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.
See drainage wind, along-valley wind systems.
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