What Leonardo da Vinci invention measured the humidity of the air?

Leonardo da Vinci designed an early hygrometer to measure the humidity of the air.

His concept used a balance between a moisture-absorbing material and a dry material. As the absorbent material took on water from humid air, its weight changed, causing the balance to move. The amount of movement could therefore indicate how damp the atmosphere was.

Leonardo’s hygrometer is an early example of turning a natural effect into a measurable instrument. It belongs to his broader investigations of air, water, weather, and mechanical balance. The design is important historically even though it was far less precise than later instruments using hair, chemical salts, or electronic sensors.

A hygrometer is not a thermometer, which measures temperature, or a barometer, which measures air pressure. Humidity can be reported in several ways, including relative humidity, but Leonardo’s balance design did not use the modern electronic percentage display familiar today. It is best understood as an early mechanical humidity indicator.