Leonardo da Vinci designed a hygrometer using a balance and absorbent material to indicate humidity.
The principle is straightforward: a material that absorbs moisture becomes heavier in damp air. Leonardo's proposed apparatus compared that changing weight with a reference object, allowing the observer to detect differences in atmospheric moisture. It was an early example of using a physical change to measure an environmental condition.
The device belongs to Leonardo's studies of weather and natural processes. It was not a modern electronic humidity sensor, and surviving evidence concerns a design rather than a widely used instrument manufactured under his name.
A hygrometer measures humidity, while a barometer measures air pressure and an anemometer measures wind. These instruments are often grouped together in weather observation, but they answer different questions about the atmosphere.