Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 433-A   November 13, 1971
Forest Preserve District of Cook County
George W, Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

For ages, shepherds and seamen have been our best amateur weather prophets because they are habitually far from shelter and have, by necessity, learned to read the meanings of clouds. Today, most of us know less of cloud lore than men did centuries ago and depend on forecasts made by professionals.

When you see your breath on a winter morning it is no different from a bit of natural cloud. The warm moist air from your lungs cools and the excess water vapor condenses into a cloud of microscopic droplets, This is because warm air can hold more water vapor, as an invisible gas, than can cold air. It is entirely possible for a cubic yard of warm air to hold an ounce of water and be cloudless, while a cubic yard of cold air may contain only a tenth of an ounce and look like a hunk of fog.

Few of us realize what enormous quantities of water evaporate into the atmosphere from oceans, lakes, streams and the land, On a summer day, a pail full of water may be evaporated from a square yard of open sea; and a gallon from one stalk of corn, Almost all of this moisture is eventually condensed into clouds and returns to the earth as precipitation -- rain, snow and sleet.

The moist air is cooled to form clouds in various ways, most often by being pushed upward - becoming about 5 degrees cooler for each thousand feet of rise, Air is sucked in on all sides and forced upward by those huge slow whirlpools in the atmosphere called "lows", These are usually hundreds or even a thousand miles across and are responsible r the main features of our cloud cover. The earth's surface, heated by the sun, produces rising columns of air in which the moisture condenses into tall fleecy white clouds At night, when the sky is clear, the earth cools to give those low-hanging fogs that slow highway traffic and close airports. When we are inside of a cloud we call it fog.

Ordinarily, when clouds form, each tiny droplet condenses on some invisible speck of dust, ash, smoke or pollen, If the air happens to be very clean, a cloud may fail to form at once but under such conditions, we sometimes see vapor trails behind airplanes. Cloud droplets are very small, averaging about a thousandth of an inch in diameter, so small that they fall less than an inch in a second, Only under special conditions do they combine to form rain, Even then it takes millions of them to make a single raindrop.

Since ancient times many attempts have been made to describe the different kinds of clouds. Most systems were so complicated that they were practically useless but, in 1803, an Englishman, Luke Howard, grouped all clouds in four main types and gave each a Latin name. These four and their combinations are the ones in use today, Cirrus, meaning "curl", is the most delicate of all clouds -- often called "mare's tails" -- casting no shadow and not obscuring the sun. Also the highest of all clouds -- averaging 6 miles up -- they are always composed of ice crystals and are always white, Stratus, meaning "layer", gives us our smooth gray skies and is our most common type, Made up of water droplets in summer and ice crystals in winter, they hide the sun and are usually less than a half-mile high, Cumulus, meaning "heap", refers to the mound-like clouds so often seen on fair days, They have flat bases, are white and billowy above, and cast moving shadows. When they tower upward -- mile on mile -- they are called thunderheads. Nimbus is any dark layer of cloud with ragged edges from which rain or snow is falling.

Clouds with silver linings are rare !

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