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The Snow Eating Wind

There really is not an easy way to explain chinook winds without getting into some pretty technical meteorology, but no discussion of Black Hills weather would be complete without mentioning the chinook. The reason is that the Black Hills of South Dakota are home to the world's fastest recorded rise in temperature, a record that has held for nearly six decades.
On January 22, 1943, the northern and eastern slopes of the Black Hills were at the western edge of an Arctic airmass and under a temperature inversion. A layer of shallow Arctic air hugged the ground from Spearfish to Rapid City. At about 7:30am MST, the temperature in Spearfish was -4 degrees Fahrenheit. The chinook kicked in, and two minutes later the temperature was 45 degrees above zero. The 49 degree rise in two minutes set a world record that is still on the books. By 9:00am, the temperature had risen to 54 degrees. Suddenly, the chinook died down and the temperature tumbled back to -4 degrees. The 58 degree drop took only 27 minutes.
The chinook was also doing strange things to the Rapid City temperature that same day. Around 10:30am, the thermometer recorded a temperature jump from 20 degrees to 56 degrees in 5 minutes, a 36 degree rise. Around noon, the temperature plummeted 47 degrees in 5 minutes from 60 to 13 above zero. About a half hour later, a 35 degree rise took place when the reading went from 15 to 50 degrees, and another precipitous drop transpired about 2:20pm when the temperature fell 41 degrees from 58 to 17 above. The chinook is known by many names in mountainous regions of the world. In North America, it is called chinook because it seemed to the first white settlers encountering the strange winds that they emanted from the territory of the Chinook Indians and the name stuck. In Switzerland, the wind is called schneefresser, meaning "the snow eater". In the Andes Mountains of South America it is known as zonda. Wherever these winds take place, there are tales of extraordinary happenings. The sudden temperature change has reportedly cracked windows, split wood and melted large amounts of snow and ice quickly. Some medical side effects of prolonged exposure to these winds have also been reported to include headaches, anxiety and irritability. Long periods of chinook winds have also been said to cause plants to come out of dormancy only to be destroyed when the cold weather returns as the chinook ends.

A chinook is a katabatic wind, meaning a wind that blows downhill. In an ideal chinook situation, the weather map would have low pressure to the northeast of the Black Hills, and mild high pressure to the southwest. This would set up a southwesterly wind flow from the high to the low across the Black Hills. As air moved up the western slopes of the hills, any remaining moisture would be lifted and condense. Condensation releases heat into the atmosphere. Then, the warmer, drier air would race down the eastern slopes of the Black Hills and the air molecules would compress as they moved downhill. A true chinook wind gets its heat from the combined effect of condensation at the crest of the mountain range and air compression as the wind moves downhill. The temperature of the air moving downhill would increase at the rate of about five and a half degrees for every thousand feet of drop. Over time, the temperature at the base of the mountains continues to climb as long as the chinook is blowing, but the most rapid rise is usually noted in the first several minutes of the event.

Predicting the exact time and extent of a chinook event is next to impossible during a Black Hills winter. While meteorologists may strongly suspect that chinook conditions will take place, pinpointing which portion of the forecast area will experience the event and how much the temperature will rise in a certain time frame is as much a matter of educated guesswork as anything else.

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