# Psychrometric formula

From AMS Glossary

## psychrometric formula

(Also known as hygrometric formula.) The semi-empirical relation giving the vapor pressure in terms of the barometer and psychrometer readings.

For temperatures in degrees Celsius, this formula is where

*T*and*T*_{w}are the dry-bulb and wet-bulb temperatures, respectively;*e*is the vapor pressure;*e*_{s}(*T*_{w}) is the saturation vapor pressure at the wet-bulb temperature; and*p*is pressure. For temperatures below freezing, with the wet bulb covered with ice, 6.60 × 10^{-4}is replaced by, approximately, 5.82 X 10^{-4}.