During the day parcels of air along valleys gets heated if valley face north(SH) or south(NH).
When the parcels are heated they become less dense and rise as Anabatic winds. Eventually,upon reaching the top of the mountain they condense producing copious rainfalls. During the night rapid terrestrial radiation leaves the top and valley sides very cold.
The air becomes heavy and start to drop down as Kanabatic winds. As the air reaches the bottom of the valley it can create dense fog or frost hollows.
