42 BULLETIN AMERICAN METEOROLOGICAL SOCIETY Some Examples of Chinooks East of the Mountains in Colorado A. W. COOK AND A. G. TOPIL U. S. Weather Bureau, Denver, Colorado ABSTRACT The mechanics of chinooks as they occur in eastern Colorado were studied with the aid of upper-air data and cross sections of equivalent-potential temperature across the Rocky Mountain region. The conventional model of the chinook involving precipitation on the windward side of the range adjacent to the chinook area, is discarded in favor of a model that does not require the precipitation. A number of cases are studied. One complete example with all important charts and diagrams is presented to show the mechanics of the chinook as it occurs in the Colo- rado area. Some data on other examples are also given. INTRODUCTION 27, 1940 (FIG. 1) is an example of the extreme HE definition of chinook used in this paper rise that can occur during the night. In a short pe- is that applied by early settlers along the riod after midnight there was a rise of 25F°, the eastern base of the Rocky Mountains. temperature reaching 12° higher than the maxi- T mum of the previous afternoon. They used the term to apply to the warm, dry wind descending the eastern slopes of these mountains The maximum temperature in a chinook is pre- [17]. It is further restricted by usage of weather dicated by the potential temperature of the air de- forecasters in the Rocky Mountain area to refer scending the mountains; this may be different to cases where westerly winds are of sufficient from day to day according to advection of warmer strength to produce forced descent and adiabatic or cooler air at the level of the mountain crests and heating. passes, or by a greater velocity of upper winds The classical theory of the chinook is predicated stirring air down from an even higher level with upon the air rising on the windward slopes and greater potential temperature. There is a much cooling at the saturated adiabatic rate, then de- smaller diurnal range in temperature on a chinook scending on the leeside at the dry adiabatic rate. day (i.e., when the chinook blows all day) than In such a process the air arrives at the lee base of the mountains with an increase in temperature equal to the latent heat liberated during the con- densation while ascending the windward slopes. This theory is credited to Hann, who studied the foehn wind in northern Switzerland. That the foehn and chinook winds have much in common, there is no question. Several authors take more or less exception to portions of the original Hann theory [6] [7] [12] [13], principally in whether the ascending or precipitation phase is necessary, and what would cause warm air at high levels to descend and displace colder, more dense air at the surface. Hann later admitted that precipitation on the windward was not essential for a foehn [18]. SOME CHINOOK EFFECTS OBSERVED IN EASTERN COLORADO Chinooks in eastern Colorado can occur at any time, day or night. They are more pronounced at night because the normal temperature is lower FIG. 1. Thermograph trace showing a rapid rise in than in the daytime and the contrast between chi- temperature after midnight, produced by a chinook wind. nook conditions and normal conditions is greater. The temperature rose far above the maximum of the The thermograph trace at Denver for January 26- previous afternoon. Unauthenticated | Downloaded 10/05/21 10:16 AM UTC VOL. 33, No. 2, FEBRUARY, 1952 43 turbulence set up as the air spills over the mountain ranges. This is essentially a warm-frontal action but it will usually not show warm-front clouds and precipitation, because of subsidence in descending the mountain slope. Probably the simplest ex- planation of why the upper air descends the east slope is that the general pressure gradient in the lower cold air directs that air eastward, and it must be replaced by warmer air from the west. The second cause is subsidence. Air that de- scends along the mountain slope is warmed dynami- cally at the dry adiabatic rate. This should bring it to the base with the same potential temperature with which it started above the top of the range. Its previous history makes no difference, for the temperature above the mountain, as well as amount of compression heating, will be approximately the same whether the air came from a valley on the other side of the range or whether it has been at FIG. 2. Illustration of the extreme temperature drop the high level for several days and thousands of that can occur when a cold front from the north passes miles of travel. Observation shows the potential Denver at the same time that a chinook wind that has temperature at the bottom of the descent is not blown throughout the night ceases. quite as high as that at the level where the air started down; this discrepancy is probably due on an average day. There is often a rapid fall in principally to contact cooling and evaporation dur- temperature when the wind ceases, even for an ing descent, with mechanical mixing and radiation hour, at night, with an abrupt rise in temperature also contributing. The surface temperature dur- if the wind again reaches the surface. ing the chinook varies with the height from which The greatest temperature drop at the end of a the air is brought down. If upper winds are suffi- chinook occurs when it is coincident with the pas- ciently strong, they can bring down air from two sage of a cold front of polar air and the wind turns to five thousand feet above the main range of into the upslope direction. The magnitude of the mountains. closing temperature fall depends on the strength of the preceding chinook wind and the temperature contrast between the two air masses. The thermo- gram for Denver on January 17, 1940 (FIG. 2) shows an excellent example; in this case the tem- perature fell from 45°F to 17°F in the first hour and continued falling to 7°F below zero. The rise in temperature during a chinook may be attributed to three causes which are often con- current. The first, which is not essentially chinook effect but which is a component part of almost any type of warming, is replacement of a cold air mass by a warm air mass, i.e., by advection of warmer air. In many cases when a dense cold cP air mass moves out of Canada its western flank is pushed firmly against the wrestern mountain ranges. Weak disturbances often fail to dislodge this cold air mass and it remains until a strong pressure gradient de- velops with 40-60 mile per hour free-air westerly winds across southern Wyoming and northern FIG. 3. Six consecutive raobs taken at Denver during Colorado. These winds remove the cold air mass a chinook wind situation. All show advection of warm partly through momentum and hydrostatic forces air up to the 400-mb level. The last two show removal of and partly by eddy excavation due to mechanical the surface inversion by the chinook wTind. Unauthenticated | Downloaded 10/05/21 10:16 AM UTC 44 BULLETIN AMERICAN METEOROLOGICAL SOCIETY The third cause of a large rise in temperature during chinook conditions is the preventing or destroying of the normal night-time ground in- version. The daytime maximum temperature in a chinook may be only a little higher than might ordinarily occur with regular daytime heating, whereas if the chinook wind continues strongly throughout the night, a ground inversion does not form and the usual nocturnal fall in temperature (average about 25°) is not realized. If an in- version has already formed during the early part of the night and the temperature has fallen to near a normal minimum value, the break through of a chinook wind to the ground causes a very striking rise in temperature. The temperature reached under such night-time conditions often approaches ordinary daytime maxima. After the chinook has FIG. 4. Thermograph trace showing day-by-day warm- once broken through to the ground during the ing during the chinook due to advection of warm air, and night and continues to blow the next day, the ad- the warm night of the 20th-21st during which the tempera- ditional rise in temperature due to insolational ture scarcely dropped below 50°. heating is usually small compared to the insola- tional effect on normal days. Where a chinook 2100 MST on January 18, the cold air mass, hav- wind at the surface lets up for a short period dur- ing just moved in, was at its lowest temperature at ing the night, even for less than an hour, the tem- all levels. In the four following soundings this air perature rapidly drops several degrees only to rise became steadily warmer at all levels—an excellent sharply as soon as the wind breaks through again. example of the advection of warm air aloft coinci- dent with a chinook. The last two raobs show the EXAMPLES OF COLORADO CHINOOK SITUATIONS characteristic lack of a ground inversion and the FIGURE 3 shows six consecutive raobs at Denver near-adiabatic lapse rate below 12,000 feet MSL at 12-hour intervals during a chinook period. At once the chinook is well established. The thermo- FIG. 5. Surface map for a chinook, showing the packing of isobars across southern Wyoming and western Colorado, January 20, 1943, P.M. Unauthenticated | Downloaded 10/05/21 10:16 AM UTC VOL.