Leonardo da Vinci designed a hygrometer to indicate humidity in the air.
A hygrometer measures moisture in the atmosphere. Leonardo’s proposed instrument used a moisture-sensitive material, such as wool, whose weight changed as it absorbed water from humid air. A balance could then show the relative change and provide an indication of atmospheric moisture.
The design reflects Leonardo’s habit of turning observations of natural phenomena into mechanical instruments. He was interested in weather, air, water, and the behavior of materials, and his notebooks contain several ideas for measuring environmental conditions.
A hygrometer is different from a barometer, which measures air pressure, and from a thermometer, which measures temperature. Leonardo also explored wind measurement, including an anemometer design. These instruments should therefore not be treated as interchangeable weather devices.