Leonardo da Vinci sketched a hygrometer to measure atmospheric humidity.
His design used materials that changed weight or condition when they absorbed moisture from the air. In descriptions of the instrument, a balance compared a hygroscopic substance, such as cotton, with a dry reference weight. Changes in the balance indicated changes in humidity.
A hygrometer measures moisture in air, whereas an anemometer measures wind speed. This distinction matters because Leonardo investigated both atmospheric moisture and moving air while studying weather and flight. His hygrometer was an early attempt to make an otherwise invisible property physically measurable.
The instrument was not the hair hygrometer later used in scientific instruments. Hair hygrometers rely on the length change of hair as humidity varies, while Leonardo’s concept depended on weight change. The exact operating details of the surviving sketch are interpreted through the drawing and historical context, rather than through a surviving original device.