AmbientWeather.com: Dew Point Calculation.
The dew point is the temperature at which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation temperature.
The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.
The standard calculation for Dew Point is here:
A well-known approximation used to calculate the dew point, Tdp, given just the actual (“dry bulb”) air temperature, T (in degrees Celsius) and relative humidity (in percent), RH, is the Magnus formula:
The more complete formulation and origin of this approximation involves the interrelated saturated water vapor pressure (in units of millibars, also called hectopascals) at T, Ps(T), and the actual vapor pressure (also in units of millibars), Pa(T), which can be either found with RH or approximated with the barometric pressure (in millibars), BPmb, and “wet-bulb” temperature, Tw is (unless declared otherwise, all temperatures are expressed in degrees Celsius):
For greater accuracy, Ps(T) (and therefore γ(T, RH)) can be enhanced, using part of the Bögel modification, also known as the Arden Buck equation, which adds a fourth constant d:
- a = 6.1121 mb, b = 18.678, c = 257.14 °C, d = 234.5 °C.