rain and hail are formed from misty clouds by a gelling process ca11ed coalescence. A hail stone, we now think, begins its icy career with the birth of a raindrop. A cloud is made of floating swarms of miniature droplets. These are so small and widely separated that coalescence seems impossible. An average droplet is 12500th of an inch, and it is separated from its floating neighbors by about ten times this distance at least a million of these tiny, scattered droplets must get together to for a smallish raindrop. This coalescence cannot occur without the help of a nucleus, a fine fragment of solid material floating among the misty droplets. When dusty particles are present, the droplets use them as nuclei and gell around them to form raindrops.
A hail stone begins, we think, when a raindrop is tossed into a zone of a turbulent cloud where the temperature is way below freezing. This changes the watery raindrop into a pellet of solid ice. In some zones of a thunderhead the air is 20 degrees or more below freezing. But the supercooled droplets of mist do not freeze solid. This is because solid nuclei are needed t0 change them into ice. Snow flakes and ice crystals, everi at way below zero, can form only when dusty particles are present in the air.
A newly frozen hail stone acts as a nucleus to gather countless supercooled droplets and change them into ice. The frozen pellet grows as it is tossed up and down, around and around by the raging winds of the turbulent cloud.
A pellet of hail was built in onion layers of hard white ice and softer grayish mush. These jackets, we think, tell the story of its trips through different zones of the cloud. The hard icy jackets are built in the coldest zones, where droplets gather on the stone and freeze all at once. The softer layers are built in warmer wet zones where the freezing process is slower.