Leonardo da Vinci is credited with designing an early hygrometer to measure the humidity of air.
The device used the changing weight of a moisture-sensitive material, traditionally described as cotton or another absorbent substance. As the material took up water from humid air, its weight changed; that change could be compared on a balance. The principle connected atmospheric moisture with a measurable physical effect.
Leonardo’s hygrometer belongs to his broader interest in weather, water, and experimental measurement. Renaissance instruments often relied on simple materials and visual comparisons rather than calibrated scales in the modern sense.
A hygrometer measures humidity, while a barometer measures air pressure and a thermometer measures temperature. Those instruments are frequently mixed up because all are associated with weather observation, but they measure different properties of the atmosphere.