Leonardo da Vinci designed an early hygrometer to indicate the moisture content of air.
His concept used a balance involving materials that respond differently to humidity, including a moisture-absorbing substance and a weight used for comparison. As the absorbing material gained or lost water, the balance changed, providing an indication of atmospheric dampness.
Leonardo's hygrometer was part of his investigations into air, water, weather, and physical change. It should not be confused with a barometer, which measures air pressure, or an anemometer, which measures wind. A hygrometer concerns humidity, usually expressed today as relative humidity.
The instrument was an early attempt to make an invisible atmospheric property measurable. Like many designs in Leonardo's notebooks, it was not necessarily a finished instrument used in routine observations. Its importance lies in combining material behavior with a readable mechanical indication.