POTENTIAL INSTABILITY

(also known as Convective Instability) is said to exist when forced lifting (e.g. ascent over mountains or broad-scale/dynamic ascent) causes a layer, initially (just) stable to such forced ascent to become unstable. Decreasing humidity aloft is required within the layer, and heavy rain/thunder can be the result. Theta-W (q.v.) difference charts are often used to find such areas of potential instability: the usual levels used are at 850hPa and 500hPa. The value at 850hPa is subtracted from that found for 500hPa, and negative values so found indicate potential instability. Only slightly negative differences can lead to some significant convective activity..all other factors being favourable of course.