Table of Contents
What Is Virtual Temperature?
Virtual temperature is the temperature that dry air would need to have in order to have the same density as moist air at the same pressure. Since water vapor (molecular weight 18) is lighter than the average dry air molecule (molecular weight ~29), moist air is actually less dense than dry air at the same temperature and pressure.
Virtual temperature is widely used in meteorology because it allows the ideal gas law to be applied to moist air using the dry air gas constant. This simplifies calculations of air density, pressure altitude, and atmospheric stability.
Virtual Temperature Formula
Where T is the actual air temperature in Kelvin and w is the mixing ratio in kg/kg (mass of water vapor per mass of dry air).
Typical Corrections
| Temp (°C) | RH 50% | RH 100% |
|---|---|---|
| 0 | +0.3°C | +0.5°C |
| 20 | +1.2°C | +2.5°C |
| 35 | +3.5°C | +7.0°C |
Frequently Asked Questions
Why is moist air less dense?
Water vapor molecules (H2O, MW=18) replace heavier nitrogen (N2, MW=28) and oxygen (O2, MW=32) molecules in the mixture. The overall average molecular weight decreases, making moist air lighter.
When does virtual temperature matter?
It is most important in tropical meteorology where moisture content is high. At temperatures below 0°C, the correction is small (less than 1°C). At 35°C with high humidity, it can exceed 5°C.
How is it used in aviation?
Virtual temperature is used to calculate density altitude more accurately. Higher virtual temperature means lower air density, which degrades aircraft performance and requires longer runways for takeoff.