Taking a reading of the wet bulb temperature is a way of measuring humidity. It is the temperature of a volume of air that has been cooled adiabatically until the humidity is at 100 percent and then compressed to the original pressure of the system prior to cooling. Using a wet bulb thermometer provides one reading, while a dry bulb thermometer is used for the second reading. The greater the difference between the temperatures read on the two thermometers, the less humid the air is. Using the two in conjunction is a single device called a psychrometer. The psychrometer measures the difference between the wet and dry bulb, and then, using established tables, the humidity of the air can be calculated.