A psychrometer is a measuring instrument used to determine the relative humidity of the air. It is considered a type of hygrometer and is often used in the fields of meteorology, air conditioning, and industry. The psychrometer is based on two standard thermometers: (1) a dry-bulb thermometer: measures the normal air temperature, and (2) a wet-bulb thermometer: a thermometer whose sensor is wrapped in wet cloth. As air passes over the wet-bulb thermometer, the water from the cloth evaporates, causing the thermometer to cool. The drier the air, the faster the water evaporates and the greater the cooling, creating a greater temperature difference between the two thermometers.