Forest Preserve District of Cook County, Illinois
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Thunderstorms
Nature Bulletin No. 72   June 29, 1946
Forest Preserve District of Cook County
Clayton F. Smith, President
Roberts Mann, Superintendent of Conservation

THUNDER STORMS
The puff of wind which lifts a piece of paper, spiraling it upward, is nothing but a thunderstorm in embryo. Why? The answer is not simple. "Nothing about weather is simple". There are winds that blow straight up. They run in all sizes and strengths and they account for many of the caprices of our weather. These columns of warm air -- invisible chimneys -- rise from hot pavements, roofs, bare hillsides and cultivated fields on sunny days.

It is these columns which eagles, hawks, and buzzards utilize to soar effortlessly. Glider pilots also use them and sometimes travel hundreds of miles by gaining altitude in one column and coasting to the next, " Bumpy air" in a plane is the series of upward pushes received as it flies swiftly through such rising columns. As these updrafts rise and the air pressure decreases they expand and cool. Some of their moisture condenses into fog droplets so tiny that it takes 10,000 to make one rain drop. Then the column is capped by a fleecy white cloud. Every updraft has a built-in brake which tends to bring it to a stop, usually at an altitude of about one mile.

But occasionally, when the lower air is warm and muggy, a particular set of conditions takes off the brake and a giant cloud continues to rise, sometimes with a velocity of 200 miles an hour. Then things happen. Instead of cooling it condenses more moisture, generates heat and keeps rising. Ice crystals form. These collect fog droplets and grow so heavy they begin to fall. Sometimes the force of the updraft carries them far upward again and again until eventually they become so heavy they drop to the earth as hailstones. More often the raindrops are torn apart by the speed of their fall. Tiny fragments come away charged with "negative" electricity and are carried back upward. The main parts keep the "positive" charge and reach the ground as rain. Thus high tension is built up and lightning jumps across as a giant spark.

Thunder is the reverberating echo of its shattering crack.


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