3. Stability

Determining Stability » Stability Types Conditionally Unstable - 2

Skew-T showing conditionally unstable lapse rate with no CAPE

Conditional instability is commonly used in two ways: the lapse-rate definition (just described) and the available-energy definition, examining the positive buoyant energy of a parcel (to be described here).

Schultz et al. (2000) described a state of conditional instability as a statement of uncertainty with regard to stability. Not all conditionally unstable atmospheres lead to unsettled weather. Because moisture is not accounted for in assessing conditional instability, some measure of the moisture profile is needed to refine the classification of stability. Thus, the concept of available energy was introduced by Normand (1938), who further subdivided conditional instability into additional classifications based on what we now term convective inhibition (CIN) and convective available potential energy (CAPE)convective inhibition (CIN) and convective available potential energy (CAPE). When a conditionally unstable atmosphere is unsaturated, CAPE must be evaluated to determine the degree of instability.

As the sounding above shows, if the conditionally unstable layer is surmounted by a stable layer such that there is no LCL and the CAPE is zero, no vertical displacement of parcels, however large, will produce any positive buoyancy.