A ratio, calculated solely from topographic (terrain elevation) data, of the diurnal temperature amplitude or range in a valley to that at the same altitude over the adjacent plain. The diurnal amplitude is larger (or “amplified”) over the valley than over the plain because the volume of air is less in the valley, but the amount of heating or cooling is approximately the same in both locations. The significance of the temperature differential caused by this amplification is that it produces a horizontal pressure gradient between the valley and the surrounding plains, or along the axis of the valley itself, that reverses twice per day, driving the diurnally varying along- valley wind system, upvalley during the day and downvalley at night. See upvalley wind, downvalley wind.