When water evaporates, it cools whatever surface it was on. For example, sweat cools your body as it evaporates. The air, however, can only hold a certain amount of water. When it is humid, the air is saturated— filled with as much water as it can contain, and water does not evaporate easily. Psychrometers make use of these principles.
A psychrometer is the simplest kind of hygrometer—a device for measuring humidity. It consists of two bulbs with thermometers: a wet bulb and a dry bulb. The dry bulb is simply left exposed to the air to measure the temperature. The wet bulb is covered with a cloth wick and dipped in water until it is ready to use.
Using a Psychrometer
When a scientist wants to measure the temperature in the room, he removes the wet bulb from the water. Depending on the design of the psychrometer, the wet bulb either swings around or remains stationary. As the water evaporates, it cools the wet bulb. By measuring the cooling of the wet bulb, the scientist can tell how much water evaporates. This, in turn, tells her how humid the air is. Moist air allows only a little water evaporate, and the wet bulb barely changes temperature. Dry air absorbs a lot more moisture, cooling the wet bulb quite a bit.