The wet bulb temperature is the reading on a mercury thermometer with its bulb wrapped in a fitted cloth sock kept wet through wicking. The difference between a wet bulb temperature and a dry bulb temperature, the latter of which a person reads from a normal thermometer, allows the user to determine the relative humidity of a location. A psychrometer combines these two thermometers into one unit.
The wet bulb temperature is an indicator of the maximum amount of cooling that ventilated, evaporative cooling allows at the temperature. If the relative humidity is at 100 percent, the wet bulb and dry bulb show the same reading. Moisture no longer evaporates from the sock. If the relative humidity is below 100 percent, evaporative cooling lowers the wet bulb temperature. Once an individual reads the two psychrometer temperatures, he compares them to a psychrometric chart to determine the relative humidity.
An individual is also able to determine the relative humidity using a mechanical hygrometer. This mechanical device uses an organic material, such as human hair, that stretches as the humidity increases and contracts as it decreases. Manufacturers connect the hair to a needle, under tension from a spring, that moves along a calibrated scale to provide a relative humidity reading.