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. See drainage wind, along-valley wind systems.