Sling Psychrometer, CambridgeBayWeather, Wikipedia

A psychrometer or sling psychrometer is a device used to measure relative humidity. It has two thermometers. One thermometer is called a wet-bulb thermometer. The bulb of the wet-bulb thermometer is covered with a damp cloth. The sling psychrometer is whirled around, using the handle. As the instrument is whirled, water evaporates from the cloth on the wet-bulb thermometer and cools the thermometer. This value is known as the wet-bulb temperature. The drier the air the more quickly the thermometer cools and hence, the lower the wet-bulb temperature. If the humidity is high, only a small amount of water will evaporate and there will be little change in temperature. The other thermometer is a dry-bulb thermometer and measures air temperature. The difference in temperature reading between the wet-bulb and dry-bulb thermometers indicates the amount of water vapor in the air. A table like the one below is used to determine the relative humidity.

Watch this video to see how to use a sling psychrometer and the table to find relative humidity.

You try!
Use the pictures below and the chart above to determine the relative humidity.