Wet-bulb temperature
Temperature & humidity psychrometric wet-bulb temperature
Wet-bulb temperature shows how far evaporation can cool under the surrounding air conditions. It is especially useful for understanding the effect of humidity.
A wet thermometer explains the idea
Wrap a thermometer sensor in a wet cloth and ventilate it. Water evaporates, using heat taken from the sensor, which cools down. The temperature it reaches is the wet-bulb temperature, expressed in degrees Celsius.
Dry air and damp air behave differently
- Dry air: Water evaporates readily. The wet thermometer reads well below a dry one.
- Humid air: Evaporation provides less cooling. The two readings are closer.
- Saturated air: Wet-bulb temperature, dew point and air temperature coincide under ordinary conditions.
What the value is useful for
The same principle explains cooling by evaporating sweat. A high wet-bulb temperature indicates limited cooling from evaporation. Actual heat strain also depends on sunshine, wind, activity and clothing.
The value is useful for snowfall too: evaporation and melting can cool air during precipitation. Wet-bulb temperature is not the heat index, nor is it WBGT, which includes further weather effects. A single number cannot supply a safety threshold that applies to everyone.