The wet bulb temperature (WBT) is a temperature associated with the moisture content of the air. Wet bulb temperature is taken by surrounding the thermometer with a wet wick and measuring the reading as the water evaporates. Because of the evaporative cooling effect, Wet bulb temperatures are always lower than dry bulb temperatures and the only time that they will be the same is at saturation (i.e. 100% relative humidity).
The wet bulb temperature (WBT) relates relative humidity to the dry bulb temperature. If the relative humidity is low and the temperature is high, moisture will evaporate very quickly so its cooling effect will be more significant than if the relative humidity was already high, in which case the evaporation rate would be much lower.
Wet bulb temperature on psychrometric chart is represented by lines that slant diagonally from the upper right of the chart (along the line of saturation) down to the lower left of the chart. These follows lines of constant enthalpy but values are read off at the upper, curved, saturation temperature boundary. The unit of measure used for wet bulb temperature is degrees Celsius (°C) or degrees Farenheight (°F).
