Leonardo da Vinci designed a hygrometer to determine humidity in the air.
A hygrometer measures the amount of moisture in the atmosphere. Leonardo’s proposed instrument used a balance involving materials that changed weight when they absorbed moisture, allowing differences in atmospheric humidity to be observed.
The design belongs to Leonardo’s broader investigation of air, water, weather, and natural processes. He often treated instruments as extensions of careful observation: a device could turn an invisible environmental change into a visible movement or shift in balance.
Leonardo’s hygrometer should not be confused with a thermometer, which measures temperature, or a barometer, which measures atmospheric pressure. Later hygrometers used hair, salts, chemical changes, or electronic sensors, but the essential goal remained the same: quantifying water vapor in air.