From AMS Glossary
(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 Tw are the dry-bulb and wet-bulb temperatures, respectively; e is the vapor pressure; es(Tw) 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.