The component of topographically generated winds that are parallel to the valley axis and can occur during conditions of light or calm synoptic-scale winds such as those near the center of anticyclones. At night, cold, dense air flowing down the river valley from the higher elevations is called a mountain wind or drainage wind with typical depths of 10 to 400 m and speeds of 1 to 8 m s−1, while the weaker return circulation aloft is called an anti-mountain wind. During the day, the gentle up-valley flow of warm air along the valley floor is called a valley wind, and the return circulation aloft is called an anti-valley wind. Compare cross-valley winds, anabatic wind, katabatic wind.