Leonardo da Vinci sketched a hygrometer for detecting changes in humidity.
A hygrometer measures the amount of water vapour in air. Leonardo’s proposed instrument used materials that changed weight or behaviour as they absorbed moisture, allowing wet and dry conditions to be compared. His interest in the subject was part of a broader study of air, weather, water, and the properties of materials.
The device is often presented as an early humidity-measuring instrument, but it was not a modern precision hygrometer. Leonardo’s notebooks contain many experimental proposals, and not every mechanism was completed or tested. The surviving idea is valuable because it shows him turning an invisible atmospheric property into something that could be observed mechanically.
Humidity is different from temperature and wind speed. A thermometer measures temperature, while an anemometer measures wind speed. A hygrometer instead concerns moisture in the air, whether expressed as relative humidity or another measure of water vapour.