Leonardo da Vinci designed a hygrometer to measure humidity by comparing the weight of two materials.
A hygrometer measures moisture in the air. Leonardo’s design used a balance with a material that absorbs atmospheric moisture on one side and a comparatively dry reference material on the other. Changes in the balance would indicate changing humidity.
The device belongs to Leonardo’s studies of weather and the physical environment. It was not an electronic sensor and did not provide a modern calibrated percentage reading. Instead, it translated moisture absorption into a visible mechanical change, an approach related to later hair and chemical hygrometers.
A hygrometer is different from a barometer, which measures air pressure, and from a thermometer, which measures temperature. Humidity can vary with temperature, so modern weather instruments often combine several measurements. Leonardo’s concept is important historically because it treats atmospheric moisture as something that can be quantified with a purpose-built instrument.