Chinook Winds: The Snow Eater

A Chinook wind can be a nice warming wind bringing instant spring in the middle of winter or they can turn into a howling destructive windstorm. Chinook winds can raise temperatures by 25 to 35 degrees in minutes, melt a foot of snow, and destroy property, sandblast cars, and blow trucks off of the road

Chinook is an Indian word that means “snow-eater”, which is exactly what they can do. Chinook winds are down slope winds and are most common on the east facing slopes of the Rocky Mountains, but also occur east of Sierra Nevada Mountains, the interior of Alaska and east of the Cascade Range in North America. These winds are caused by the same pressure differences that cause the Santa Ana winds of Southern California.


Why Chinook Winds are Warm


With Chinook winds, a high-pressure area will be to the west of the Rocky Mountains causing the winds blow down the eastern side to lower barometric pressure. The wind travels downhill picking up speed and heats the air as it goes. As the Chinook wind blows downhill, the air heats up by a process known as adiabatic heating or more accurately the dry adiabatic lapse rate.

The heating of the air occurs at a rate of 5.5 degrees F per 1,000 feet (10 C per 1000 Meters). For every 180 feet downhill the temperature can increase by 1°F and as the air warms, the humidity also drops.

To the west of the continental divide, the air moves up mountains producing snow. There can be a snowstorm in Vail, Colorado and just to the east of the Continental Divide it can be warm and very windy. As the air moves up on the western slope of the mountains, it drops all its moisture.

Then on the way down the eastern side of the Rocky Mountains it picks up speed and heats up as it blows down the mountains. When it hits the base of the foothills it spreads out as a strong Chinook wind, warming and drying the air.

Wildfires aren’t as much a danger with Chinook winds as with the Santa Ana winds since they usually occur in winter with snow cover on the ground. The Chinook winds can be much more powerful than the Santa Ana winds reaching over 100 mph with numerous instances of the winds blowing over 140 mph (225 kph) at cities like Boulder, Colorado and Livingston, Montana.

That would be the same as a category 4 hurricane. The main threat from the Chinook wind is the damage to buildings and cars. It is not unusual for cars to be sandblasted or having their windows blown out while driving on the Boulder Turnpike between Denver and Boulder during a Chinook windstorm.

What causes Chinook winds graph

The Snow Eater Wind


Chinook winds have caused some amazing temperature changes. Temperature rises of 20° to 40° F in one hour are common during a Chinook windstorm. Some dramatic temperature rises due to Chinook winds include:

·        The greatest 24-hour temperature change occurred on January 14-15, 1972, in Loma, Montana when the temperature rose 103 degrees in 24 hours from -54°F to 49°F.
·        On January 11, 1980, a temperature rise of 47° in seven minutes occurred in Great Falls, Montana.

The Black Hills of South Dakota, just east of the Rocky Mountains also have Chinook winds and wild temperature changes.

A world record for the most rapid temperature rise occurred in Spearfish, South Dakota, when on January 22, 1943, the temperature went from –4 degrees F (-20 C) to 49 degrees F (10 C) in just three minutes.

In Colorado, I have seen the temperature go from a bitter –25° for the morning low to an afternoon high in the mid 40s because of Chinook winds.

The Chinook wind cause the air to become so dry, you can see the static electricity arcing when touching a doorknob. And be careful touching your dog’s wet nose during these dry Chinook windstorms.

These dry Chinook winds can also have a negative affect on how you feel and your health. Chinook winds create positive ions, and positive ions put people in a bad mood, police departments always report an increase in violence during these dry winds.

Chinook Winds and Lenticular Clouds

Lenticular cloud in NM
Lenticular clouds in N.M. / Brandy Jenkins

Since the Rocky Mountains are so high, the jet stream can actually influence the strength of a Chinook windstorm. This also causes the air and the clouds to stay almost stationary over the mountains.

These stationary clouds over the 14,000-foot mountain peaks create what are called standing wave clouds or lenticular clouds. Their official cloud name is Altocumulus Standing Lenticular. These clouds are called lenticular because they are shaped into clouds that look like lenses and can be absolutely beautiful. They have also been called flying saucer clouds since they can resemble a flying saucer. 

Lenticular clouds can stay over one area for hours or most of the day. There can be a single Lenticular, or stacked lenticular clouds on top of each other. 


Chinook winds also cause what are called Chinook arch clouds or mountain wave clouds. These clouds are stationary and usually hang over the area just east of the Rocky Mountains all day and always seem to move east during the late afternoon, causing beautiful sunsets.

Mountain wave cloud at sunset
Chinook arch or mountain wave cloud at sunset

Chinook winds are called by different names in other parts of the world. In the Swiss Alps they are called the Foehn, Zonda in Argentina, Halny Wiatr in Poland and in Java they are called Koembang.

© 2009 Sam Montana
 

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