The term ‘conditional instability’ refers to the state of a column of air when its vertical distribution of temperature is such that the layer is stable for dry air but unstable for saturated air. This occurs when moist air has a lapse rate between the dry and wet adiabatic rates (between 0.5°C and 1°C per 100 meters).
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In Figure 32.3, for the first 1750 meters the ascending air parcel is cooler than the surrounding air and it is, therefore, considered stable. Since above the condensation level the latent heat of condensation is returned to the rising air, further cooling takes place at the wet-adiabatic rate which is less than the environmental lapse rate.
However, the lapse rate being greater than the wet adiabatic rate, from the level of free convection upward the rising parcel of air is warmer than the surrounding air. From this level along its ascent the parcel would continue to rise without any impulse and is considered unstable.
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Thus, the air that was initially stable is made unstable by forced ascent during which latent heat of condensation is added to it. A column of rising air may be stable and unstable at different elevations as a result of condensation after sufficient uplift and cooling.
The word “conditional’ is prefixed because only if the air is forced upward initially can it become unstable. Conditional instability is said to be the most common type of instability.
In conditional instability the lapse rate is less than the dry-adiabatic and greater than the moist-adiabatic lapse rates. Besides, there are two most important prerequisites for this state of equilibrium: (a) the ascending air parcel should be moist or saturated, and (b) the air parcel should get a very strong initial uplift, so that it would reach the condensation level under the impulse.