Leonardo da Vinci designed a hygrometer to measure humidity in the air.
His proposed instrument used a balance: one side held a moisture-sensitive material, while the other held a reference weight. As the material absorbed water from humid air, its mass changed, shifting the balance and indicating atmospheric moisture.
A hygrometer measures humidity, whereas an anemometer measures wind speed and a barometer measures air pressure. Those instruments are frequently grouped together in weather discussions, but they measure different properties.
Leonardo's design belongs to his broader interest in quantifying natural phenomena. It was a conceptual mechanical instrument rather than the electronic humidity sensors used in modern weather equipment.