Leonardo da Vinci designed a hygrometer to measure humidity in the air.
A hygrometer measures the amount of water vapor in the atmosphere. Leonardo’s proposed device used the changing weight of a moisture-sensitive material, such as a wool-like substance, to indicate differences in humidity. Drier air and more humid air would produce different measurements.
The design belongs to Leonardo’s early studies of weather and atmospheric conditions. He was interested in observing natural phenomena quantitatively, using physical changes as clues to invisible properties such as moisture.
A hygrometer is not the same as a thermometer, which measures temperature, or a barometer, which measures air pressure. Leonardo is sometimes credited with an early hygrometer, but the modern electronic instruments used today are far more precise. His device is best understood as an early scientific measuring concept.