Leonardo da Vinci designed a hygrometer to show changing atmospheric humidity.
A hygrometer measures moisture in the air. Leonardo’s design used a balance with moisture-sensitive material on one side and a dry reference material on the other. As the material absorbed water from humid air, its weight changed, causing the balance to move. A scale could then indicate the change in atmospheric moisture.
The instrument is usually described as an early hygrometer rather than a precise modern humidity meter. The sensitivity would have depended on the selected material, the balance, and the calibration. Leonardo’s interest in humidity formed part of broader investigations into weather, air, water, and natural processes.
The design is sometimes confused with an anemometer, which measures wind speed, or a barometer, which measures air pressure. Those instruments respond to different atmospheric properties. Leonardo’s hygrometer is notable because it turned a difficult-to-see environmental change into a visible mechanical movement.