Wet-bulb temperature

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Wet-bulb temperature - there are several meaning of this term: (1) the temperature read from red bulb thermometer; (2) isobaric wet-bulb temperature - the temperature an air parcel would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it, all latent heat being supplied by the parcel, (3) adiabatic wet-bulb temperature - the temperature an air parcel would have if cooled adiabatically to saturation and then compressed adiabatically to the original pressure in a moist-adiabatic process (AMS Glossary).

[edit] Wet-bulb temperature measured by wet bulb thermometer

Wet-bulb temperature is measured using a thermometer that has its bulb wrapped in cloth—called a sock—that is kept wet with water via wicking action. Such an instrument is called, not surprisingly, a wet-bulb thermometer. At relative humidities below 100%, water evaporates from the bulb which cools the bulb below ambient temperature. To determine relative humidity, ambient temperature is measured using an ordinary thermometer, better known in this context as a dry-bulb thermometer. At any given ambient temperature, less relative humidity results in a greater difference between the dry-bulb and wet-bulb temperatures; the wet bulb is colder. The precise relative humidity is determined by finding one's wet-bulb and dry-bulb temperatures on a psychrometric chart (or via complex calculation).

$e_s = 6.112 \exp \left( {17.67T \over T+243.5} \right)$

$e_w = 6.112 \exp \left( {17.67T_w \over T_w+243.5} \right)$

$e=e_w-p_{sta} \left(T-T_w\right) 0.00066 \left[1+(0.00115 T_w) \right]$

$RH= 1000 {e \over e_e}$

$T_d={243.5 \log(e/611.2) \over 17.67 - \log(e/611.2)}$

where RH is relative humidity and $T d$ is dew point. Hygrometers are instruments comprising both wet-bulb and dry-bulb thermometers. A hygrometer can also be used in combination with a globe thermometer (for measuring solar radiant heat) in the calculation of the wet bulb globe temperature.