Ferne Reynolds, age 12, of Calgary, Alberta, Canada, for her question:
What kind of wind is the chinook?
In Calgary you live where the chinook wind blows. In winter, nothing is more welcome than its warm, dry breezes. Old timers can read the cloudy patterns in the sky and predict when the balmy chinook is on its way down the eastern slopes onto the plains.
The major weather fronts circle the globe in the belts of the prevailing winds. In this major meteorological picture there are minor disturbances known as local weather events. Most of them are produced by local geographic items, such as seas and mountains. The chinook wind is a local weather event, a child of the western mountains. A similar wind arises on the northern slopes of the Alps. There it is called the foehn wind.
Our North American foehn was named for the Chinook Indians, alias the Flatheads, who bound their growing skulls to conform with their ideas of human beauty. The southwest wind blew from the direction of the Chinook's domain and it seemed logical to call it the Chinook wind. Often it swoops down the eastern slopes of the Rocky Mountains in mid winter with gusty breezes warm enough to melt deep snow drifts. In a few hours, it may raise the wintry temperature 50 or even 60 degrees.
As it comes, the advancing warm, dry air clears wintry clouds from the sky. Sometimes' it sweeps in a swooping curve. Part of the sky is clear, leaving a curtained arch of clouds on one side. Old timers look to the western sky for this sign of the waning Chinook. Most people are contented merely to enjoy it. But some want to know what causes it.
The welcome wind is started by pressure differences on the western and eastern sides of the mountains. On the windward side, the prevailing westerlies bring layers of moisture¬laden air to the western slopes. Sooner or later it is pushed from behind and forced to climb. As it rises, it expands and cools. Higher up it drops its moisture and the peaks are crowned with fresh layers of snow.
Over the hump, the weather conditions change. The light, cool air toboggans down the slopes, piling up as it goes. The crowded molecules in the descending air become warm. And the warm Chinook wind swoops all the way down the slopes and on over the plain. As a rule, it sheds its moisture on the peaks. It has no more vapor to create snow or rain and almost always the warm Chinook is a dry wind.
The Chinook follows a general rule of air that is forced to climb over a mountain range. As it rises, the air expands and as it expands it cools. As it descends down the other side, it warms up by a weather process called adiabatic heating. When it reaches the eastern slopes, the Chinook is warmer and drier than it was at the same level on the western slope. Climbing the mountains and going down the other side made it warm and thirsty.