Just to elaborate on the "thermometer in a wet cloth" explanation... If you have water at the wet bulb temperature exposed to ambient air, then evaporation and

Just to elaborate on the "thermometer in a wet cloth" explanation... If you have water at the wet bulb temperature exposed to ambient air, then evaporation and condensation are in equilibrium. Above the wet bulb temperature, there is more evaporation, transferring heat from the water to ambient air. Below it, there is more condensation, transferring heat from ambient air to the water. Except at 100% humidity, this is higher than the dew point, which is the temperature where water spontaneously condenses out of the air.