Leonardo da Vinci designed a hygrometer to measure humidity by comparing changes in material weight.
A hygrometer measures the moisture content of air. Leonardo is often credited with designing an early hygrometer that used a moisture-sensitive material, such as cotton or wool, balanced against a dry material. As the absorbent material took up water from the air, its weight changed and disturbed the balance.
This was a qualitative or comparative approach rather than the precise electronic measurement used by modern instruments. Leonardo's proposal belongs to his wider interest in atmospheric science, weather, and the physical properties of water. Renaissance investigators often relied on visible changes in weight, length, or balance to detect environmental conditions.
A hygrometer is not the same as a thermometer, which measures temperature, or a barometer, which measures air pressure. Leonardo's weather-related studies also included attempts to understand wind and water movement, but those instruments measured different properties.