Leonardo da Vinci designed an early hygrometer to estimate humidity by comparing the weight of moisture-absorbing materials.
His concept used a balance with materials such as cotton or wool on one side and a dry reference material on the other. As the absorbent material took in moisture from the air, its weight changed, causing the balance to move. The movement provided a visual indication of atmospheric dampness.
A hygrometer measures humidity, whereas a barometer measures air pressure and an anemometer measures wind. These instruments are often confused because all belong to the history of weather observation, but they measure different properties of the atmosphere.
Leonardo’s design was qualitative or approximate rather than a modern calibrated relative-humidity sensor. Even so, it applied a clear physical principle: water vapour in the air can change the mass of a hygroscopic material. The idea helped connect everyday observations of dampness with measurable instrument readings.