by Elizabeth Waddell

While the prevailing winds may blow from the west, mountain valleys act as a wind funnel, providing locally divergent wind patterns. When air is funneled around mountains, locally strong winds are created (just as skyscrapers may channel wind in an urban setting.) Summits and crests are perpetually windy. But these blustery winds are mild compared to two famous winter-time winds.

Chinook – The Snow Eater

Anyone who’s lived in the area for very long is familiar with the strong, dry, warm winds that sometimes develop in the winter quickly raising the temperature by 50 degrees Fahrenheit or more. These winds are called “Chinooks” after a Native American tribe in the Northwest where the air masses originate.

As warm, moist air blows ashore on the coast, it runs into a barrier of mountains. As the air is forced to climb upwards, over the mountains, it cools down. During this rising and cooling, the airmass will release most of its moisture as rain or snow. Once the now dry airmass crests the Continental Divide, it begins to move downhill and the process reverses itself as the airmass drops down the eastern side of the mountains and warms. Now comes the interesting part – moist air warms and cools less than does dry air. The sinking, dry air warms rapidly resulting in an airmass that is significantly warmer and drier than the original airmass that moved onshore in the Pacific Northwest. The Chinook usually begins with a sudden change in wind direction towards the west or southwest, and a rapid increase in wind speed. Snow and ice melt (or sublime) rapidly – hence the name “snow eater”.

Bora Winds – Arctic Blast

Bora winds are also downslope winds. Like Chinook winds, they are relatively dry (due to the descending air), but, unlike Chinook winds, Boras are cold. Whereas Chinooks start as warm, moist air over the Pacific, Bora winds originate in very cold, dry inland areas of Canada and the Arctic and remain cool. Despite the warming caused by the air sinking down from the mountains, the Bora is still a cold wind because the air was so chilly and dry to begin with before crossing the Rockies. Bora winds are often caused by a strong cold front passing over Colorado from the northwest. After the front passes, the winds can reach speeds of 50-60 mph.

Last updated: March 31, 2012

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