A hygrometer is used to measure relative humidity. Relative humidity is the amount of water vapor present in the air compared to the maximum amount that the air could hold at a given temperature. Hygrometers measure this by sensing changes in the electrical resistance or capacitance of a material as it absorbs moisture from the air. They are commonly used in weather forecasting, indoor climate control, and in industries where humidity control is important, such as food processing and pharmaceuticals.