Leonardo da Vinci designed a hygrometer to measure humidity in the air.
His proposed instrument used a balance to compare a dry material with the same material after it had absorbed moisture. The change in weight indicated how damp the surrounding air was. This principle anticipated later hygrometric methods based on moisture absorption.
A hygrometer measures humidity, whereas an anemometer measures wind speed and a barometer measures atmospheric pressure. Those instruments are often grouped together because they all belong to the history of weather observation, but they answer different questions.
Leonardo’s device was part of his wider interest in making natural phenomena measurable. The surviving drawings show an ingenious concept rather than a modern electronic sensor, and the instrument should not be confused with later hair, wet-bulb, or digital hygrometers.