Cecile Girard, age 12, of Fairfield, Connecticut, for her question:
What is a chinook wind?
In pioneer days, the region around the great Columbia River was known as the country of the Chinook Indians. Later some of the settlers noticed that wonderful winds sometimes blew from this direction. They came bearing spells of mild dry weather, just when the wintery season was at its worst. Surely such welcome winds deserved a name and surely the most suitable name was chinook winds.
Winter brings blizzards and deep snows to Montana and Wyoming, to the Dakotas and southwestern Canada. But in these regions, the people have come to expect a mid winter treat. It arrives when the chinook wind blows down the eastern slopes of the Rockies, wafting mild dry air over the snows.
Often the people look toward the west, hoping to see its sign in the sky. When a great triangle of clouds is blown like a curtain across half the blue sky, they know that the chinook is on the way. As the air blows down the eastern slopes, it grows warmer. When it reaches the plains, the temperature may rise 40 degrees. In a few hours, more than a foot of the deep snow may melt or more likely evaporate into the warm dry air. Often one chinook wind follows another and in a few unlikely places, cattle may graze outdoor through much of the winter.
Naturally this kindly wind is part of the global weather system. Actually, it is a feature of the prevailing westerlies that sweep across the Pacific Ocean. These cool moist winds reach the western mountains and to continue on their way they must ascend and descend over the hump. This causes changes in air pressure and the cool wet wind becomes a warm, dry wind.
As the ocean winds ascend the western mountains, they shed their moisture as rain on the slopes and often as snow on the high peaks. When it reaches the top, the breezy air is much drier, but still cool.
When the wind continues on down the eastern slopes, its air is warmed by a process called adiabatic heating. For every 180 feet it drops down the mountains, the temperature may rise one degree Fahrenheit. If the chinook wind climbs over a range 5,400 feet high, it gets to be about 30 degrees warmer than it was on the other side.
By the time it reaches the level plains, the chinook is a rather gusty wind of moderate strength. But it is warm enough to relieve the worst winter weather with a reminder of springtime.
Winds of this sort blow over mountainous regions in other parts of the world. Some meteorologists call them foehan winds, which is what they are called in Switzerland. There these warm, dry, west winds blow up, over and down the steep slopes of the Alps.