What humidity-measuring device did Leonardo da Vinci design?

Leonardo da Vinci designed a hygrometer, an instrument for measuring humidity.

His concept used the change in weight of a moisture-absorbing material to indicate the amount of water vapour in the air. As the material absorbed moisture, its increased weight could be compared with a scale or balance to show changing atmospheric conditions.

A hygrometer measures humidity, while a thermometer measures temperature and a barometer measures air pressure. These instruments are often grouped together in weather stations, but they measure different physical properties.

Leonardo’s work on the hygrometer formed part of his broader investigations into air, water, weather, and natural processes. Later hygrometers used other materials and mechanisms, including hair, wet-and-dry bulb systems, and electronic sensors. Leonardo’s design was an early attempt to turn an invisible atmospheric property into a measurable quantity.