Water on a surface evaporates into the air, but not completely. The more humid the air, the more water will be left on the surface. When the water does evaporate, it carries off heat, cooling the object. When the humidity is low, water evaporates very easily and completely, providing a lot of cooling. When the humidity is high, the water cannot cool the surface as much. This is why muggy days feel so much hotter than "dry heat." On muggy days, sweat doesn't cool the body very effectively. This is also the principle that underlies psychrometers.
A psychrometer uses two thermometer bulbs. One is kept wet, usually by attaching wet fabric to it. The other is kept dry. Air is blown over both thermometers to speed up evaporation. Then the temperatures are compared. The wet psychrometer will always have a lower temperature than the dry one because of the cooling effect of the evaporating water. By comparing the two temperatures to a psychrometric chart, a scientist can determine the humidity of the air.
There are two main types of psychrometers. Sling psychrometers are the simplest type. The two bulbs are attached to a rod or a pair of strings and swung around in a circle. This moves air past the bulbs to speed the evaporation. The Assman psychrometer works on the same principles but uses a more sophisticated design. Both thermometers are mounted inside a metal tube. An electric or mechanical fan blows air past them.
The main advantage to psychrometers is their simplicity. A fairly good sling psychrometer, for example, can be made in a few minutes with a bit of string, a couple of thermometers, tape and a cotton swab. Assuming the thermometers are reliable, psychrometers are also fairly accurate.
A psychrometer depends on the accuracy of its thermometers. If one or both of the thermometers is off, it will throw off the reading. Sling psychrometers can also be thrown off by heat sources. Radiant heat from the sun or warm objects in an environment can affect the thermometers and throw off the humidity reading. This affects Assman psychrometers less, because they are insulated by metal tubes. The biggest drawback to psychrometers, however, is simply that better hygrometers exist. Electronic hygrometers are extremely accurate and can continuously and automatically record the humidity.