A psychrometer is an instrument that measures the amount of humidity in an atmosphere. The instrument uses two thermometers which, when compared, determine relative humidity.
One of the thermometers is dry when it is exposed to the air, while the other thermometer has a wet cloth around its bulb before being exposed to the air. By comparing the two different temperatures, users can determine the relative humidity of the surrounding atmosphere. A psychrometer is sometimes referred to as a wet-and-dry-bulb thermometer. It is a simple type of hygrometer, which is a device that measures the moisture content of the air.