Leonardo da Vinci designed an early hygrometer to measure humidity, making it a precursor of the modern hygrometer.
The instrument used a balance with materials whose weight changed differently as atmospheric moisture varied. A moisture-sensitive substance, often described in accounts of the design as cotton or wool, was compared with a material less affected by humidity. Changes in balance could indicate whether the air was becoming wetter or drier.
Leonardo’s design belongs to his broader investigations of air, weather, water, and measurement. It was not a modern electronic sensor and did not provide humidity readings in the precise numerical form used today. Instead, it was an early attempt to create a repeatable relationship between atmospheric moisture and a visible mechanical change.
A hygrometer measures humidity, while a thermometer measures temperature and a barometer measures atmospheric pressure. Leonardo’s instrument is sometimes confused with his anemometer, which concerned wind speed. These devices show his effort to turn observations of weather into measurable physical data.