6 - Hydrostatic equilibrium

Summary

Changes of thermodynamic state that accompany vertical motion follow from the distribution of atmospheric mass, which is determined ultimately by gravity. In the absence of motion, Newton's second law applied to the vertical reduces to a statement of hydrostatic equilibrium (1.16). Gravity is then balanced by the vertical pressuregradient force. This simple form of mechanical equilibrium is accurate even in the presence of motion because the acceleration of gravity is, almost invariably, much greater than vertical acceleration of individual air parcels. Only inside deep convective towers and other small-scale phenomena is vertical acceleration large enough to invalidate hydrostatic equilibrium.

Because it is such a strong body force, gravity must be treated with some care. Complications arise from the fact that the gravitational acceleration experienced by an air parcel does not act purely in the vertical. It also varies with location. According to the preceding discussion, gravity is large enough to overwhelm other contributions in the balance of vertical forces. The same holds for the balance of horizontal forces. Horizontal components of gravity that are introduced by the Earth's rotation and other sources must be balanced by additional horizontal forces. Unrelated to air motion, those additional forces unnecessarily complicate the description of atmospheric motion.