Winds that flow over the top of mountains will often come down the leeward side the range. When the airflow moves from the higher elevation to lower, there is compressional warming. If you compress air, it heats up and when it expands it cools. That is why oxygen under pressure comes out cold when released. In the American West, such a downslope wind is referred to a Chinook Wind. The word Chinook means “Snow Eater” because Native Americans had noticed that when a warm wind blew down the mountains, the snow would disappear. That would be from rapid sublimation, or the snow turning directly to vapor as a result of the influx of warm, dry air. It is said that in places on the Eastern Slope of the Rockies, Chinook Winds are so common that houses have sliding wooden shutters to protect windows from flying debris. Flying debris? Are the winds that strong? Yes…sometimes they are that strong.
On this date in 1972, The National Center for Atmospheric Research in Boulder, Colorado recorded a wind gust of 144 mph. That is nearly category 5 hurricane strength. South of Boulder at Rocky Flats, winds gusted to 105 mph and 90 mph gusts greeted downtown Boulder. Stapleton Airport in Denver reported gusts to 53 mph on January 11, 1972 and 47 mph the following day. In Boulder, more than 25 mobile homes were destroyed…some by wind and others by fire that erupted after they were overturned.
Buildings suffered major damage, car windows were blown out, traffic lights blown away and trees, power lines and utility poles were down. Some 75 families had to leave their apartment building due to severe structural damage. Flying debris caused much of the damage, much as one might expect in severe weather or a hurricane. Industrial complexes, government and private offices were evacuted out of fear of the results of flying matter and broken glass. 15 airplanes were damaged and hanger damage reported at Jefferson County Airport. In Boulder, the damage added up to 2 million dollars and total reports for damage in the area came to about $25 million.
So, what about the “snow eater” part. In Denver, temperatures climbed to the mid 50′s. It is not unusual for temperatures to rise 20 to 40 degrees in a matter of minutes. The winds typically come from the northwest or west with the mountain crest sporting the “chinook arch.” At the outset of a chinook, temperatures can vary wildly as warm air mixes with cold. So, one spot may feel a 30 degree temperature rise in 15 minutes only to feel the rush of much colder air before the warming begins again. In the Black Hills of South Dakota, the town of Spearfish in 1943 experienced a 49 degree rise in temperature in just 2 minutes. 90 minutes after that, the mercury dropped 58 degrees in less than a half hour. The low for the time frame was -4 F and a high of 54 F. The system in 1972 was not isolated as on January 15, 1972 there was a 103 degree rise in temperature in 24 hours in Loma, Montana where the mercury went from 54 below zero to 49 degrees above zero. Here are some other significant temperature rises: Granville, ND on Feb. 21,1918 went from -33F to 50F in 12 hours, Fort Assiniboine, MT rose from -5 F to 37F in 15 minutes, Dec. 1 1896 folks in Kipp, MT felt the mercury rise of 34 degrees in just two minutes and a total daily rise of 80 degrees and 30 inches of snow disappeared in half a day.
The United States is not the only place to get this type of phenomena, but North America is the only place where they are called Chinook. Elsewhere, they are known as Foehn Winds, but Chinook is much more fun. There is some notion that the term Chinook derived from people of a region in the Columbia River Valley, the Chinooks, who first reported the winds to the white settlers. But I like the Snow Eater story better.
Weather Bottom Line: The Monday snow was pretty lame. Not enough moisture. Now, some folks may get above freezing on Tuesday afternoon as the clouds break but I suspect that the official end of the below freezing temperatures will be on Wednesday afternoon or maybe even midday. The long wave pattern is finally changing and the cold air will retreat back into Canada. That will mark about 12.5 days since we were above freezing in Louisville, one of the longest stretches on record, though I believe the longest such freezing streak runs to 17 days. It’s going to be short of my early prognastication in late 2009 that we wouldn’t get above freezing until the 15th, but the 13th is close enough…or long enough. Now, we get to the 40′s Wed through Friday. There is still that big low that will develop in the Gulf. The southern branch of the jet stream will swing down out of the southwest into Texas and a low will form. It will then eject into the SE US. The jury is still out but at this time, most modeling data does not drag down cold air sufficiently to give us snow…but we’ll see.