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Why is the Chinook warmer than the air around it?


ChrisChinook
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Hi! Please can somebody explain to me why the falling wind of the Chinook is warmer than the rest of the air near the ground East of the Rockies? I get that it warms back up as it falls from the top of the mountains and does so quicker when dry than moist as it rises on the west side. But how can it warm to higher temperatures than air that is already there? Does it start off that much warmer when it comes from the Pacific? Is there arctic influence East of the Rockies that there isn't West of the Rockies? 

Please if you know, share your wisdom. I must have read everything on google :) thank you!

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  • 2 months later...

It has to do with air pressure. Air moving from a higher to a lower elevation compresses, making the air warmer. Many times the difference is on the order of 5 degrees F per 1000 feet, which can create quite a difference- for example, if the Continental Divide averages around 12,000 feet in CO and the Plains are around 5000 feet, that's a 35 degree difference just from compression. There are of course other factors, but that's the basic reason.

As to the part of your question about why it's warmer than the air that has been sitting around at a given elevation for a while, it has to do with cool air sinking. At night, if it isn't windy, cooler air will layer out. So if it's 20 F in Denver on a calm morning, then a westerly wind kicks up from the mountains, you get warmer, compressed air invading the cooler air. Many times the temp can go up 30 F in a couple hours with the combination of a Chinook wind and mixing the layers out. This time of year, you can get what we call "two-coat days": perhaps 18 F (winter coat) at 6 AM,  50 F (spring coat) at 9 AM, and 68 F (no coat) at 2 PM. 

Then again, you can get 8 F within a couple of days after 70 F (this happened Sat PM-Tues AM at Centennial Airport) when cold air rushes down the lee side of the mountains. Tough place to be a plant!!

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  • 4 months later...

Winds push the air up the mountains.  The air expands and cools and latent heat is released when the cooling air causes condensation in the form of rain or snow, which falls on the mountains.  Same reason Gulf moisture is responsible for Tornado Alley... latent heat.  Air continues down the eastern slopes of the mountains into Denver, latent heat released but sans precipitation that would otherwise then be evaporating and cooling (re-absorbing the latent heat), so after it has re-compressed to the pressure at 5000 feet it is now warmer than it was back over in Grand Junction.  Same pressure (more or less) but now drier and warmer.

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