Photo: Sandy Girl (flickr)
Legend has it that when warm, dry Chinook winds race down the eastern slopes of the Rockies, people on the plains start acting a little crazy.
But maybe they’re just trying to figure out why that blustery wind off the mountains is so warm and dry. Doesn’t heat rise? And aren’t mountaintops often damp and cold?
To understand Chinook winds, we need to start in the land of the Chinook Indians, on the western side of the Rockies.
As moist ocean air climbs the western slopes, it expands and cools, becoming saturated with water vapor, which then condenses out as rain or snow. By the time the air passes over the high ridges, it’s dry and cold.
Gravity In Action
At the same time, air over the eastern slopes has its own ideas. At night, the air blanketing the slopes becomes cooler and more dense than air at the same altitude to the east, farther from the mountains.
Gravity quickly pulls that cool, dense layer of air down the slope, sucking the dry mountaintop air behind. This mad rush downhill compresses the air, generating a lot of heat, but it’s moving way too fast to lose heat to the surrounding air or ground.
So the compressed, fast-moving current actually heats up about 5.5 degrees Fahrenheit for every 1,000 feet it descends. Chinook winds can be a whopping fifty degrees warmer than the air they displace!
In winter, a Chinook can melt a foot of snow on the plains, exposing much-needed grass for grazing animals. But they’ve also been known to gust off the slopes at up to eighty miles per hour, enough to drive anyone crazy.