Leonardo da Vinci's humidity-measuring instrument was a hygrometer.
A hygrometer measures moisture in the air. Leonardo's early design used a balance comparing a material that absorbs moisture, such as cotton, with a material less affected by humidity. As the absorbent material gained water from the air, its weight changed, causing the balance to move. This provided a way to detect changing atmospheric moisture before electronic sensors existed.
Leonardo's device was an early humidity indicator rather than a modern precision instrument. It is sometimes confused with a barometer, which measures air pressure, or a thermometer, which measures temperature. The instrument illustrates Leonardo's interest in turning physical changes into measurable signals. Later hygrometers used hair, wet-and-dry bulbs, chemical substances, and electrical sensors, but the basic question remained the same: how much water vapor is present in the atmosphere?