What instrument measures relative humidity in the atmosphere?

A hygrometer is the instrument used to measure relative humidity in the atmosphere. Relative humidity compares the amount of water vapour in air with the maximum amount the air could hold at the same temperature, expressed as a percentage.

Many hygrometers use electronic sensors that respond to changes in electrical properties as moisture changes. Traditional instruments include hair hygrometers, which use the small expansion and contraction of human or animal hair, and psychrometers, which compare readings from wet-bulb and dry-bulb thermometers.

Humidity is closely linked to temperature. Warm air can contain more water vapour than cold air, so relative humidity can rise when air cools even if its actual water-vapour content stays unchanged. A hygrometer therefore differs from a thermometer, which measures temperature, and a barometer, which measures atmospheric pressure.