# Equivalent temperature

In atmospheric science, equivalent temperature is the temperature of an air parcel from which all the water vapor has been extracted by an adiabatic process.

Air contains water vapor that has been evaporated into it from liquid sources (lakes, sea, etc...). The energy needed to do that has been taken from the air. Taking a volume of air at temperature T and mixing ratio of r , drying it by condensation will restore energy to the airmass. This will depend on the latent heat release as:

${\displaystyle T_{e}\approx T+{\frac {L_{v}}{c_{pd}}}r}$

where:

• ${\displaystyle \,L_{v}}$ : latent heat of evaporation (2400 kJ/kg {at 25C} to 2600 kJ/kg {at -40C})
• ${\displaystyle \,c_{pd}}$ : specific heat at constant pressure for air (${\displaystyle \approx }$ 1004 J/(kg·K))

Tables exist for exact values of the last two coefficients.