Humidity measures the amount of moisture present in the air. Usually, you can measure this with a hygrometer, a simple meter that tells you what percentage of water vapor the air contains. However, if you don’t have a hygrometer or you want to figure out the humidity without one, there are other ways. The simplest way is to use wet- and dry-bulb temperatures. Wet and dry bulb refers to different kinds of thermometers; dry-bulb thermometers are common thermometers while wet-bulb thermometers have a moistened piece of cotton or cloth wrapped around the bottom. A wet-bulb thermometer is easy to make and use.
Shake both thermometers to make sure the mercury is as far down toward the bulbs as possible. Soak a cotton ball in room-temperature water and tape it around the bulb of a regular mercury thermometer. Set this thermometer and a second mercury thermometer with no cotton ball in the same place in a room or outdoors.
Wait several hours, if not overnight. Check the temperatures of both thermometers and write the temperatures down.
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Subtract the wet-bulb temperature from the dry-bulb temperature to get the humidity percentage. For instance, if your dry bulb reads 75 degrees and your wet bulb reads 40, humidity is at approximately 35 percent. The dry bulb thermometer records how hot or cold it actually is in the room. As water evaporates from the wet bulb, the cotton ball's temperature drops. The more water evaporated from the wet bulb, the cooler it will read. The dryer the air, the more water will evaporate from the cotton.
Try the experiment again in different rooms with humid and dry air.