Stability Types, Lapse Rates, and Virtual Temperature (Tv)

The stability of an air parcel in a given layer of the atmosphere can be assessed on a thermodynamic diagram (for example, the skew-T/log-P diagram) by comparing the lapse rate of the virtual temperature (Tv) curve for the given layer with the lapse rate of the corresponding dry adiabat (for an unsaturated parcel) or saturation adiabat (for a saturated parcel).

In present forecasting practice, however, the ambient temperature (T) curve is used for these comparisons instead of the more exact Tv curve to allow faster evaluation of stability or instability. It should be noted, however, that in very warm, moist environments the substitution of T for Tv can be a source of significant error in evaluating stability.

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