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Icicle Cycles
| Article
# : |
20151 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
2 / 1992 |
2,060 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." |
After a summer shower, rows of glistening water drops can be seen suspended from branches and wires. All these pendant drops are about the same size, which is limited by the competition between gravity pulling them downward and surface tension holding them fast to their moorings. The force of gravity acting on a drop--its weight--increases as its diameter cubed, whereas the surface tension acting on it increases as its diameter. As a consequence, a drop can grow only so large before it loses its grip and falls because its weight exceeds surface tension.
Pendant drops are almost hemispherical with diameters close to the width of a child's finger. During winter, however, when air temperatures fall below freezing, dripping water can form icicles that dwarf single drops. Sometimes these icicles can attain the length of a man. Icicles hang from eaves and from ledges, ephemeral versions of their Methuselahanic cousins the stalactites, also formed by the relentless dripping of water.
Frozen drops are embryos for icicles, and dripping water feeds their growth. Yet, although icicles begin their lives as drops, they grow not as ice dome but as spires. And when icicles are given more than a casual glance they are seen to possess a remarkably intricate structure. A water drop is to an icicle as a drink is to a castle. Like castles, icicles are wrinkled with crenellations ad honeycombed with halls and chambers.
Birth And Growth
Growing icicles need a supply of water, room to grow, and subfreezing air temperatures. The feedwater may be snowmelt dripping from a roof warmed by sunshine or by a poorly insulated house. Icicles are often suspended from eaves because they overhang unconstrained space and because they are fed by large catchments on which snow can accumulate and subsequently melt.
When a water drop freezes it becomes more cohesive, more resistant to the downward pull of gravity. Although the size of a suspended liquid drop is severely limited, a vastly greater mass of ice can remain suspended without rupturing under its own weight. Water flowing onto an embryonic frozen drop coats it more or less uniformly with a thin film except at tits tip, where water accumulates as a pendant drop. Some of this water may drip from the nascent icicle or eventually freeze, providing a new surface on which inflowing water can accumulate and freeze. As long as liquid water feeds the icicle it can grow ever longer and
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