Wet-bulb temperature (Twb, Tw or WBT) is the temperature recorded by a thermometer that has its bulb wrapped in cloth and moistened with distilled water. It can be expressed in Celsius (C), Fahrenheit (F) or Kelvin (K).
The rate of evaporation from the wet bulb, and so the temperature it records varies depending on the humidity of the air it is exposed to. The higher the humidity, the lower the rate of evaporation and so the higher the temperature recorded. Wet-bulb temperatures are the same as dry-bulb temperatures at a relative humidity of 100%, but otherwise they will be lower than dry-bulb temperatures due to the cooling effect of evaporation (described as wet-bulb depression).
Wet-bulb temperature is an indicator of the temperature felt when damp skin is exposed to the air and so can be used to express a component of thermal comfort. The wet bulb globe temperature index (WBGT) is widely used for the assessment of heat stress and combines wet-bulb temperature, dry-bulb temperature and globe temperature (mean radiant temperature).
A sling psychrometer holds both wet and dry-bulb thermometers and can be used to express the physical and thermal properties of moist air on a psychrometric chart.
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