Thermal and three-cell circulation
You know equatorial areas receive more direct solar radiation than do polar areas. Also, in the lower latitudes, the ratio of land to water is low. Since water loses heat slowly, more heat is received than lost through radiation. In the polar regions, just the opposite is true. The land to water ratio is high and more heat is lost due to radiation than is received. Since the Tropics do not become progressively hotter and polar regions colder, there must be a transfer of heat between the two regions or a general circulation. The mechanism that sets up Earth’s general circulation pattern is the latitudinal transfer of heat.
Thermal circulation
If the Earth’s surface were smooth, uniform, and stationary, atmospheric circulation would be very simple. The atmosphere would act as a contained fluid and movements within this fluid would be the convective currents caused by temperature and density differences. The latitudinal transfer of heat would result in a single circulation cell.