Section 4.3 Atmospheric stability
Warm parcels of air will ascend through the atmosphere, but cool as they ascend. Whether they will continue to rise or subside depends upon the stability of the atmosphere.
the environmental lapse rate (ELR) describes the actual atmospheric temperature change with altitude at a specific time and location
the dry adiabatic lapse rate (DALR) describes how a dry parcel of air (that is, a parcel where no condensation is occurring) will cool as it rises and expands. The DALR is typically \(10^\circ\) C/km.
the wet adiabatic lapse rate (SALR) describes how quickly a saturated parcel of air (where condensation is occurring) will cool as it rises and expands. The SALR is typically \(6^\circ\) C/km.
In other words, the SALR is lower than the DALR because the condensation of water releases latent heat to the surroundings. Saturated air thus tends to demonstrate more buoyancy than dry air.
Whether or not the atmosphere is stable depends upon the relative values of ELR, DALR, and SALR. The atmosphere is
absolutely unstable if DALR<ELR; in other words, even a dry parcel will cool more slowly than the surrounding environment and thus remain warmer (and more buoyant) than the surrounding atmosphere
absolutely stable if ELR<SALR; in other words, even a saturated parcel will cool more quickly than the surrounding environment and thus subside
conditionally unstable if SALR<ELR<DALR where saturated parcels will keep rising but dry parcels will subside
To summarize, the basic steps for cloud formation are:
lifting of air (via free convection or forced convection)
as the air rises, the temperature decreases
as the temperature decreases, the relative humidity increases
the relative humidity reaches 100%