A hygrometer has two bulbs: one wet and one dry. Both bulbs are like thermometers, though one is covered with a wet or dry towel. After a period of time, the water on the bulb evaporates and at that time, the temperature is measured on each bulb. The difference between the temperatures is noted. Then each of the temperatures are used on a chart to find the relative humidity of that temperature and area. Relative humidity is a ratio, so it has no unit. A small difference between the temperature of the bulbs shows a high relative humidity coming from a low evaporation rate. In dry air, evaporation takes place faster showing a large difference in temperature thus giving a low relative humidity.