A hygrometer is the humidity-measuring instrument commonly attributed to Leonardo da Vinci.
Leonardo’s proposed device used a balance to compare the weight of a moisture-absorbing material with a dry reference. As the material absorbed water from the air, its weight changed, providing an indication of atmospheric humidity.
The design reflects Leonardo’s habit of converting observations of nature into mechanical measurements. Humidity mattered to his interests in weather, materials, painting, and engineering, although the surviving evidence comes from drawings and notes rather than a documented working instrument.
A hygrometer should not be confused with an anemometer, which measures wind speed. Modern hygrometers may use electronic sensors, hair, or other materials, but they serve the same broad purpose: measuring moisture in the air.