Oftentimes during the winter, rapid and dramatic temperature fluctuations may occur in a region to the east of the Rocky Mountains that are called "chinook winds". One such rapid temperature rise is mentioned on pages 191 and 192 of the DataStreme Part A: Narrative. Another impressive increase in temperature occurred at Great Falls, MT on 11 January 1980 when the temperature rose 27 Celsius degrees (49 Fahrenheit degrees) in 7 minutes from -36 degrees Celsius (-33 degrees Fahrenheit) to -9 degrees Celsius (16 degrees Fahrenheit).
While the exact origin of the name is unclear -- either from the Chinook tribe from Oregon or the Native American word for "snow eater" -- this wind refers to a warm dry wind that descends from the mountains. The effect is often felt in a "Chinook belt" that extends roughly 250 to 300 km eastward from the crest of the Rockies and runs from central Alberta south to central New Mexico.
Although several different weather events can produce a chinook, one common situation results when a storm system and trailing cold front sweeps eastward from the Pacific and moves inland, ultimately crossing the northern Rockies. Some of the impressive chinook events occur in the chinook belt east of the Montana Rockies when the low pressure system redevelops on the eastern slopes of Alberta and a large high pressure cell remains farther south in the Great Basin. As a result of the strong pressure difference between these two systems, the winds become strong especially once the cold front passes across the Rockies and the winds turn to a more westerly or downslope direction.
Prior to the passage of the cold front, winds in central Montana may be weak and near-surface temperatures may be cool. Once the wind changes to a westerly direction, the temperature often times rises rapidly as a strong west wind over the peaks of the Montana Rockies causes air to descend from as high as 3000 m (10,000 feet) to 1000 m (3000 feet) on the high Plains. As the air sinks, it is compressed by increased air pressures at lower levels. The compression heats the air at the dry adiabatic rate, ending perhaps 20 Celsius degrees (35 Fahrenheit degrees) warmer than it begins. The water vapor content of the air remains near the initial amount so that the warming produces final relative humidities that are extremely low, thereby "eating the snow" through sublimation and evaporation.
In addition to rapid increases in temperature, chinook winds in the Montana chinook belt can also produce ground blizzards, damaging winds, a "false spring" which can cause plants to prematurely bud and drought conditions if repeated and prolonged chinook events persist.
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 6 Chapter Progress Response Form, under section B. Daily Summary.
For more information describing the various sets of lines on a Stüve Diagram and how you can make graphical determinations from a Stüve Diagram, you may consult the optional material in Thursday's electronic Supplemental Summary Information .