Leonardo da Vinci designed a hygrometer to show changes in atmospheric humidity.
A hygrometer measures the amount of water vapor in air. Leonardo’s concept used materials whose weight changes when they absorb moisture, allowing the difference to be indicated by a balance. In descriptions of the design, absorbent substances are compared with materials less affected by humidity.
The invention belongs to Leonardo’s broader investigations of weather, air, water, and measurement. Renaissance scholars were increasingly interested in turning observations of nature into quantities that could be compared. A humidity indicator was useful because dampness affected materials, health, agriculture, and mechanical work.
The hygrometer is different from a barometer, which measures atmospheric pressure, and from a thermometer, which measures temperature. Leonardo’s design was an early conceptual instrument rather than the standardized electronic or calibrated hygrometers used today. Historical accounts sometimes differ over which earlier thinkers deserve priority for humidity-measuring devices.