Leonardo da Vinci designed an early hygrometer to measure moisture in the air.
His apparatus used a balance with materials whose weight changed when they absorbed atmospheric moisture. By comparing the changing weight with a reference, the instrument could indicate whether the air was relatively dry or humid. This made humidity a measurable physical property rather than merely a subjective sensation.
Leonardo’s hygrometer was an early contribution to meteorological instrumentation. It was not a modern electronic humidity sensor, and the surviving descriptions do not provide the precision expected from later calibrated instruments. Nevertheless, the design shows his interest in connecting observation, mechanics, and natural philosophy.
A hygrometer measures humidity, while a thermometer measures temperature and a barometer measures atmospheric pressure. Those instruments are often grouped together in weather stations, but they answer different questions about the atmosphere.