From AMS Glossary
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(Also called density temperature.) The virtual temperature Tv = T(1 + rv/ ε)/(1 + rv), where rv 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 Tv. 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 rv, the virtual temperature may be approximated by Tv = (1 + 0.61 rv) T. Some authors incorporate the density increment due to liquid or solid water into virtual temperature, in which case the definition becomes Tv = T(1 + rv/ε)/(1 + rv + rl) ≈ T(1 + 0.61rv - rl), where rl is the liquid or liquid plus solid water mixing ratio.