What instrument measures the moisture content of air?

A hygrometer is an instrument that measures the moisture content of air.

Humidity can be expressed in several ways, including absolute humidity, specific humidity and relative humidity. Relative humidity compares the water vapour in air with the maximum amount the air could hold at the same temperature.

Early hygrometers used materials such as hair, which changes length as humidity changes. Modern instruments commonly use electronic sensors that detect how moisture alters electrical properties.

A hygrometer is not the same as a barometer, which measures air pressure, or an anemometer, which measures wind speed. Weather stations often combine humidity measurements with temperature to calculate dew point, the temperature at which air would become saturated.