Determining Stability from Lapse Rates
In addition to tracing the path of air parcels, the stability can also be determined by comparing the environmental lapse rate to the dry and saturated adiabatic lapse rates.
Absolute stability occurs when the environmental lapse rate is less than the saturation adiabatic lapse rate.
Absolute instability occurs when the environmental lapse rate is greater than the dry adiabatic lapse rate. We refer to such lapse rates as superadiabatic.
Neutral equilibrium occurs when the environmental lapse rate equals the dry adiabatic lapse rate in an unsaturated environment or when the environmental lapse rate equals the saturated adiabatic lapse rate in a saturated environment.
If the environmental lapse rate falls between the dry and saturation adiabatic
lapse rates, then the environment is conditionally unstable. To assess the stability
for this case, we need to know something about the moisture content of the parcel.
If the parcel is saturated, it will exhibit moist absolute
instability.
If the parcel is unsaturated, then the stability will depend
on its CAPE:
If CAPE = 0, then the parcel is absolutely
stable.
If CAPE > 0, then the parcel becomes unstable
when lifted beyond the level of free convection (LFC).