The Whirling psychrometers are used to measure relative humidity and also equipped with mounted on ABS plastic frame, plastic handle, water reservoir-water proof tetron cloth soft carrying case with nylone string & hygrometric tables Weather observers can use a sling psychrometer to measure the amount of water vapor in the air - that is, its humidity. It consists of two glass thermometers containing a liquid. One thermometer measures the air temperature while the other one measures the wet-bulb temperatures. After the wick is dipped in distilled water, a weather observer whirls the sling psychrometer around, using the handle.
As the instrument is whirled, water evaporates from the wick on the wet-bulb thermometer and cools the thermometer. The wet-bulb thermometer cools to the lowest value possible in a few minutes. This value is known as the wet-bulb temperature. The drier the air the more the thermometer cools and hence, the lower the wet-bulb temperature.