A psychrometer is a device used to measure relative humidity of a certain area. A psychrometer has two bulbs: one wet and one dry. 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. Psychrometers are very popular in Canada and Australia.