Winds in excess of 15 knots are rare and only occur when the mountain breeze is severely funneled. Drainage winds are cold and dry. Adiabatic heating does not sufficiently heat the descending air because of the relative coldness of the initial air and because the distance traveled by the air is normally short. Drainage winds have a very localized circulation. As the cold air enters the valley below, it displaces the warm air.
Temperatures continue to fall.
If the flow achieves speeds of 8 knots or more, mixing results between the warm valley air and the cold descending air that results in a slight temperature increase. Campers often prefer to make summer camps at the base of mountains to take advantage of the cooling effect of the mountain breeze. There are two types of tertiary circulation caused by local heating valley breezes and thermals. When the valley walls and mountain slopes are heated by the sun during the morning hours, the air next to the ground is heated until it rises along the slopes. Rocky or sandy slopes devoid of vegetation are the most effective heating surfaces.