The “tendency equation” is applied to analyze the pressure changes produced in wave-shaped westerly flow. With sufficiently strong westerlies horizontal divergence and convergence occur in such distribution as to cause eastward displacement of troughs and crests. When the westerlies drop under a “critical speed” the distribution of divergence and convergence is reversed. In the normal case of supercritical speed and increasing west wind with height, incipient waves will develop thermal asymmetry and will intensify. The same kind of irreversible growth of incipient waves would occur in easterlies which increase with height. Therefore, the temperate westerlies in all seasons, and the subtropical easterlies in summer and fall, are the regions of dynamic instability, where, respectively, the extratropical and the tropical cyclones are generated. The tendency equation applied to closed circulations shows such distribution of pressure rise and fall as to make the patterns drift westwards unless they are strongly “eccentric.” The main reason for the usual eastward drift of closed isobar patterns in the temperate latitudes lies, however, in the fact that the superimposed wave pattern in the upper layers produces overcompensating accumulation of air where the low levels show depletion, and depletion of air where the low levels show accumulation. The behavior of the composite low and high-level depression is discussed from this viewpoint.