Leonardo da Vinci sketched a hygrometer to measure changes in air moisture.
His design used an absorbent material, traditionally described as wool, whose weight would change as it took up moisture from the air. A balance could then indicate relative humidity. Leonardo recorded the idea in his notebooks while studying weather and natural processes.
A hygrometer is not the same as a barometer, which measures air pressure, or a thermometer, which measures temperature. Leonardo's instrument was an early humidity-measuring concept rather than a modern electronic sensor. The surviving drawings show that his interest extended beyond machines for transport and warfare to instruments for observing the atmosphere.