To understand large-scale motions of the atmosphere, it is essential that the Aerographers Mate study the primary or general circulation of the atmosphere as a whole.

The suns radiation is the energy that sets the atmosphere in motion, both horizontally and vertically. The vertical motion is caused by the rising and expanding of the air when it is warmed, or the descending and contracting of the air when it is cooled. The horizontal motion is caused by differences of atmospheric pressure; air moves from areas of high pressure toward areas of low pressure. Differences of temperature, the cause of the pressure dif-ferences, are due to the unequal absorption of the Suns radiation by Earths surface. The differences in the type of surface; the differential heating; the unequal distribution of land and water; the relative position of oceans to land, forests to mountains, lakes to surrounding land, and the like, cause different types of circulations of the air. Due to the relative position of Earth with respect to the Sun, much more radiation is absorbed near the equator than at other areas, with the least radiation being absorbed at or near the poles. Consequently, the principal factor affecting the atmosphere is incoming solar radiation, and its distribution depends on the latitude and the season. Unit 3 discusses general circulation in lesson 1; secondary circulations in lesson 2, including a discussion on the vertical extension of pressure systems; tertiary circulations in lesson 3; and concludes with a practical training exer-cise for the entire unit.