Leonardo da Vinci designed a hygrometer to estimate atmospheric moisture using changes in the weight of materials.
A hygrometer measures humidity, the amount of water vapor in the air. Leonardo’s design used a substance that could absorb moisture, such as cotton or wool, and compared its weight with that of a dry reference material. As humidity increased, the absorbent material became heavier and shifted a balance.
The instrument reflects Leonardo’s wider interest in measuring natural phenomena. Rather than relying only on visual observation, he sought mechanical ways to make invisible environmental changes detectable. His work belongs to the early history of humidity measurement, before modern electronic sensors and psychrometers.
Leonardo’s hygrometer should not be confused with an anemometer, which measures wind, or a barometer, which measures air pressure. Historical accounts differ about the exact construction and date of early hygrometers, so the design is best described as an early proposed instrument rather than a modern calibrated device.