Leonardo da Vinci sketched a hygrometer, an instrument intended to measure atmospheric moisture using a balance.
The design compared the weight of a moisture-absorbing material with a reference weight. As the material took in water from humid air, its weight changed, shifting the balance. That change could be read as an indication of humidity. The principle is straightforward: more atmospheric moisture produces a measurable increase in mass.
Leonardo’s hygrometer belongs to his wider work on meteorology and natural processes. He carefully observed clouds, rain, rivers, and air movement because these subjects were relevant both to science and to engineering. His notes often connect apparently separate topics, such as weather observation and the design of flying machines.
The device should not be confused with a barometer. A barometer measures air pressure, whereas a hygrometer measures humidity. Leonardo’s proposal was an early mechanical approach, not the electronic humidity sensors used today.