The amount of moisture in the air is usually stated in terms of relative humidity. This term refers to the percentage of moisture the air holds relative to the amount it can hold at a particular temperature. Relative humidity is measured using a instrument called a psychrometer. A psychrometer is composed of two thermometers—a wet-bulb thermometer and a regular thermometer. The wet-bulb thermometer in actuality is a regular thermometer. However, the bulb of the thermometer is covered with a piece of wet cloth. The other thermometer is called a dry-bulb thermometer. The two thermometers are attached to each other and swung around in the outside air.
When the air blows over the wet-bulb thermometer, the water in the cloth evaporates. Evaporation requires heat energy; therefore, the evaporation of water from the cloth cools the bulb of the thermometer. If the humidity is low, evaporation will occur rapidly and the temperature of the wet-bulb thermometer will fall at a fast pace. If the humidity is high, evaporation will occur at a slower rate causing the wet-bulb thermometer’s temperature to remain high.
In order to determine the relative humidity you must first find the difference between the dry-bulb temperature and the wet-bulb temperature. The differences between the two temperatures are calculated by using a table made by and available from the National Weather Service in Washington, D.C. From this information it is also possible to tell at just what temperature the dew point will be reached under all conditions of relative humidity.