The Leonardo da Vinci instrument intended to measure changes in atmospheric humidity was a hygrometer.
Leonardo designed an early hygrometer using materials whose weight could change as they absorbed moisture from the air. In the commonly described arrangement, a balance compared a moisture-sensitive material with a dry reference. Changes in the balance indicated changes in atmospheric humidity.
A hygrometer measures moisture in the air, while a barometer measures atmospheric pressure and a thermometer measures temperature. These instruments are often grouped together because all are used in weather observation, but they measure different physical properties.
Leonardo’s design was an early scientific instrument concept rather than a modern electronic humidity sensor. Its importance lies in recognizing that humidity could be estimated through a measurable change in mass. Later hygrometers used hair, chemical materials, and other moisture-sensitive substances to improve practical readings.