Measuring the moisture content of air requires a hygrometer. Early versions used human hair, which lengthens as humidity rises. Modern instruments include capacitive sensors, chilled-mirror dew-point hygrometers, and psychrometers that compare wet-bulb and dry-bulb temperatures.

Accuracy matters for weather observing, industrial processes, and indoor climate control. Some hygrometers display relative humidity directly; others report dew point. Calibration against known standards keeps the readings reliable.

Weather stations incorporate hygrometers as part of their standard suite of instruments. Portable versions help assess conditions for agriculture, storage of sensitive materials, and personal comfort. In research, high-precision hygrometers track subtle moisture changes in the atmosphere.

The data they provide are essential for forecasting fog, clouds, and precipitation.