Leonardo da Vinci’s humidity-measuring device was a hygrometer.
Leonardo described a hygrometer that used a material affected by moisture in the air. As humidity changed, the material’s weight or position could change, providing an indication of atmospheric moisture. The concept belongs to his wider study of weather and measurable natural phenomena.
A hygrometer measures humidity, usually relative humidity or the amount of water vapor in air. Leonardo’s design was an early attempt to create an instrument for this purpose, long before electronic sensors and calibrated meteorological instruments became standard.
The device should not be confused with Leonardo’s anemometer. An anemometer concerns wind, while a hygrometer concerns moisture. Leonardo’s scientific notebooks often combined observations from several fields, so instruments for air movement, humidity, and temperature may appear near one another in discussions of his engineering work.