Leonardo da Vinci improved a hygrometer using wool and a balance to indicate humidity. Because wool absorbs moisture from the air, its changing weight could provide a visible comparison of damp and dry conditions.
A hygrometer measures the moisture content of air. Leonardo's design used the sensitivity of a material rather than a modern electronic sensor or a calibrated psychrometer. As the wool gained moisture, the balance would shift; the direction or amount of movement could be used as an indication of atmospheric humidity.
The instrument belongs to Leonardo's broader investigations of natural processes and measurement. It is often confused with his anemometer, which concerns wind, but the two devices measure different properties of air. The design was an early practical attempt to make humidity observable through a controlled mechanical effect.