Downloaded 10/05/21 01:30 PM UTC Enced a Chinook

transition zone (V)—the zone at a than the dry adiabat; in the case discontinuity wherein the proper- of saturated air, greater than the ties are characteristic neither of one saturation adiabat . air mass nor the other, but lie some- where between the two. It is now vapor pressure (II)—the partial pres- customary to assume that all the sure of the air exerted solely by air in the transition zone belongs the water vapor molecules. to the colder air mass, the air in ivarm front (V)—the discontinuity at warm sectors being considered more the front of a warmer air mass nearly homogeneous. which is displacing a retreating unstable (I)—a vertical distribution colder air mass. of temperature such that particles warm sector (VII)—the air enclosed of air, because of their lesser or between the cold and warm fronts greater density than the surround- of a cyclone. ing air, will rise or sink of their wave disturbance (VII)—a deforma- own accord once given an initial tion produced along a front. These impetus up or down. For dry air waves travel along the discontinuity the unstable lapse rate is greater surface producing new disturbances. THE DRY CHINOOK WIND1 By HARLEY N. JOHNSON, U. S. Weather Bureau, Rapid City, S. D. Authorities agree that the name increased at least several degrees by "chinook" came from a tribe of In- the chinooks, which is markedly ad- dians who formerly resided in the vantageous for cattle, crops and in- vicinity of the mouth of the Columbia dustries. In the early spring, how- River. The name was originally ap- ever, the wind may be detrimental, plied to a warm, moist, southwest as several days of warm wind may wind in the north Pacific states, that cause tree buds to open and occasion- was supposed to come from the ally blossoms to burst forth, only to Chinook Indian villages,2 but in this be destroyed later by low tempera- discussion only the dry chinook of the ture. Consequently, fruit raising is eastern slope of the Rocky Mountains generally unsuccessful over the east- and the Black Hills is considered, ern slope of the northern Rocky which wind, because of its similarity Mountains, and trees are compara- to the tempering effect of the warm tively short lived due to the great winds of the North Pacific states, fluctuation in temperature in the was called "chinook" by early settlers winter and early spring. of Montana and Alberta. The real home of the chinook wind The frequency and duration of the is in Wyoming, Montana, Alberta, chinook wind is a factor of great im- and Saskatchewan, and in winter it portance as it affects the climate of is probably the most spectacular the region on the eastern slope of weather phenomenon in the region. the northern and central Rocky That the temperature will rise in a Mountains, especially in the winter few hours from many degrees below season when the severe cold of this zero, on a day when the ground is latitude is materially tempered by this covered by a foot or more of snow dry, balmy wind. The average an- and the air filled with ice needles, nual temperature of this region is to a temperature well above the freezing point, and the snow disap- xPaper read at the Minneapolis meeting, June, 1935. pear as if by magic and the water 2See Elliott, T. C. ; The Chinook Wind, run in the gutters, is hardly believ- Oregon Hist. Quart., Vol. 33, No. 3, 1932 ; able until one has actually experi- abstr. in BULLETIN, NOV. 1934, p. 269. Unauthenticated | Downloaded 10/05/21 01:30 PM UTC enced a chinook. A notable record, sections in the same latitude un- in this connection, was made near affected by the chinooks. Havre, Montana, on January 19, The winter temperatures of the 1892, when the temperature rose 43 region are among the lowest in the degrees in 15 minutes—from severe United States, and the snow cover cold, —6°, to mild, balmy air, 37° occasionally reaches a depth of from above with the arrival of the chinook. 6 to 14 inches on the level and of The chinook as applied to the east- several feet where it has been blown ern slope of the Rocky Mountains is by the wind into the gulches and a warm, dry westerly or southwester- canyons. At periods during the win- ly wind that blows down the moun- ter the extremely frigid air and the tain side and occasionally far to the snow render it difficult for range eastward over the plains, its velocity stock to get at the short-grass; the ranging from a gentle zephyr to high cattle drift with the wind, their legs winds. Its approach and the period become sore and bleeding from being of its continuation is uncertain. It cut by the crusted snow, and loco- may last a few hours or it may last motion is difficult; but in general the several