Explanation:

A hygrometer, also known as a psychrometer, is a device that is used to measure the humidity in the air. A common way these devices work is by using a material that attracts moisture, and that changes depending on how moist it is.
A hygrometer has two bulbs: one wet and one dry. Both bulbs are thermometers, though one is covered with a wet towel. After a period of time, the water on the wet bulb evaporates and at that time, the temperature is measured on each bulb. The difference between the temperatures is noted. Then each is used on a chart to find the relative humidity of that temperature and area. A small difference between the temperature of the bulbs shows a high relative humidity coming from a low evaporation rate. In dry air, evaporation happens faster showing a large difference in temperature thus giving you a low relative humidity.