Leonardo da Vinci designed a hygrometer to measure humidity by comparing the weight of dry and wet materials.
A hygrometer measures the amount of moisture in air. Leonardo’s proposed instrument used a balance: a moisture-absorbing material would become heavier in damp conditions, and the change could be compared with a dry reference. The movement of the balance would indicate relative atmospheric moisture.
Leonardo’s idea belongs to his broader investigations of air, water, weather and the physical properties of materials. The instrument was not an electronic sensor and would not have produced the precise digital readings available today. It was an early attempt to turn an invisible atmospheric condition into a measurable mechanical effect.
A hygrometer is different from a barometer, which measures air pressure, and a thermometer, which measures temperature. Leonardo is sometimes credited with early meteorological instruments generally, but individual attribution should be made carefully because some surviving notebooks contain proposals rather than documented completed devices.