Hygrometer, a device for determining the humidity of the atmosphere. It can indicate relative humidity (the percentage of moisture in the air), absolute humidity (the amount of moisture), or both. Some crude hygrometers will simply tell whether the air is dry or damp. An important use of hygrometers is in weather forecasting. Hygrometers are also used for monitoring the humidity in laboratories, storage areas, and manufacturing plants where specific levels of humidity must be maintained. Some hygrometers form part of devices called humidistats, which are used to control humidifiers or dehumidifiers for regulating the humidity of the air.

There are several types of hygrometers. One common type contains animal fibers, usually human hair, that stretch when the relative humidity increases and contract when it decreases. The fibers are usually attached to levers that turn the pointer on a dial.

The psychrometer, sometimes called the wet-and-dry bulb hygrometer, consists of two thermometers mounted side by side. One of the thermometers—the dry-bulb thermometer—measures the temperature of the air. The bulb of the other thermometer is covered with a sleeve of muslin or candle-wick. The cloth is wetted and the water is then made to evaporate by blowing air over the cloth with a fan or by whirling the psychrometer in the air. As the water evaporates, the bulb is cooled. In general, the drier the air, the greater is the drop in temperature. The relative humidity is determined by comparing the readings of the two thermometers with published psychrometric tables or charts.

Electrical hygrometers typically measure the electrical resistance of a substance, such as lithium chloride, whose resistance to an electric current varies with the humidity. In the dew-point hygrometer, a smooth, shiny surface is cooled until water vapor in the air begins to condense on it. The humidity is determined by comparing the temperature at which the condensation occurs (the dew point) with the temperature of the air. Chemical hygrometers use a substance such as phosphorus pentoxide to absorb moisture from a given volume of air. The difference in the weight of the substance before and after its exposure to the air indicates how much moisture is present.