Leonardo da Vinci designed a hygrometer to measure humidity by observing moisture absorbed by a material.
Leonardo’s proposed instrument used a moisture-sensitive substance, traditionally described as cotton or wool, balanced against a dry reference. When the material absorbed water from the air, its weight changed. That change could provide an indication of atmospheric humidity. The design reflects Leonardo’s habit of turning everyday physical effects into measurable mechanical systems.
A hygrometer measures humidity, while an anemometer measures wind and a barometer measures air pressure. Leonardo’s device should not be treated as equivalent to a modern electronic humidity sensor or a highly standardized laboratory instrument. Its importance lies in recognizing that moisture in air could be estimated through a controlled change in mass. Leonardo’s investigations of weather, water, and flight connected practical engineering with early quantitative observation.