Leonardo da Vinci proposed a hygrometer, an instrument intended to measure humidity in the air.
His design used materials whose weight changed as they absorbed moisture. A balance compared a moisture-sensitive substance, such as cotton, with a dry reference material. Changes in the balance could indicate whether the surrounding air was more or less humid. This approach connected a visible mechanical movement with an otherwise invisible atmospheric property.
A hygrometer measures humidity, whereas an anemometer measures wind speed and a barometer measures air pressure. Those instruments are often discussed together because all three belong to the history of weather observation, but they answer different questions. Leonardo’s hygrometer was an early proposal rather than a modern precision instrument. Later hygrometers used hair, metal, chemical salts, electrical sensors, and other materials that respond predictably to moisture. The concept nevertheless shows Leonardo applying careful observation and mechanical reasoning to atmospheric science.