Nature Bulletin No. 72 June 29, 1946
Forest Preserve District of Cook County
Clayton F. Smith, President
Roberts Mann, Superintendent of Conservation
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.
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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Update: June 2012