Valley Winds

Valley winds are diurnal winds that flow upvalley by day and downvalley at night. They are the result of local pressure gradients caused by differences in temperature between air in the valley and air at the same elevation over the adjacent plain or larger valley. This temperature difference, and the resulting pressure difference and airflow, reverses from day to night . During the day, the air in the mountain valleys and canyons tends to become warmer than air at the same elevation over adjacent plains or larger valleys.

One reason for the more intensive heating of the mountain valley air is the smaller volume of air in the valley than over the same horizontal surface area of the plain. The rest of the volume is taken up by landmass beneath the slopes. A valley may have only from one-half to three-fourths the volume of air as that above the same horizontal surface area of the plain.

Another reason is the fact that the mountain valley air is somewhat protected by the surrounding ridges from the general wind flow. The valley air is heated by contact with the slopes, and the resulting slope-wind circulation is effective in distributing the heat through the entire mass of valley air. As the valley air becomes warmer and less dense than the air over the plain, a local pressure gradient is established from the plain to the valley, and an upvalley wind begins.

Whereas upslope winds begin within minutes after the sun strikes the slope, the upvalley wind does not start until the whole mass of air within the valley becomes warmed. Usually this is middle or late forenoon, depending largely on the size of the valley. The upvalley wind reaches its maximum speed in early afternoon and continues into the evening. Upvalley wind speeds in larger valleys are ordinarily from 10 to 15 m.p.h. The depth of the upvalley wind over the center of the valley is usually about the same as the average ridge height.

Strong upvalley and upcanyon winds may be quite turbulent because of the unstable air and the roughness of the terrain. Eddies may form at canyon bends and at tributary junctions. Along upper ridges particularly, the flow tends to be quite erratic. Wind speed and direction may change quickly, thus drastically affecting fire behavior.

Slopes along the valley sides begin to cool in late afternoon and, shortly after they come into shadow, cool air starts flowing downslope. Cool air accumulates in the valley bottom as more air from above comes in contact with the slopes and is cooled. Pressure builds up in the valley, causing the upvalley wind to cease. With continued cooling, the surface pressure within the valley becomes higher than the pressure at the same elevation over the plain, and a downvalley flow begins.

The transition from upvalley to downvalley flow takes place in the early night--the time depending on the size of the valley or canyon and on factors favoring cooling and the establishment of a temperature differential. The transition takes place gradually. First, a downslope wind develops along the valley floor, deepens during the early night, and becomes the downvalley wind. The downvalley wind may be thought of as the exodus or release of the dense air pool created by cooling along the slopes. It is somewhat shallower than the upvalley wind, with little or no turbulence because of the stable temperature structure of the air. Its speed is ordinarily somewhat less than the upvalley wind, but there are exceptions in which the downvalley wind may be quite strong. The downvalley wind continues through the night and diminishes after sunrise.

Valley winds and slope winds are not independent. A sloping valley or canyon bottom also has slope winds along its length, although these winds may not be easy to distinguish from valley winds. Proceeding upstream during the daytime, the combined flow continually divides at each tributary inlet into many upravine and upslope components to the ridgetops. As the valley-wind system strengthens during the day, the direction of the upslope wind is affected. The first movement in the morning is directly up the slopes and minor draws to the ridgetop. Then, as the speed of the valley wind picks up, the upslope winds are changed to a more upvalley direction. By the time the valley wind reaches its maximum, the slope winds, on the lower slopes at least, may be completely dominated by the upvalley wind. Along the upper slopes, the direction may continue to be upslope, because the upvalley wind does not always completely fill the valley.

Nighttime downslope winds are similarly affected. When the downvalley wind is fully developed, it dominates the flow along the slopes, particularly the lower portion, so that the observed wind direction is downvalley.