Chinooks blow

The Front Range of Colorado is under a high wind warning today — we’re having a Chinook!

I’ve talked about chinook winds before at https://amylaw.blog/2014/02/15/snow-eaters-melt-the-snow/ But in the intervening six years, I’ve learned a bit more about them, as well as upgrading my graphics and getting some better photos of what I’m talking about.

Chinook winds blow when warm moist air crosses a range of mountains. As the air mass crosses the mountains, it rises; as it rises, it dries.

As it crosses the ridge of the mountains, the now warm very dry air mass is held down by a strong jet stream. The warm dry air is trapped between the ground below and the jet stream above.

Lenticular clouds above the Flatirons west of Boulder Colorado. Fohn wall is behind them, to the left.

As the air moves from cold to warm, the clouds form and evaporate.

Fohn arch.

When the now warm very dry air hits the ground, we can have winds from 30-90 mph (48-145 km per hour). Chinooks blowing at these speeds can blow over semi-trucks. In the 1970s, extremely violent chinooks hit the National Center for Atmospheric Research, just west of Boulder, and pegged their anemometer (wind meter) at 120 mph (193 km per hour).

Broader, longer Fohn arch. The mountains are behind the trees.

Chinooks can raise the air temperature 40o (4o C) in minutes. Because they are dry winds, they can evaporate and sublimate (evaporate directly from snow or ice, without melting into water) snow at an astonishing rate. This is nice for getting ice off sidewalks, but chinooks have caused us to go from having a comfortable amount of snow in the mountain snowpack to needing water restrictions within the space of a day. This is why the Blackfeet Indians of Montana call it “snow eater.”

Chinooks blow on the lee side of mountains all over the world. In Europe, they are called Fohn winds; in California, Santa Anas. Loo winds blow off the Kirthar Range in Pakistan east onto the Indo-Ganges Plains in summer, making a hot season hotter.

Published by Amy Law

Amy Law is a science geek. She feels about science the way some people feel about music, or art, or sports – a total and complete emotional connection. She thinks in science. For Amy, there’s nothing better than helping people see the beauty of science as she does. She loves to untangle a complicated subject into its parts, explaining it so that anybody can understand what’s happening. Let her show you her world...

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