Leonardo da Vinci’s hygrometer design measured humidity.
A hygrometer measures the amount of moisture in air. Leonardo proposed an instrument that compared the behavior of materials affected differently by moisture, using their changing weight or condition to indicate atmospheric dampness.
His interest in humidity connected to broader studies of weather, flight, and natural processes. Renaissance scientists lacked modern electronic sensors, so early hygrometers depended on substances such as wool, salt, or other moisture-sensitive materials.
Leonardo’s device should not be confused with a barometer, which measures air pressure, or a thermometer, which measures temperature. Humidity is specifically the moisture content of the atmosphere, usually reported today as relative humidity.