Leonardo da Vinci designed an early hygrometer to measure atmospheric moisture.
His concept used a moisture-sensitive material, such as wool, whose weight would change as it absorbed water from the air. By comparing that material with a dry reference, the instrument could indicate whether the surrounding atmosphere was more or less humid.
A hygrometer measures humidity, while a hydrometer measures the density or specific gravity of a liquid. That distinction is a common source of confusion. A barometer, by contrast, measures air pressure, and an anemometer measures wind speed.
Leonardo’s hygrometer belongs to his broader program of experimental observation. He repeatedly tried to convert natural phenomena—including water, air, motion, and temperature—into measurable quantities. Later hygrometers used hair, chemical substances, or electronic sensors, but the basic goal remained the same.