Leonardo da Vinci’s hygrometer attempted to measure humidity.
Leonardo investigated how moisture in the air affected materials. His hygrometer concept used an absorbent substance, traditionally described as cotton or wool, whose weight would change as it took up moisture. Comparing the material’s weight under different conditions could provide an indication of atmospheric dampness.
A hygrometer is different from a thermometer. A thermometer measures temperature, while a hygrometer measures humidity, usually expressed today as relative humidity. Leonardo’s instrument was an early example of using a physical change in a material to estimate a quantity that cannot be seen directly.
The design was part of Leonardo’s broader interest in weather and natural processes. It was not a modern electronic sensor and would not provide the precision of contemporary instruments. Nevertheless, it shows his characteristic approach: observe a physical effect, connect it to a measurable change, and use that relationship to study nature.