It is a stronger and deeper wind than the mountain breeze. It is difficult to isolate the valley breeze effect because of the prevailing gradient winds. Consequently, the valley breeze is much more likely to be superposed as a prevailing wind than is the mountain breeze, which by its very nature can develop only in the absence of any appreciable gradient wind. The valley breezes are generally restricted to slopes facing south or the more direct rays of the sun, and they are more pronounced in southern latitudes. They are diurnally strongest in the late afternoon and are seasonally strongest in summer.
Thermals are vertical convective currents that result from local heating.
They stop short of the condensation level. Thermal convection is the usual result of strong heating of the lower atmosphere by the ground surface. A superadiabatic lapse rate immediately above the ground is necessary to the development of strong thermals. They form most readily over areas of bare rock or sand, and in particular over sand dunes or bare rocky hills. In the presence of a moderate or fresh breeze, especially in a hilly terrain, it is impossible to distinguish between turbulent and thermal convection currents.