The instability of a deep layer of air is automatically enhanced when the layer as a whole is raised by hills or a front, for instance. This is because the layer expands in thickness as it moves to lower pressures, and so the top of the layer rises more than the base. Therefore the top cools more, which implies destabilisation of the layer.
An additional factor arises if the layer is dry but conditionally unstable, because cooling by uplift will make it saturated and therefore actually unstable. Destabilisation by uplift is further enhanced if the top part of the layer is dry whilst the bottom is saturated. This condition is known as potential instability (or convective instability). The top cools at 10 K/km during ascent whilst the bottom cools less rapidly, at the appropriate SALR. So the layer becomes less and less stable. It can be shown that a layer is potentially unstable if the equivalent potential temperature (Section 7.2) decreases with height, just as absolute instability occurs when the potential temperature decreases with height. Potential instability occurs on the Australian east coast or the coastal plains of Uruguay and Argentina, for instance, when warm moist air from the tropics flows under a dry westerly airstream aloft.
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