Leonardo da Vinci designed an early hygrometer to indicate changes in humidity by observing changes in weight.
His proposed instrument used an absorbent material that became heavier when it took up moisture from the air. A balance or pointer could then show the change, providing a simple way to compare damp and dry conditions.
Leonardo’s idea belongs to the early history of meteorological instruments. It did not measure relative humidity with the calibrated precision of modern electronic sensors, but it captured the key principle that atmospheric moisture can alter the mass or dimensions of certain materials.
A hygrometer measures humidity, while a thermometer measures temperature and a barometer measures air pressure. Leonardo investigated many weather-related phenomena, but his humidity instrument should not be confused with later hair hygrometers, which used the length change of human or animal hair.