The virtual temperature T_v = T(1 + r_v/ ε)/(1 + r_v), where r_v is the mixing ratio and ε is the ratio of the gas constants of air and water vapor, ≈ 0. 622. The virtual temperature allows the use of the dry-air equation of state for moist air, except with T replaced by T_v. Hence the virtual temperature is the temperature that dry dry air would have if its pressure and density were equal to those of a given sample of moist air. For typical observed values of r_v, the virtual temperature may be approximated by T_v = (1 + 0. 61 r_v) T. Some authors incorporate the density increment due to liquid or solid water into virtual temperature, in which case the definition becomes T_v = T(1 + r_v/ε)/(1 + r_v + r_l) ≈ T(1 + 0. 61r_v − r_l), where r_l is the liquid or liquid plus solid water mixing ratio.