Leonardo da Vinci designed a hygrometer to indicate changes in humidity.
The instrument used materials whose weight changed as they absorbed moisture from the air. Leonardo proposed comparing a moisture-sensitive substance with a dry reference, creating a way to observe whether atmospheric humidity had risen or fallen. The design belongs to his wider investigations of weather and natural processes.
A hygrometer measures humidity, usually the amount of water vapor in air. Leonardo’s device was not a modern electronic sensor and did not provide the precise relative-humidity percentages used in weather reports today. It was an early conceptual instrument for detecting moisture in the atmosphere.
The design is often confused with Leonardo’s anemometer. The anemometer concerns moving air and wind speed, while the hygrometer concerns water vapor. Both reflect Leonardo’s habit of converting observations of nature into mechanical measuring devices.