Definitions

Chinook    n.  3. a warm, dry wind that blows at intervals down the eastern slopes of the Rocky Mountains.    

severe downslope wind,    a warm, dry air flow which travels down-terrain, from higher to lower elevations and which exceeds 58 miles per hour.    

Introduction

How do severe downslope winds affect Fort Collins?

The Front Range cities of central Colorado are built along the foothills of the Rocky Mountains at elevations that average 5,000 feet, or more. In contrast, the elevation at the eastern Colorado border runs generally around 3,500 feet. If the low-level wind flow (say from the surface to 5,000 feet above the ground) is from the east, then the low-level air is forced upward by the terrain. We call this flow upslope. Winds from the west are called downslope.

Due to the laws of chemistry, rising air cools and subsiding air warms. That is why it typically feels cooler in the mountains than down here on the Plains. When upslope flow develops, air reaching Fort Collins has been cooled by 5 to 10 degrees Fahrenheit. Because the moisture that's being carried along with this rising air remains constant, the air feels damper. The relative humidity has actually gone up. Sometimes the relative humidity reaches 100%, and precipitation develops.

When downslope flow occurs, mountain air is transported down from the mountains into Fort Collins. This air is warmed, and becomes relatively drier. In the winter, warm, dry downslope flow can turn a cold day to warm in a matter of minutes and melt several inches of snow in a few hours. When this happens we refer to the wind as a "Chinook."

Along the Front Range these effects are magnified due to the starkly rising terrain of the nearby foothills. Upslope flow can result in copious precipitation, as well as very cold, snowy air in the winter. Downslope effects are also magnified. The warming can be dramatic. Also, if the downslope flow develops in winter, (when the winds at mountain top levels are much stronger) a low pressure trough often develops out on the Plains. When this happens, the downslope winds can become extremely strong. If the wind gusts exceed 58 mph, we refer to the event as a severe downslope wind.

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Here is a tree downed by a severe downslope wind in Fort Collins.

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Here is a building under construction that was destroyed by a severe downslope wind in Fort Collins.