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
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!
To return to the Nature Bulletins Click Here!
Update: June 2012