Leonardo da Vinci designed a hygrometer to measure humidity using a balance and moisture-sensitive material.
The instrument was intended to compare the weight of a dry substance with the weight of the same substance after it absorbed moisture from the air. Leonardo proposed using materials such as wool, whose mass changes as it takes up water vapour. A balance could then reveal the difference.
Humidity describes the amount of water vapour in air, so the hygrometer belongs to a different family of instruments from the anemometer, which measures wind speed. Leonardo's design did not provide the precise digital readings used by modern weather instruments, but it established a clear physical method for detecting atmospheric moisture.
Leonardo's interest in humidity connected with his studies of clouds, rain, evaporation, and the movement of water through nature. His notebooks frequently treated weather as a measurable physical system.