conditional instability An atmospheric condition in which otherwise stable air, on being forced to rise (e.g. over a mountain barrier), cools at a rate less than that at which the temperature drops with height in the surrounding air. The rising air therefore becomes warmer than the surrounding air, and so continues to rise. This lesser rate of fall of temperature in the rising air is owing to the condensation that occurs, as this is accompanied by a release of the latent heat of condensation. In such cases the instability is thus conditional upon the relative humidity of the rising air. See also instability.

conditional instability Atmospheric condition in which otherwise stable air, on being forced to rise (e.g. over an orographic barrier), cools at a rate less than that at which the temperature drops with height in the surrounding air. The rising air therefore becomes warmer than the surrounding air, and so continues to rise. This lesser rate of fall of temperature in the rising air is due to the condensation that occurs, as this is accompanied by a release of the latent heat of condensation. In such cases the instability is thus conditional upon the relative humidity of the rising air. See also INSTABILITY.