The wet bulb temperature (WBT) relates relative humidity to the ambient air or dry bulb temperature. When moisture evaporates, it absorbs heat energy from its environment in order to change phase (via latent heat of vapourisation), thus reducing the temperature slightly. The WBT will vary with relative humidity. 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. The difference between the wet bulb and dry bulb temperature therefore gives a measure of atmospheric humidity.