Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 177-A   January 30, 1965
Forest Preserve District of Cook County
Seymour Simon, President
Roland F. Eisenbeis, Supt. of Conservation

In warm weather, on calm clear nights, the sun-warmed surface of the earth quickly cools as it radiates its heat up into the sky, Heavy, colder air then settles into every valley and depression. This layer of air becomes further cooled to the point where, if it is damp and the ground is moist, it loses some of its moisture and this is deposited on plants and other objects near the ground as "dew". On cloudy nights when there is less surface cooling because the clouds act like a warm blanket, and on windy nights when the air is kept mixed and prevented from settling, little or no dew will form.

Under conditions otherwise favorable for dew, but when the temperature during the night goes below the freezing point, then "hoar frost" forms. In late fall and early spring it usually appears as a white coating of fine ice-needles, spine-like or feathery; sometimes as tiny flat six-sided flakes resembling snow crystals. Grass, weeds, shrubs and trees will be silvered with a delicate fur of frost; roofs of buildings will be coated white. Sometimes, when the temperature falls several degrees below 32 Fahrenheit, but the air is too dry or windy for hoar frost to form, we have a "black frost" -- so-called because many plants freeze and later become limp and black when thawed out by the sun. Frost damage to a plant is not caused ordinarily by the frost crystals themselves, but by freezing of the plant juices, which disrupts the plant's living substance and the tubes bringing nourishment to it.

Some garden plants, such as tomatoes, and especially tropical plants like peppers and eggplant, are extremely sensitive, even to a light frost. The length of the growing season for most hardier plants is measured from the date of the last killing frost in spring until the first killing frost in autumn. This is vitally true of corn, our principal crop in the Middle West and the most valuable crop in America. Occasionally a late spring frost will make it necessary to replant large acreages; more often planting is delayed by continued rains and cool weather. With such a late start, a severe frost early in September may so damage the immature corn as to greatly reduce the yield for that year.

Other crop plants like wheat, rye, alfalfa and clover, which commonly live thru the winter, may be injured by frost in the ground -- especially in periods of alternate freezing and thawing -- causing the soil to swell and "heave", perhaps several inches, breaking the roots and exposing them to the air. Sometimes the trunk of a large tree will be split by "frost", with an explosion like the crack of a cannon, due to freezing of the moisture within it. On the other hand, the seeds of some plants -- such as the Alpine Willow, some common weeds and certain nut trees -- cannot germinate until they have been frozen. Frost helps keep soils loose and crumbly. Frost also plays an important part in splitting rocks, causing them to disintegrate into fine particles which, carried by wind and water, become part of soils.

The windowpane patterns of frost, on a bitterly cold morning, are beautiful, unique, and never the same. An imaginative person can see funny faces, beautiful landscapes, fairy castles, or forests of towering ferns. Geometrical designs, a spider's web, tiny blossoms and leaves, or intricate lace may be etched in gossamer lines that sparkle in the sunlight. Try blowing on such a frosty pane until your breath melts the crystals into a film of water! Then step back and watch the frost reform a new design. We are told that the air of the room must be moist; that various combinations of temperature and moisture affect the formation of the frost crystals; and that they are affected by the thickness of the glass, its structure, and its cleanliness.

Phooey! Let' s give Jack Frost a big hand!

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