Hygrometer_Approaching_02
One type of hygrometer is a wet-and-dry bulb. It consists of two thermometers set side by side. One of the thermometers is called the dry-bulb thermometer. This thermometer measures the temperature of the air. The bulb of the other thermometer is covered with a damp cloth. It is called the wet bulb. When air blows over the wet bulb, the water evaporates off the cloth. One way to do this is to use a fan facing toward the device.

A sling psychrometer is another type of hygrometer. Sling psychrometers are spun (or slung) around in the air. It is this motion that creates a breeze over the wet bulb. Whether the air is blown over the wet bulb with a fan or slung around in the air, air is forced over the wet bulb. The moving air allows the water to evaporate from the wet bulb. When water evaporates it has a cooling effect. In general, the drier the air, the faster the water will evaporate off the wet cloth. When the wet bulb is dried quickly, the faster it will cool the thermometer it covers. As a result, one thermometer will have a cooler reading than the other. The relative humidity is found by using the difference in temperatures and a table.
Humidity Table