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. Pacific coast and settles over the The Black Hills is an elongated Salt Lake basin and intensifies. The anticlinal mass, irregular in outline, pressure areas are now ideally lo- in length about 125 miles trending cated to produce a chinook wind over north-northwest and south, by about the eastern slope of the Rocky Moun- 65 miles in width, culminating in tains in Wyoming and Montana. peaks 7200 feet in elevation above This pressure situation results in a sea-level, being the highest point be- steady flow of air ranging in direc- tween the Rocky Mountains proper tion from east through southeast and and the Atlantic seaboard. Ever- south to southwest or west. The green trees, spruce in the gulches and movement of the cold easterly wind pine on the mountain slopes up to is usually light, while the southwest timber line, cover the greater portion or west winds may develop a rather of the area, and the elevation is suffi- strong gale, dependent upon the ciently great that the westerly winds steepness of the barometric gradient blowing over it from a high to a low

Unauthenticated | Downloaded 10/05/21 01:30 PM UTC pressure area develop chinook condi- Probably the most noteworthy rec- tions on the eastern slope. ords of sudden temperature rises at Since the chinook winds of the Rapid City, due to chinook winds, are Black Hills are often swift and the as follows: January 13, 1913, a rise heating rapid, it follows that the of 64°, from —17° to 47° in 14 most pronounced of them are quite hours; and December 29, 1933, a rise likely to be accompanied also by fre- of 67°, from zero at 8 p.m. Decem- quent strong gusts and general tur- ber 28th to 67° above at 2 p. m. De- bulence. Ordinarily the abrupt or cember 29th, an 18-hour period, being sudden rises in the temperature at the greatest temperature rise in any Rapid City are due to the same gen- 24-hour period of record at Rapid eral processes involved in the Rocky City. The humidity readings during Mountain chinook, or possibly in some the period of this phenomenal tem- cases a continuation of the Rocky perature rise were as follows: 1:45 Mountain chinook. In the more de- a. m., 95 % ; 6 a. m., 30 % ; 9 :45 a. m., cided cases there is precipitation on 29%; Noon, 52%; 1:45 p.m., 21%; the western slope of the Black Hills, 6:00 p. m., 33%. which would not seem likely if they Aside from the direct chinook were but continuations of chinooks winds, there are two general causes from the Rockies, since the air masses for temperatures in the Black Hills would be too dry upon reaching the in the winter season being higher than Black Hills. Practically every winter on the surrounding plains. brings to the Black Hills several of Many periods of high temperatures these abnormal temperature rises, occur on the eastern slope of the many of which are more pronounced Black Hills that are less pronounced than when they made their appear- in the suddenness of the temperature ance on the eastern slope of the rise, and more prolonged in duration Rockies, being perhaps accentuated than those of the typical chinook wind successively upon coming in contact periods. These are caused by a cur- with Big Horn Mountains of Wyo- rent of warm air overriding a mass ming and later the Black Hills. of stagnant cold, dense surface air During 1915 the several outstand- in the basins of Wyoming and adher- ing chinooks in the Black Hills pre- ing to earth upon and after coming vented the mean temperature from in contact with the Black Hills. The reaching what otherwise probably air pressure conditions are similar would have been the coldest year of to those of the abrupt temperature record at Rapid City; during January, change type of winds but the warm November and December there were air on descending the eastern slops of six days upon which the temperature the Rocky Mountains comes in con- changes were, so unusual as to merit tact with the cold mass and glides mention: along its sharp upper surface; con-

Date Temp. Rise Max. Wind Prev. Dir. Jan. 4 21° in 2 hrs. 26 miles SW Jan. 12 28° in 30 min. 33 S & SW Nov. 18 20° in 1 hr. 44 W Nov. 22 22° in 1 hr. 34 W Nov. 23 27° in 15 min. 19 W Dec. 22 21° in 1 x/4 hrs. 39 W

Unauthenticated | Downloaded 10/05/21 01:30 PM UTC tinuing eastward in passing over the attributes ordinarily associated with Big Horn Mountains the chinook pro- chinook winds. A necessary combina- cess is repeated with further drying tion is, of course, a pressure condi- and warming as a result, and in the tion such as to cause a strong west- lee the warm currents again override to-east circulation and the attendant the cold, dense air in the basins. mechanical downdraft on the lee side After passing over the summit of the of the Hills. Undoubtedly at times Black Hills the mechanical turbulence a subsidence effect enters into the caused by the Hills, wears away by general phenomena, the inevitable friction the cold land layer on the temperature rise where the air lee side and here the warm air first spreads out aloft accentuating the reaches the surface east of the so-called chinook effect. Rockies. Therefore, the conclusion is t/iat In the winter season it is not an the dry chinook winds of the pro- unusual condition to have steep in- nounced type occur on the eastern versions in temperature in the gen- slope of the Rocky Mountains and eral area of country embracing the over the Black Hills area under Black Hills, with the temperatures definitely defined meteorological con- near the surface decidedly lower than ditions and a forecast of their ap- at elevations of 1000 to 6000 feet, proach is no more uncertain of veri- especially is this true in the early fication than the ordinary daily mornings. It may be inferred that weather forecast; but the sudden this rather normal winter inversion local temperature changes in the effect, undoubtedly at times in com- Black Hills, due to inversions in tem- bination with other effects, plays an perature and sibsidence effects, not important role in causing the occa- wholly indicated by the attendant sional rapid rise in temperature at meteorological conditions, are more Rapid City, not strictly attributable difficult to foretell with only surface to chinook effects, nor having all the observations available.

ABSTRACTS, MINNEAPOLIS MEETING1 Practical Uses of Weather Data in Public Utility Operations By A. C. BRAUN, Assistant Engineer, Wisconsin Michigan Power Co., Appleton, Wis. Actual uses to which weather data generation. In budget preparation has been put, using records obtained operating costs are affected by both from 8 years' operation of an ob- rainfall and temperature trends. Coal server's weather station maintained storage piles gain in tonnage with by the Wisconsin Michigan Power heavy precipitation and spontaneous Company, are related in this paper. combustion occurs or is absent as a In the operation of an extensive result. Steam boiler operating costs electric distribution system, load dis- are directly affected to some extent. patchers make use of weather maps supplemented by local anemometer Gas sales curves are in direct rela- and barometer indications. Kilowatt tion to monthly mean temperatures. hour generation is influenced by pre- Soil temperatures affect the accuracy cipitation and bears a direct relation of gas meter measurements. to the amount of hydro or steam The number of bus passengers varies directly with good or bad 1For program, see June-July, 1935, BUL- weather. LETIN.

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