Hygrometers are instruments used for measuring humidity. The simplest form of a hygrometer consists of two thermometers, one of which has its bulb constantly kept wet. Evaporation from the bulb lowers the temperature so that this thermometer usually shows a lower temperature. However, when the air temperature is below freezing, it is possible for the "wet bulb" (actually a thin coating of ice) to be warmer than the dry bulb. Relative Humidity is computed from the ambient temperature as shown by the "dry bulb thermometer" and the difference in temperature shown by the "wet bulb" and dry bulb thermometers. One device that uses the wet/dry bulb method is the sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes.