The prevailing fundamental atmospheric circulation on a planetary scale that must exist in response to 1) radiation differences with latitude, 2) the rotation of the earth, and 3) the particular distribution of land and oceans; and that is required from the viewpoint of conservation of energy. Primary circulation and general circulation are sometimes taken synonymously. They may be distinguished, however, on the basis of approach; that is, primary circulation is the basic system of winds, of which the secondary and tertiary circulation are perturbations, while general circulation encompasses at least the secondary circulations. See macrometeorology.