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Analog and digital hygrometer mounted on wood, surrounded by green leaves, displays about 80 percent relative humidity and a temperature reading of 83.7 degrees Fahrenheit. A table showing relative humidity values dry-bulb temperature values on the y-axis and the difference between the dry-bulb and wet-bulb temperatures on the x-axis.
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Analog and digital hygrometer mounted on wood, surrounded by green leaves, displays about 80 percent relative humidity and a temperature reading of 83.7 degrees Fahrenheit.
hygrometer

Hygrometer

meteorological instrument
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Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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Hygrometer, instrument used in meteorological science to measure the humidity, or amount of water vapor in the air. Several major types of hygrometers are used to measure humidity.

General types

Mechanical hygrometers make use of the principle that organic substances (particularly finer substances such as goldbeater’s skin [ox gut] and human hair) contract and expand in response to the humidity. Contraction and expansion of the hair element in a mechanical hygrometer causes the spring to move the needle on the dial.

Electrical hygrometers measure the change in electrical resistance of a thin layer of lithium chloride, or of a semiconductor device, as the humidity changes. Other hygrometers sense changes in weight, volume, or transparency of various substances that react to humidity.

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Dew-point hygrometers typically consist of a polished metal mirror that is cooled at a constant pressure and constant vapor content until moisture just starts to condense on it. The temperature of the metal at which condensation begins is the dew point.

Psychrometer

A table showing relative humidity values dry-bulb temperature values on the y-axis and the difference between the dry-bulb and wet-bulb temperatures on the x-axis.
Calculating relative humidity using a values taken from readings made by dry-bulb and wet-bulb temperatures.A location's relative humidity may be obtained by using a matrix of values that relate dry-bulb temperature readings and the difference between the dry-bulb and wet-bulb temperature readings.

The psychrometer (q.v.) is a hygrometer that utilizes two thermometers—one wet-bulb and one dry-bulb—to determine humidity through evaporation (see also wet-bulb temperature). Digital psychrometers are often used by technicians for heating, ventilation, and air conditioning applications, whereas sling psychrometers (in which an operator manually spins a dry-bulb thermometer and a wet-bulb thermometer) are used by meteorologists, firefighters, and researchers in the field. A wetted cloth wraps the wet-bulb thermometer at its enlarged end. By rapidly rotating both thermometers, or by blowing air over the bulbs, the temperature of the wet-bulb thermometer is cooler than that of the dry-bulb thermometer. The difference in temperature between the wet- and dry-bulb thermometers can be used to compute the amount of water vapor in the air.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by John P. Rafferty.