Changing Stability » Vertical Motion Vertical Displacement of a Dry Stable Layer
Consider the case of a thin layer within some air mass. The air in the layer is dry (unsaturated) and has a uniform lapse rate. We then lift that layer with no divergence or convergence, so that the horizontal area of the layer doesn't change. As we lift the layer, the pressure drops, so its density decreases, and its volume increases correspondingly. To accommodate the increased volume, the layer deepens. Now the question is, did the layer become more or less stable when we lifted it?
To answer the question, let's first look at the temperature at the bottom of the layer. As the base of the layer rose, its temperature dropped, following the dry adiabatic lapse rate. Meanwhile, at the top of the layer, the temperature also dropped by the dry adiabatic lapse rate. However, the top of the layer ascended more than the bottom of the layer because the layer depth increased as the layer ascended. Therefore, the temperature at the top decreased more than the temperature at the bottom. The lapse rate moved toward the dry adiabatic lapse rate. If we lift the layer more and more, its lapse rate will progressively approach dry adiabatic. Thus, in the absence of divergence or convergence, the stability of a dry, ascending layer decreases, while the stability of a descending layer increases.