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Definition:
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A nocturnal, thermally forced along-valley wind produced as a result of nocturnal cooling of the valley air; a nocturnal component of the fair- weather mountain-valley wind systems encountered during periods of light synoptic or other larger-scale flow. Valley cooling is accomplished by the combined effects of draining cold air off the slopes by early-evening downslope (katabatic) winds, and upward motion with upward cold-air advection from the convergence of katabatic flows in the valley center. Air in the valley thus becomes cooler than air at the same level over the adjacent plain ( see topographic amplification factor), producing higher pressure in the valley. The pressure gradient drives a downvalley wind that begins one to four hours after sunset, persists for the rest of the night until after sunrise, and often reaches 7-10 m s-1 or more above the surface. The downvalley wind tends to fill the valley, that is, its depth is approximately the depth of the valley, and where mountains end and a valley empties onto the plains, the downvalley wind can become a cold-air valley outflow jet flowing out of the mouth of the valley. (Same as mountain breeze.)
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