What instrument did Leonardo da Vinci devise to measure humidity?

Leonardo da Vinci devised an early hygrometer to measure humidity in the air.

His proposed instrument used a material that would gain weight when it absorbed moisture. Leonardo suggested comparing that weight with a dry reference, allowing changes in atmospheric moisture to be estimated. The idea belongs to his wider interest in measuring natural phenomena with physical instruments.

A hygrometer measures humidity, usually relative humidity in modern meteorology. Later hygrometers used hair, paper, salts, or other moisture-sensitive materials, but Leonardo's concept relied on weighing an absorbent substance.

This device is sometimes confused with a barometer. A barometer measures air pressure, while a hygrometer measures moisture. Leonardo's design was an early scientific instrument, not a weather forecast machine.