Because of the expansion of warm air, this column of air is much thicker and lighter than over the poles. At the poles Earth receives little insolation and the column or air is less thick and heavier. This differential in insolation sets up a circulation that transports warm air from the Tropics poleward aloft and cold air from the poles equatorward on the surface. Modifications to this general circulation are discussed in detail in Unit 3. Some of the radiation makes several trips, being absorbed, reflected, or reradiated by Earth or the atmosphere.
Insolation comes into the atmosphere and all of it is reradiated.
How many trips it makes while in our atmosphere does not matter. The direct absorption of radiation by Earth and the atmosphere and the reradiations into space balance. Although radiation is considered the most important means of heat transfer, it is not the only method. There are others such as conduction, convection, and advection that also play an important part in meteorological processes. These will be discussed in more detail later-in this unit .