Standing water evaporates into the air. When it does, it carries off heat, cooling off whatever it evaporates from. The dryer the air, the more the water evaporates and the greater its cooling effect. A psychrometer compares the temperature of a dry thermometer and a wet one to determine the humidity of the air. In dry air, the wet thermometer will be much cooler, whereas in moist air the two thermometers will have very similar temperatures. By comparing the temperatures to a chart, a scientist can determine the relative humidity.
Most psychrometers use moving air to speed up the rate of evaporation. The simplest design is a sling psychrometer. Both thermometers are attached to a rope or handle. The scientist then spins the psychrometer around to make air flow past the thermometers before comparing their temperatures. The Assman psychrometer is a more sophisticated design. Both thermometers are mounted inside hollow metal tubes, and a fan blows air past each.
Psychrometers are an extremely simple and fairly dependable way to measure humidity. More precise instruments require delicate electronic equipment and a power supply. A sling psychrometer, by contrast, can give you a decent reading of the humidity in nearly any area with nothing but some water, a couple thermometers and a chart.
Psychrometers need to be carefully calibrated to make an accurate reading. Because they depend on recording small differences in temperature between two thermometers, a slight inaccuracy in either thermometer can throw off the results. Sling psychrometers can also be affected by radiant heat sources nearby. Warm objects radiate infrared energy, which can throw off the readings of the thermometers. Assman psychrometers are insulated, which makes them much less susceptible to this effect.
Psychrometers are not the preferred method of making really precise humidity measurements. Instead, meteorologists use a device called a mirror hygrometer. At a certain temperature, the water in the air will start to settle out on a surface. This temperature is called the dewpoint and depends on how much water is in the air. A mirror hygrometer slowly lowers the temperature of a mirror until it reaches the dewpoint. By calculating the dewpoint, the hygrometer can calculate the humidity.