What Leonardo da Vinci device was designed to show changes in humidity?

Leonardo da Vinci designed a hygrometer to show changes in humidity.

His concept used a material that could gain or lose weight as moisture in the air changed. A balance could then reveal the relative difference, giving observers an indication of whether the air was becoming wetter or drier. The device belongs to Leonardo’s studies of atmospheric conditions and measurement.

A hygrometer measures humidity, meaning the amount of water vapor in air. Leonardo’s design was an early attempt to turn that invisible atmospheric change into a visible mechanical reading. It was not the same as a modern electronic humidity sensor, and it did not necessarily provide the precise numerical readings common today.

The invention is frequently mixed up with Leonardo’s anemometer. The anemometer concerns wind, while the hygrometer concerns moisture in the air.