The valley breeze is a local thermal wind blowing upslope during the day. The sun heats the valley slopes, which in turn warm the air in contact; this less dense air rises, creating an ascending flow similar to a large-scale anabatic wind.
Daily cycle
At night, the process reverses: slopes cool by radiation, cold air drains to the valley floor (katabatic wind), establishing the mountain breeze. This daily cycle is fundamental for valley ventilation, dispersal of urban pollutants, and the formation of early morning radiation fog on the valley floor.