A hygrometer is a device that is used to measure the amount of water vapor contained in the air. The amount of water vapor in air is called humidity, and is expressed as a percentage, indicating the amount of water vapor present at that temperature compared to the maximum amount it can hold at that temperature. Hygrometers may take a metal strip wrapped around a paper cone design; there is also a hygrometer that uses human hair as a testing strip to measure the humidity. There are more specific hygrometers of an electronic design that deliver the humidity as a digital display. In my class room, I have several Weathermaster units that have an array of data collection devices in them, of which a hygrometer is one. This allows students to gather data over the course of a week, then compare the data to the actual physical manifestations of weather patterns that actually occured. Hygrometers are used in areas that have a need to specifically measure the amount of humidity in the air, such as agricultural settings and industrial settings.