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# Wet-bulb temperature

Wet-bulb temperature - there are several meanings of this term:

1. The temperature read from a wet 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).

## 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).

$e_s = 6.112 \exp \left( {17.67T \over T+243.5} \right)$
$e_w = 6.112 \exp \left( {17.67T_w \over T_w+243.5} \right)$
$e=e_w-p_{sta} \left(T-T_w\right) 0.00066 \left[1+(0.00115 T_w) \right]$
$RH= 100 {e \over e_s}$
$T_d={243.5 \log(e/6.112) \over 17.67 - \log(e/6.112)}$

where:

RH is relative humidity and Td is dew point in degrees Celsius
T and Tw are the dry-bulb and wet-bulb temperatures respectively in degrees Celsius
es, ew and e are the saturated water vapour pressure at the dry-bulb, wet-bulb at saturation pressure and normalized to sea level pressure wet-bulb (hPa)
for greater accuracy use the Arden Buck Equation to find the water vapour pressures

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.