The instrument that measures humidity is called a hygrometer. Hygrometers come in various forms and types, including analog and digital models, and can utilize different methods to determine humidity levels. Some hygrometers measure humidity based on the expansion or contraction of certain materials, like a human hair, while others may use electronic sensors. Accurate humidity measurements are crucial in various fields, including meteorology, agriculture, and HVAC (heating, ventilation, and air conditioning) systems, as humidity levels can significantly impact comfort, health, and the environment.