A daytime, thermally forced, along-valley component of along-valley wind systems from the direction of the plains or valley toward the mountains, produced by diurnal heating of the valley air; a daytime along-valley component of the mountain-valley wind systems encountered during periods of light synoptic or other larger-scale flow. The mechanism of the upvalley wind is as follows. Air in the valley heats faster than an equivalent vertical column of air over the adjacent plains as a result of 1) the difference in the ratio of the volume of air heated in each location to the horizontal area intercepting radiation and 2) subsidence over the middle of the valley compensating for the flow up the slopes. The more effective heating in the valley produces a pressure drop in the valley relative to the plains, and this in turn produces an upvalley wind, called the valley breeze, beginning in late morning one to four hours after sunrise. It often reaches 3–5 m s⁻¹ at the surface and >5 m s⁻¹ above the surface. Mature upvalley flows tend to fill the valley, that is, the depth of the upvalley flow is approximately equal to the depth of the valley. See Also topographic amplification factor.