The mountain and valley breeze circulation is a thermal induced air circulation. When the mountain sides are warmed due to the sun they in turn warm the lowest level of air. The warmed air begins to ascend accelerated the mountain side as a valley breeze. The ascending air masses are replaced by the air from the valleys. In the evening and at night the circulation changes, because the mountain sides cool more than the free atmosphere and the lowest air layers. Consequence is the mountain breeze, which blows down to the valley.