Psychrometer

Sling psychrometerThe air flow that produces the wet-bulb temperature is achieved by rapidly rotating the thermometer, often as part of a psychrometer—that is, a hygrometer composed of two similar thermometers.
Illustration of two psychrometers: on the left, a sling psychrometer with two thermometers and a handle for swinging; on the right, a stationary psychrometer with labeled dry-bulb and wet-bulb thermometers, wicks, and a water container.
PsychrometerPsychrometers are tools used to measure the temperature of ambient air and the temperature derived from evaporation. The difference between these temperatures is known as the wet-bulb depression.

Psychrometer, a type of hygrometer that measures humidity by comparing temperature readings on two similar thermometers. The bulb of one thermometer is kept wet (by means of a thin, wet cloth wick) so that the cooling that results from evaporation makes it register a lower temperature than the dry-bulb thermometer. When readings of both thermometers are taken simultaneously, it is possible (with the use of psychrometric tables) to determine the relative humidity and dew-point temperature of the air. A decrease in the humidity of the air brings an increase in the difference between dry- and wet-bulb temperatures, called the wet-bulb depression.

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.

There are several types of psychrometers. The sling psychrometer is operated by spinning wet-bulb and dry-bulb thermometers in a circular direction to hasten evaporation. Other types use external fans to move air at a constant speed over the thermometers or contain materials such as metal springs that contract or expand as temperature changes occur. Digital psychrometers use electronic sensors collect the temperature values of each thermometer to return various weather data (such as relative humidity, air temperature, surface temperature, and dew point temperature) without the need for psychrometric tables.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by John P. Rafferty.
Citation Information
Article Title: Psychrometer
Website Name: Encyclopaedia Britannica
Publisher: Encyclopaedia Britannica, Inc.
Date Published: 11 November 2026
Access Date: November 14, 2026