by Dr. Oakleigh Thorne, II
As I was starting to write this article on an early October day, we were having a strong Chinook wind in Boulder, particularly at our Thorne office at the Sombrero Marsh Environmental Education Center. So what is a Chinook wind?
It was originally named after the native Chinook people in the Pacific Northwest’s lower Columbia River basin in reference to a warming, moist wind from the ocean. It later became transferred as a term for the downslope winds we have on the eastern slope of the Rocky Mountains. In Germany, these same winds are referred to as the “foehn.” The cause of a Chinook wind was once explained to me in a college ecology class. When an air mass, moving rapidly from west to east, piles up against the Continental Divide (like west of Nederland), it forms a localized high pressure area. Then Boulder automatically becomes a low pressure area. So the air then rushes down the slope from the high pressure area at 12,000 feet to the low pressure area in town at 5,300 feet causing a Chinook wind. These winds are relatively warm and very dry.
If the air moves downslope from the very high mountains to the much lower altitude of Boulder, it undergoes what scientists call “adiabatic warming.” That is, by the laws of physics, it becomes much warmer and dryer as the air moves downslope. Because of this, these winds will often quickly melt any snow that is on the ground and also absorb much of the moisture. Folklore, therefore, has it that the word Chinook means “snow eater!”
In the many decades that I have lived in Boulder, I have actually seen the temperature rise so rapidly when a Chinook hits that one can actually observe the liquid moving up in the thermometer. Boulder is only about 18 miles from the Continental Divide to the west, so it’s a very steep gradient from Arapahoe Peak, for example, downslope to the city of Boulder. That makes us more subject to these extreme winds. The greatest 24-hour recorded change due to a Chinook wind was on January 15, 1972 in Loma, Montana when the temperature went from 54 degrees below zero to 48 degrees above zero!
Another characteristic of Chinook winds is that they are not steady, but rather are extremely gusty. A number of years ago when I was teaching field ecology at Naropa University, I took the class up to the National Center for Atmospheric Research (NCAR) on Table Mesa during a strong Chinook wind. The wind gusts were so strong that we could hardly stand up against them. Inside the NCAR building, we watched the wind gauge go from zero miles per hour (mph) to 104 in a matter of a few seconds! Many years ago, when I had my own wind gauge on top of our TV antenna in central Boulder, I recorded one brief Chinook gust at 142 mph. Fortunately for us, because these winds are so light and dry, they do not do as much damage as a heavy, moist hurricane does.
During a Chinook, you can usually see a wall or bowl of clouds over the high peaks to the west. This is called a “Chinook arch.” Look for it the next time the wind blows hard from the west.
Sometimes, when an air mass is not moving fast enough to create a Chinook wind, we will get a “Chinook condition” during the winter that gives us a lovely warm day with only a slight breeze. So Boulder definitely has what we call “a Chinook climate.” At any particular day during the winter, the ground is usually snow free. I think our weather is great!
Dr. Thorne is founder and honorary president of Thorne Ecological Institute in Boulder. They have helped “connect kids to nature” for more than 55 years. For more information about Thorne Natural Science School classes for children, check www.thornenature.org or e-mail email@example.com or call (303) 499-3647.