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 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.

Snow-eaters melt the snow

Where did all the snow go? Last week at this time, the Front Range was entirely blanketed in several inches of the white stuff. Today, it’s almost gone. Where did it go?
The Answer: Chinooks.
Chinooks are warm dry winds. They get their name from a Pacific Northwest Indian word for “snow eater,” because when the chinook blows, the snow goes. With gusts that blast up to 120 miles per hour, pushing the temperature up 40 degrees F in minutes, chinooks along the eastern side of the Rockies are the most violent in the world. You always suspected that, didn’t you?
What causes a chinook? Chinooks are cool air pushed down by the weight of a high pressure front above them. Squeezing the cool air warms it up, to become warm dry, air. In winter and spring, chinook winds often buffet the areas just east of the mountains from Canada all the way to Mexico.14-Chinook
At ground level, the now-warm dry air is no longer under pressure. Like any compressed gas, it expands when released, and rips along the surface at 50 to 120 miles per hour as a chinook, melting and evaporating snow as it goes. By definition, chinook winds are warm and dry, and they suck the moisture out of plants, soils and people as they blow.

Foehn wall hanging above the Front Range (not visible in this photo).

Foehn wall hanging above the Front Range (not visible in this photo).

When a chinook blows, look toward the mountains. A bank of smooth clouds often hangs above the highest peaks. This is known as a foehn wall; it is created as the cool, wet, air crosses the mountains and is then forced down by high air pressure above that causes the chinook as well.
As the cool wet air containing the foehn wall is pushed down, it is compressed by the air above it; as it is compressed, it heats; as it heats, the amount of water it can hold increases (dew point rises) and the foehn wall cloud evaporates.
Chinooks also have the odd habit of “bouncing” over the land. They swoop down, blast some poor place with hurricane-force winds and then bounce back up, totally skipping over a locale just a few miles down the road.
In California chinooks are called Santa Aña winds, and in Europe, foehn (German for “warm”, pronounced “phone”. Actually, I heard a native German-speaker pronounce this word and it was closer to the way Peter Sellers and Steve Martin did it in the “Pink Panther” movies: “fun” as in “Your fun is ringing.”).