Temperature to which absolutely dry air would have to be brought in order for it to have the same density as moist air, considered at the same pressure.
The temperature a sample of dry air at pressure would have in order that its density equal that of the sample of moist air at temperature , pressure , and water vapor mixing ratio . It is given by a If there is also liquid water in the air this is modified as where is the liquid water mixing ratio (in grams of liquid water per gram of dry air). The virtual temperature is thus defined because it allows the ideal gas law to be used for situations in which the air is not dry, i.e. moist air of temperature behaves identically to dry air of temperature .
The temperature at which dry air would have the same density as moist air. The introduction of moisture makes the air less dense. The difference between virtual and actual temperature is very small in cold air which cannot hold much moisture, but in tropical air the virtual temperature may be several degrees higher than the actual temperature.