Leonardo da Vinci’s hygrometer aimed to measure humidity.
His design used the changing weight of an absorbent material to indicate moisture in the air. As the material took up water, its mass changed; that change could be compared with a dry reference material to estimate atmospheric moisture.
A hygrometer measures humidity, especially the amount of water vapour in air. Leonardo’s concept was not a modern digital instrument and did not provide the standardized readings used by contemporary weather stations.
Leonardo’s interest in atmospheric measurement reflects his wider studies of water, air, weather, and natural processes. Later hygrometers used materials such as hair, whose length changes with humidity, and eventually electronic sensors.