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Mists
| Article
# : |
11546 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
10 / 1986 |
752 Words |
| Author
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Barbara Tufty
Barbara Tufty is a free-lance natural science writer
who lives in Washington, D.C. |
Now the chilled air of autumn flows down from the North, forming the strange silent phenomenon of fog as it encounters lakes, steams, and wetlands till warm from languid days under the summer sun. As the cool twilight falls, fogs and mist rise.
Mysterious as a shroud hanging over an abandoned graveyard or radiant as a vision when the morning sun lights it up, fog is the visible condensation of warm moisture in cool air - small, buoyant drops of water that float on air currents. The mist is also fog, but more ethereal, with finer, lighter drops of water.
Fog is an earthbound cloud that touches the land the sea, softening the sharp edges and noises of the landscape. Fogs can creep through forests, across cooling lakes and sedge-draped edges of a river, up mountain slopes and through saddles of hills, spilling into the next valley.
Two ingredients are needed to form fogs: moisture and the meeting of two different temperatures, warm and cold. Such conditions occur when cool air blows across warm water; when warm air laden with moisture blows over cold land or sea; or when moist warm air rises into cool, still air.
Depending on its temperature, air can hold a certain amount of moisture before water is precipitated out as rain. Warm air can hold far more water vapor than can cold air. As moist air cools, it reaches the saturation point at which the air can no longer hold moisture in the form of invisible water vapor. Then, minuscule - two to fifty microns in diameter - spherical droplets of water are squeezed out, so fine and buoyant that they are suspended and drift gently on the slightest air current. Some fogs may contain very few droplets per square inch and seem like a thin white veil; other fogs are so thick they are called "pea soup," obscuring visibility so severely that they frequently become the indirect cause of collisions of land vehicles, ships at sea, and planes in the air.
Some of the heaviest and most persistent fogs occur where warm ocean currents flow like rivers into colder climates - on the west coast of Alaska, for instance, where the warm Aleutian current brings rain and fog much of the year; or along the coasts of Oregon, Washington, and Canada where the westerly winds blow warm air from the North Pacific current onto the coastal mountains. Other familiar fog-enshrouded spots occur where the warm winds and waters of the Gulf Stream strike cooler air and water currents from the Arctic - along the coast of New
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