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Wesr E!k Mine • surface-based inversion is destroyed by solar heating, more vertical mixing of the air which is then associated with light and variable winds is able to occur and the surface air joins with the faster, more freely moving gradient flow at higher elevations. This effect occurs at all three sites, with the winds at Site 3 coupling about an hour sooner than those at Sites 1 and 2. Another contributor to the rise in wind speed during this period is upslope flow conditions arising primazily from convection due to the more intense heating of the canyon or mountain sides and tops than heating of the valley or canyon bottoms. This convection upslope flow is generally in the direction of the gradient flow and is further reinforced by the mechanism described previously. As the sun drops later in the day, the vertical mixing begins to diminish. This results in less vertical transfer of horizontal momentum from the upper level gradient flow to the surface layer and hence the wind speeds diminish at all three sites. As the sun sets, the nocturnal surface-based inversion begins to form. Once again, this results in the beginning of a drainage flow situation. As these drainage flows funneling through the canyon again begin to dominate at Sites 1 and 2, the average wind speed increases to its nighttime maximum. Being above the intense drainage layer, Site 3 experiences light drainage winds from higher ten•ain directly to the south which remain about 4 and 5 mph throughout the night. Wind Speed and Direction Because of the extreme topographical influence on the wind flows over the azea, joint frequency • distribution data between wind speed and direction, or wind roses, were developed for different segments of the day at each site. Complete data listings of all of these frequency distributions azr. compiled and aze graphically presented in Exhibit 24. Each wind rose shows the percentage of time over the reporting period that the wind blew from a particulaz direction by plotting a baz in the compass direction from which the wind blew, with a length proportional to that percentage of occurrence. At Sites 1 and 2 between midnight and 8:00 a.m., the drainage flow through the canyon is well established and virtually all winds during this period come out of the southeast. Although the major valley axis at this point runs from east-southeast to west-northwest, there is a bend in the canyon about 3,000 feet to the east which forces the flow to curve around and flow from the southeast. In addition, the minor tributary of Sylvester Gulch empties into the major valley immediately upstream from the monitoring site. Drainage from Sylvester Gulch flows due north, and as it merges with the drainage through the major valley itself, lends asouth-southeasterlycomponent to the flow. Between 8:00 a.m. and 4:00 p.m., three wind regimes aze present. During the eazlier part of this period, drainage flows continue. Because the minor drainage component from Sylvester Gulch, which most likely ceases almost immediately after sunrise, is not present, the flow exhibits more of aneast-southeast behavior. The light drainage through the main valley ceases about 10:00 a.m. After a short transitionperiod, upslope flows begin to dominate. • These upslope flows continue until about 4:00 p.m. The period from 4:00 p.m. to 8:00 p.m. is anotherperiod of transition. During this time, upslope flows decrease and downslope flows become 2..04-116 1 /94 7R05; 8/94 rR71; 6/95 PR06