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 are larger and stronger.
These winds are opposite from each other. While valley breezes occur when the warm air rises up the sides, warm air in a mountain breeze will rise up the middle.
Mountain and valley breezes form through a process similar to sea and land breezes. During the day, the sun heats up valley air rapidly while mountain remains relatively cooler. Convection causes it to rise, causing a valley breeze. At night, the process is reversed. During the night the slopes get cooled and the dense air descends into the valley as the mountain wind. These breezes occur mostly during calm and clear weather.