Leonardo da Vinci designed a hygrometer to measure atmospheric humidity.
His device used materials whose weight changed as they absorbed moisture from the air. Leonardo compared a moisture-sensitive material with a dry reference, allowing the instrument to indicate whether the air was relatively damp or dry.
The design belongs to Leonardo’s wider study of weather, water, and atmospheric phenomena. It was an early humidity instrument, although it was not a modern calibrated electronic hygrometer and did not provide today’s familiar percentage readings.
A hygrometer should not be confused with a barometer, which measures air pressure, or a thermometer, which measures temperature. Leonardo’s invention is significant because it shows his interest in turning observations of nature into measurable physical quantities.