Conditional instability

From Glossary of Meteorology

conditional instability

  1. The state of a layer of unsaturated air when its lapse rate of temperature is less than the dry-adiabatic lapse rate but greater than the moist-adiabatic lapse rate.

    Under such conditions a parcel of air at the environmental temperature is unstable to upward vertical displacements if it is saturated, unstable to downward displacements if it is saturated and contains cloud water, but stable to all small vertical displacements if it is unsaturated. For descending air containing only rain water, the stability depends on both the lapse rate and the drop-size distribution. This definition does not require that such a parcel be obtained by adiabatic displacement from any level. It also does not require that the energy released from latent heating (CAPE) be greater than the convective inhibition (CIN) required to bring the parcel to its level of free convection.

  2. Similar to definition 1 except that it must be possible for a parcel displaced adiabatically from some level and with conservation of total water mixing ratio to attain the environmental temperature in a saturated state.

    The choice of usage of the term "conditional instability" has been uncertain and sometimes controversial for at least 50 years. Haurwitz defined it approximately as definition 1, and this has been most frequently accepted. However, Byers used a definition similar to definition 2. Beers separated the definition into three subdefinitions, "stable type," corresponding to definition 1 when a moist parcel cannot be obtained, and "pseudolatent"and "real latent," corresponding to definition 2 but with the last requiring essentially that CAPE be greater than CIN. Dutton subscribes to the Haurwitz definition, while Emanuel develops a definition similar to definition 2, but with elaboration similar to that of Beers.

    Haurwitz, B. 1941. Dynamic Meteorology. McGraw–Hill.

    Byers, H. 1944. General Meteorology. McGraw–Hill.

    Beers 1945. Atmospheric Physics. Handbook of Meteorology. eds. Berry, Bollay, and Beers.

    Dutton, J. 1995. Dynamics of Atmospheric Motion. Dover Press.

    Emanuel, K. 1994. Atmospheric Convection. Oxford Univ. Press, . 580 pp.