Leonardo da Vinci’s early humidity-measuring instrument was a hygrometer.
A hygrometer measures the moisture content of air. Leonardo’s proposed device used a material that changed weight when it absorbed moisture, allowing the change to serve as an indication of humidity. His design is usually described as an early hygrometer rather than a precision instrument in the modern scientific sense.
Leonardo was interested in atmospheric conditions because they affected weather, engineering, and flight. His notebooks contain observations and proposed instruments for wind, water, heat, and moisture. These studies show him treating natural phenomena as measurable processes rather than relying only on descriptive observation.
A hygrometer should not be confused with an anemometer. An anemometer concerns wind movement, while a hygrometer concerns water vapor in the air. Later hygrometers used hair, wet-and-dry thermometers, and electronic sensors, but Leonardo’s moisture-sensitive weighing idea belongs to the early history of humidity measurement.