3. Stability

Changing Stability » Vertical Motion Vertical Motion vs Divergence/Convergence

In the preceding pages, we demonstrated that ascent without divergence and convergence without vertical motion both act to destabilize a layer. Similarly, descent without convergence and divergence without vertical motion both act to stabilize a layer.

This fact has a practical significance in the lower atmosphere, where, because of the constraining boundary at the earth's surface, divergence must always accompany descent and convergence must always accompany ascent. Therefore, in this region it is customary to regard the effects on the lapse rate of divergence-combined-with-descent (or of convergence-combined-with-ascent) to be of the same kind and essentially inseparable.