A hygrometer measures humidity, or the amount of water vapor present in the air. Hygrometers are standard tools for weather forecasters, providing information about the potential for precipitation. They are also vital parts of climate control systems for homes and businesses, helping to maintain safe and comfortable humidity levels.
The amount of water vapor that air can hold varies according to its temperature. Warm air can hold more moisture than cold air, and if that air cools rapidly, it can cause condensation on objects and surfaces. Since this condensation can cause moisture damage to walls and furnishings inside a home, dehumidifiers often include a hygrometer for on-the-fly measurements of the humidity levels inside a structure. If the humidity level approaches a danger zone, the dehumidifier activates and removes excess moisture to reduce humidity levels.
Most modern hygrometers use electronic means to measure humidity, but older models used a variety of different means. One of the most common implementations was the wet bulb-dry bulb hygrometer. Two thermometers measure ambient temperature, with one thermometer bulb remaining dry while the other is covered in a wet fabric. Evaporation from the fabric reduces the temperature of the wet bulb, and the difference between the two thermometer readings can indicate how fast evaporation is taking place and by extension, how much humidity is present.