Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 458-A   May 20, 1972
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

Because we are so rapt up with our modern inventions, we forget that primitive man, ages before the beginnings of civilization, made one of the greatest discoveries of all time -- the use of fire. However, for thousands of years before he learned to kindle his own fires by friction or by striking sparks from flint, he snatched flaming firebrands from forest fires started by lightning. These he carried away and carefully fed with dry wood, year in and year out, to warm his cave, frighten away wild beasts and cook his food. Around the world, in every year, countless thousands of fires are started by lightning.

On a sultry summer's evening a distant storm cloud with its flickering tracery of lightning and its far-off muttering rumble is one of nature's finest spectacles. But one overhead, with its blinding flashes of light, the snap and crash of thunder, whipping winds and drumming rain is awesome and terrifying. In olden times an electrical storm was looked on as a show of power by an offended god. To the American Indians, lightning was the fiery glances from the eye of the Thunderbird. In Norse mythology it was the mighty hammer blows of Thor. From these naming spears of Zeus, king of the Greek gods, Prometheus stole fire and gave it to man. Not until the time of Benjamin Franklin with his famous kite experiment was it proven that lightning is a greatly elongated electrical spark.

A thunderstorm starts as an ordinary fleecy white cloud which forms when an updraft of warm air is cooled enough for part of its moisture to condense into a cap of fog droplets at a height of about a mile. Occasionally, when the lower air is warm and muggy, this column of air surges upward at speeds of 100 miles an hour or more and the cloud piles up mile after mile. The fog droplets combine to form large rain drops which start to fall but are torn into mist and whirled upward to reform again and again as rain or hail in the blast of this invisible chimney. In this churning cloud, enormous charges of static electricity are built up between its different parts or between the cloud and the earth. When great enough they discharge as giant sparks sometimes miles long. For example, a lightning stroke starts upward from some tall tree, building or other high point on the earth in a series of jerky steps called the "leader". Instantly, the main charge of electricity flows back along this path with a burst of light and a clap of thunder. Thunder is merely the explosive expansion of the air along that channel.

Alive tree struck by lightning may be blown to pieces or only a narrow strip of bark may be ripped off. Such damage is caused by the heating of the tree's sap into steam causing the wood to explode or be cooked to death. Regardless of species, it has been found that the taller trees in a forest and trees standing alone or in small groves are most likely to be struck.

Lightning, overall, is a major cause of fires. Each year in the United States it takes the lives of about 400 people and cripples another thousand. Hunters, fishermen, sportsmen, vacationers, farmers and others who spend a lot of time in the open are particularly vulnerable. A gun or fishing rod in your hands attracts lightning. Mountains, rowboats and golf courses are especially dangerous. If you are caught in the open with lightning snapping and cracking all around, stay away from fences and do not seek shelter under a lone tree. It is best to lie flat on the ground until the storm has passed over.

Thunder five seconds after the flash means it missed you a mile.

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