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Givers and Takers of Light
| Article
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18727 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
8 / 1991 |
2,196 Words |
| Author
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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." |
Clouds bring both brightness and darkness: They are symbols of contrary moods. Samuel Taylor Coleridge found "a fair luminous cloud" a source of joy, but to a brooding Edgar Allan Poe, "clouds hung oppressively low in the heavens [on] the whole of a dull, dark, and soundless day." Why can clouds appear so different, now the brightest objects in sight, now the embodiment of gloom? What endows clouds with the power both to give light and to take it away?
No all-encompassing answer can be given to these questions. Each cloud observation is a separate knot to be unraveled. There is not one way, but many.
Absorption is not the answer
Light that interacts with matter suffers one of two possible fates: scattering or absorption. Scattered light is redirected light, its radiant energy may be recovered. Absorbed light ceases to exist as light, although its energy lives on in whatever absorbed it.
A clean white shirt is easily smudged by careless and dirty fingers or by an inadvertent brush with a sooty stovepipe. This transformation from white to black results mostly from absorption. Many common objects near to hand--black stoves, black dresses, black ties--owe their darkness to absorption. This has engendered the misconception that all dark objects, distant and untouchable dark clouds in particular, are similarly the result of absorption. Yet a moment's reflection suggests that if there are two ways in which light interacts with matter, there may be at least two paths to darkness.
Clouds often appear so palpable that it seems they might be walked upon if one were to tread gently. But they are really quite tenuous: Only about one millionth the volume of a cloud is occupied by liquid (or solid) water. How much water is contained in clouds?
Weather, as this term usually is understood, is largely confined to the lower 10 to 15 kilometers of the atmosphere. If a hypothetical cloud extending throughout this entire region were to be compressed suddenly into a liquid layer, its thickness would be about a centimeter. A corollary is that rainfall accumulations many times this amount result from considerable water passing through a cloud, in the same way that much more water can flow through than be held by a spring-fed reservoir.
A few tens of centimeters or
...
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