days, depending on the move- winter range would be worthless and ments of the pressure areas which are starvation of cattle would be inevit- largely responsible for the wind. It able were it not that the snow seldom may occur at any season of the year, remains on the ground any length of but the occurrence of the well-de- time, despite the severity of the cli- fined radical-temperature-change-type mate, due to the remarkable influence is largely confined to the winter and of the chinooks. spring months, and such seldom last Any surface winds, regardless of more than a day or two at a time. the origin, in passing from an anti- Contrary to the general assumption, cyclone towards a cyclone center, are the radical type is seldom followed by necessarily forced upward over inter- either precipitation or a severe cold vening mountains, thereby causing wave. expansion and dynamic cooling. The The coming of a chinook in the ascending air soon reaches the dew winter season is welcomed by the point, it becomes cloudy, and precipi- stockmen, for its remarkable power to tation occurs on the windward side melt snow soon cleans the ranges, ex- of the mountain. This cooling pro- posing the short-grass to the benefit ceeds at the approximate rate of 1.6 of the range cattle. The abnormal degrees per each 300 feet of ascent, evaporation into the very dry air up to the point where condensation rapidly takes up the moisture from begins, above which the liberated heat the snow. Seldom is the ground of condensation reduces this cooling muddy after the disappearance of the rate about one-half. Once the rising snow. In t,he earlier days cattle air reaches the top of the mountain roamed at will in this region, both the precipitation ceases, the clouds summer and winter, and without become less or disappear, and it starts doubt it would be practically impos- downward on the lee side of the sible for unsheltered stock to have mountain, being adiabatically heated subsisted in the winter season, except as it descends at approximately the for the favorable influence exerted by dry rate for the whole of the descent. the chinooks. As a range country for During the descent the air rapidly wintering stock this section excels the becomes very dry. Therefore, upon Unauthenticated | Downloaded 10/05/21 01:30 PM UTC reaching the foot of the mountain the from the high area over the western air temperature may be 30 or 40° Plateau to the low centered over Sas- higher and the humidity much lower katchewan or the Dakotas, and the than it was at the same level on the rapidity with which the low area opposite side of the mountain where moves to the eastward. the air started its upward movement. With the approach of this westerly A typical set-up of pressure condi- or chinook wind there is usually a tions necessary for the development flat layer of cirro-stratus clouds in- of and indicative of the occurrence creasing in density to alto-stratus; the of a chinook wind is as follows: An wind starts with considerable sudden- extensive area of high pressure at- ness and the velocity quickly in- tended by low temperatures moves creases; the temperature rises rapidly, from the Canadian Northwest across and the humidity falls abruptly. As Alberta to Saskatchewan. In the the chinook progresses there is a meantime there is an area of moder- tendency for the clouds to disappear, ately low pressure and high tempera- although occasionally the sky remains tures over the inter-mountain region. partly cloudy to cloudy during the This results in an influx of abnorm- period of its duration. If the clouds ally cold air to the low, attended by disappear the sky takes on a deep much cloudiness and perhaps some blue color and the visibility becomes snow in Montana and Wyoming. The unlimited. high area then proceeds from Sas- To those who make weather maps katchewan southeastward to the Cen- daily, it is common knowledge that tral Valleys and an area of low pres- the temperatures reported from the sure from the north Pacific coast eastern slope of the Black Hills are advances eastward over British Co- often higher and out of agreement lumbia and Alberta to Saskatchewan, with the temperatures reported from then southeastward over northeastern the surrounding stations located on Montana, the Dakotas, and to the the Plains having otherwise appar- Missouri River Valley. Meanwhile ently the same general weather con- the low area over the Inter-n^ountain ditions. To understand these seem- region has shifted to the southern ing abnormalities of temperature the Plateau and a moderate high area chinook must be considered, especi- moves in from the middle and north ally during the winter season.

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