What Leonardo da Vinci device measured the moisture in air?

Leonardo da Vinci designed an early hygrometer to measure moisture in the air.

His device used a balance with materials whose weight changed when they absorbed humidity. Leonardo proposed comparing a moisture-sensitive substance with a dry reference material. A change in balance indicated that the surrounding air contained more or less water vapor.

A hygrometer measures humidity, not air pressure, wind speed, or temperature. Those functions belong to a barometer, anemometer, and thermometer respectively. Leonardo’s instrument was an early attempt to make an invisible atmospheric property measurable through a physical change.

The design belongs to Leonardo’s broader investigations of weather, water, and natural forces. It was not a digital sensor and did not provide the precise relative-humidity readings used today. Nevertheless, the underlying idea—using a material response to estimate atmospheric moisture—remains central to many humidity instruments.