A mountain breeze and a valley breeze are two related, localized winds that occur one after the other on a daily cycle. They are not the same as katabatic winds, which usually occur on a larger scale.
Mountain and valley breezes form through a process similar to sea and land breezes. During the day, the sun heats up valley air rapidly. This causes it to rise, causing a warm, upslope wind. At night, the process is reversed. Mountain air cools rapidly at night and "falls" downslope, causing a wind going in the other direction.[1]