<P> The presence of water within the atmosphere complicates the process of convection . Water vapor contains latent heat of vaporization . As a parcel of air rises and cools, it eventually becomes saturated; that is, the vapor pressure of water in equilibrium with liquid water has decreased (as temperature has decreased) to the point where it is equal to the actual vapor pressure of water . With further decrease in temperature the water vapor in excess of the equilibrium amount condenses, forming cloud, and releasing heat (latent heat of condensation). Before saturation, the rising air follows the dry adiabatic lapse rate . After saturation, the rising air follows the moist adiabatic lapse rate . The release of latent heat is an important source of energy in the development of thunderstorms . </P> <P> While the dry adiabatic lapse rate is a constant 9.8 ° C / km (5.38 ° F per 1,000 ft, 3 ° C / 1,000 ft), the moist adiabatic lapse rate varies strongly with temperature . A typical value is around 5 ° C / km, (9 ° F / km, 2.7 ° F / 1,000 ft, 1.5 ° C / 1,000 ft). The formula for the moist adiabatic lapse rate is given by: </P> <Dl> <Dd> Γ w = g 1 + H v r R s d T c p d + H v 2 r R s w T 2 = g R s d T 2 + H v r T c p d R s d T 2 + H v 2 r ε (\ displaystyle \ Gamma _ (w) = g \, (\ frac (1 + (\ dfrac (H_ (v) \, r) (R_ (sd) \, T))) (c_ (pd) + (\ dfrac (H_ (v) ^ (2) \, r) (R_ (sw) \, T ^ (2))))) = g \, (\ frac (R_ (sd) \, T ^ (2) + H_ (v) \, r \, T) (c_ (pd) \, R_ (sd) \, T ^ (2) + H_ (v) ^ (2) \, r \, \ epsilon))) </Dd> </Dl> <Dd> Γ w = g 1 + H v r R s d T c p d + H v 2 r R s w T 2 = g R s d T 2 + H v r T c p d R s d T 2 + H v 2 r ε (\ displaystyle \ Gamma _ (w) = g \, (\ frac (1 + (\ dfrac (H_ (v) \, r) (R_ (sd) \, T))) (c_ (pd) + (\ dfrac (H_ (v) ^ (2) \, r) (R_ (sw) \, T ^ (2))))) = g \, (\ frac (R_ (sd) \, T ^ (2) + H_ (v) \, r \, T) (c_ (pd) \, R_ (sd) \, T ^ (2) + H_ (v) ^ (2) \, r \, \ epsilon))) </Dd>

When do you use the moist adiabatic rate