Leonardo da Vinci designed a hygrometer, a device intended to measure humidity in the air.
His concept used the changing weight of a moisture-absorbing material. As the material took in water from humid air, its weight would change; that change could then be compared with a scale. This approach connected an observable physical effect to the otherwise difficult-to-see amount of moisture in the atmosphere.
A hygrometer measures humidity, while a barometer measures air pressure, a thermometer measures temperature, and an anemometer measures wind speed. These instruments are often grouped together because they all belong to weather observation, but they measure different properties.
Leonardo’s design is significant because it shows his interest in experimental measurement and meteorology. The device is generally treated as an early hygrometer concept, not as a modern electronic sensor. Later hygrometers used materials such as hair and other moisture-sensitive elements in more standardized instruments.