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Sep 13, 2013 6:24 PM by Stephen Bowers

"Upslope" Breeze: What is that, anyway?

When you watch the weather forecast on News 5, you often hear Mike, Stephen, Stephanie, or John talk about an "upslope breeze." It makes sense, but when I saw the upslope breeze in action, I thought it might be fun to break it down for you.

Look at this picture of the Wet Mountains.

Those clouds are resting on the east-facing slopes of the mountains and are a good example of clouds forming in the upslope breeze I talk about a lot. The wind hits the mountains and cannot go THROUGH, so it goes UP. That upward motion in the wind cools the air (that process is actually called "adiabatic expansion") and increases the humidity until clouds and eventually rain forms.

Right against the mountains, that upslope is most intense. This same thing happens with the Palmer Divide, on which Colorado Springs sits. It can also happen more gradually on a larger scale.

The downslope wind, the opposite of the upslope (duh, right?) WARMS the air and LOWERS the humidity. The downslope wind also tends to have more momentum since dry air is more dense than humid air (it's true... the molecular weight of water vapor is much less than that of dry air, making humid air much less dense and much more buoyant). That means the downslope wind often leads to windy days in Colorado. Remember May and June? The downslope wind was really kicking in those months and actually helped to fuel the Black Forest Fire.

Pueblo sits in a position just south of the Palmer Divide, and a downslope wind frequently comes out of the mountains or down the Palmer Divide. The drying that comes with it chokes off snow or rain. Pueblo is going to get the best rain or snow off a wind blowing from the east. That will happen most frequently with low pressure to the west or even a little southwest with high pressure to the north. 

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