???? A hygrometer is an instrument used to measure humidity or the amount of ambient water vapor in the air. The first hygrometers used human hair, since hair expands with increasing humidity, and a hygrometer made with human or animal hair actually will work reasonably well. Typically, however, meteorologists rely on more sophisticated instruments that use change in electrical resistance or capacitance to make sensitive measurements. Hygrometers serve a number of different purposes in different industries.

???? For weather forecasters, measuring humidity is an important part of trying to predict tomorrow's weather. Relative humidity can help assess the chance of rain, snow or fog. Humidity is one of the important parameters meteorologists track (together with other variables like temperature, pressure and wind speed) when making a weather forecast; typically they rely on hygrometers to measure humidity.