Leonardo da Vinci sketched a hygrometer for measuring humidity in the air.
His proposed instrument used the change in weight of an absorbent material to indicate moisture. A substance that took on water from humid air would become heavier, allowing the amount of moisture to be estimated with a balance or scale. This was an early attempt to make atmospheric humidity measurable rather than merely observable.
Leonardo’s hygrometer belongs to his wider investigations of weather, air, water, and natural processes. The instrument was not equivalent to the electronic humidity sensors used today. It also did not provide the modern concept of relative humidity unless calibrated against temperature and reference conditions.
A hygrometer measures moisture in air, whereas an anemometer measures wind speed and a barometer measures air pressure. Leonardo’s design is historically important because it shows the movement from qualitative weather descriptions toward instruments that could record numerical observations.