Leonardo da Vinci developed an early hygrometer to estimate atmospheric humidity.
His design used a moisture-sensitive material, traditionally described as wool, balanced against a dry reference such as a ball of wax. As the wool absorbed moisture from the air, its weight changed, causing the balance to move. The movement provided a comparative indication of dampness.
Leonardo’s instrument was not a modern calibrated electronic hygrometer, and it did not provide humidity readings in today’s standard units. Its value was methodological: it turned an invisible atmospheric property into a measurable physical effect.
A hygrometer measures humidity, while a barometer measures air pressure and a thermometer measures temperature. Those instruments are often grouped together in weather stations, but they answer different questions about the atmosphere.