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A Letter From the Sky
| Article
# : |
15686 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
2 / 1989 |
1,057 Words |
| Author
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Herbert Levine
Herbert Levine is a theoretical physicist on the faculty of
the University of California, San Diego. He has specialized
in the study of pattern formation in systems including snow
crystals, chemical reaction fronts, and flow in underground
oil reservoirs. |
"They had six little teeth, like clockmakers' wheels...formed as perfectly and symmetrically as one could possible imagine." This description of falling snow by Rene Descartes in 1637 began the scientific study of snow crystals. Generations later, modern physics has begun to unravel the mystery of the snowflake pattern. Yet understanding does not diminish appreciation, and the fascination with these intricate, lacy creations of the heavens continues unabated.
A snowflake begins its journey as a microscopic speck of dust high up in the atmosphere. Water vapor molecules that chance upon this seed will stick to it, forming an ever-growing ice crystal. Eventually, the weight of the snowflake causes it to fall to earth. Most flakes melt, break apart, or clump together with other snowflakes. It is, or course, the other ones that excite the observer.
To understand snowflake structure one must study the physical mechanisms underlying crystal growth. At the microscopic level, water vapor molecules attach to a growing crystal by falling into place in the existing crystalline array. For ice, this array is a hexagonal close-packed structure; that is, there are planes in which each molecule forms the center of a hexagon of neighboring molecules. At low humidity, the attachment process is very slow and thus controls the overall growth. Not surprisingly, the macroscopic pattern merely mimics the microscopic one, bringing the formation of hexagonal plate-like snowflakes.
As the atmospheric humidity increases, a new behavior emerges. Now the attachment rate is sufficiently rapid to ensure that most molecules reaching the nascent crystal will be immediately incorporated. The most critical step is the arrival of new water molecules at the crystal's surface. In the atmosphere, each molecule exhibits random motion, being buffeted by the wind and colliding with other molecules. Occasionally, this random motion will cause the molecule to land on the crystal and allow it to grow. The rate of molecular diffusion is thus the governing rate of the process.
The change to diffusion-controlled growth has dramatic consequences for the snowflake. It is a well-known phenomenon that sharp protruding tips are more effective at gathering randomly moving molecules than are the neighboring flat regions of the crystal surface. A familiar use of this effect is in the design of lightning rods--they are made with sharp points so that they "attract" the atmospheric electrical discharge. For snowflakes, needle-like tips begin to grow from the
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