One thermometer is called the dry bulb – it measures the normal air temperature.
The other is the wet bulb – it has a wet cloth wrapped around its tip.
Soak the wet bulb: You dip the cloth in water so it's wet.
Spin the hygrometer: You whirl it around for about 30 seconds to a minute. This helps air move quickly over the bulbs.
Evaporation cools the wet bulb: As the water on the wet bulb evaporates, it cools down (just like how sweat cools your skin).
Read both thermometers: After spinning, you look at the temperatures of both the dry bulb and the wet bulb.
If the air is very dry, the water on the wet bulb evaporates quickly, and the wet bulb temperature drops a lot.
If the air is humid, not much water can evaporate, so the wet bulb stays closer to the dry bulb temperature.
You subtract the wet bulb temperature from the dry bulb temperature and use a chart or calculator to find the relative humidity.
