Valley winds, sometimes referred to as mountain breeze, are diurnal produced by local temperature and pressure differences with the valley or between a valley and nearby plain. The resultant flow is upvalley during the day caused by thermal heating and downvalley at night as a result of natural cooling.
Source: MetEd. 2011. MetEd Education and Training pages. Accessed 23 November 2011. https://www.meted.ucar.edu/