What device did Leonardo da Vinci design to measure changes in humidity?

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

His early device used materials whose weight or condition changed when they absorbed moisture from the air. By comparing the material’s change with a scale or reference, the instrument could indicate whether the surrounding air was becoming more or less humid.

The hygrometer belongs to Leonardo’s investigations of atmospheric conditions, alongside his studies of wind, clouds, water, and flight. It was an observational instrument rather than a modern digital sensor, and its readings were less precise than those of later scientific devices.

A hygrometer measures humidity, while a hydrometer measures the density or specific gravity of a liquid. The similar names cause frequent confusion. A rain gauge, by contrast, measures precipitation collected over time.