|
|
|
| Current Issue |
|
|
| Resources |
|
|
|
Winter Frosting
| Article
# : |
18972 |
|
|
Section : |
NATURAL SCIENCE
|
| Issue
Date : |
2 / 1991 |
1,967 Words |
| Author
: |
Craig F. Bohren
Craig F. Bohren is Distinguished Professor of Meteorology at
Pennsylvania State University. More of his thoughts on clouds
and light can be found in his recently published What Light
Through Yonder Window Breaks?, a sequel to Clouds in a Glass
of Beer, for which he was the first recipient of the Louis J.
Battan Author's Award of the American Meteorological Society.
In 1988 Bohren was elected a fellow of the Optical Society of
America for "outstanding contributions in radiative transfer
and atmospheric optics." |
Appearances to the contrary, frost does not fall in the same way that snowflakes are attracted earthward. A coating of frost is mainly the child of chance, the consequence of myriads of atmospheric water molecules coming together by way of independent and haphazard paths. Although snowflakes are directed by the relentless pull of gravity, water molecules hardly know it exists, moving so rapidly that it shapes their paths only slightly. They come under the influence of forces organizing them into frost only when exceedingly close to surfaces.
The path of a snowflake from its birthplace high in the atmosphere to where it is incorporated into snow on the ground is a more or less smooth vertical line. But a water molecule that finds itself on a blade of grass fringes with frost arrived there by a zig-zag path, a tangled string of dashes, the direction of its motion having been changes frequently and abruptly by collisions with other air molecules.
Air is a mixture of gases, water vapor one of its constituents. A thimbleful of even the driest Saharan air contains a million times more water molecules than the population of the earth, and they are moving at average speeds greater than that of a bullet exiting the muzzle of a rifle.
Any surface exposed to such a dense and frantic swarm as water molecules will be bombarded by them at a stunningly high rate. Some molecules will stick or condense, but some leave the surface after a brief visit there, evaporating back into the air whence they came. The surface will remain perceptibly free of water if the rates of evaporation and condensation are equal. If condensation exceeds evaporation, water accumulates as tiny drops of dew or; if the surface is cold enough, as delicate ice crystals of frost.
But why doesn't frost from every wintry night when temperatures drop so low that water in a pail placed outside sundown will have acquired a hard costing of ice by sunrise? It seems that subfreezing temperatures are necessary but not sufficient. for frost formation. Indeed, frost may form when the air temperature is above freezing.
The greater the concentration of water molecules in the air, the greater the rate at which they bombard a surface, hence condense onto it. Yet regardless of the amount of moisture in the air, for frost to accumulate on a surface its temperature must drop below air temperature. Only when this happens may the balance between condensation of water molecules onto a surface and
...
Read Full Article
Look for this article in Ask.com
|
|
