A hygrometer is an instrument that measures how much water vapor is in the air, which scientists call humidity. When the weather forecaster says “80% humidity today,” a hygrometer is what helped measure that.

You've felt what different humidity levels are like even if you didn't know the word for them. On a humid summer day, the air feels heavy and sticky because it's packed with moisture. Your sweat doesn't evaporate easily, so you stay hot and uncomfortable. On a dry winter day, the air feels crisp and your lips might get chapped because there's so little moisture around. A hygrometer measures these differences precisely.

Some hygrometers use human hair, which gets longer when it's humid and shorter when it's dry. Others measure electrical changes that happen when moisture is in the air. Museums use hygrometers to protect paintings and artifacts, because too much or too little humidity can damage them. Greenhouses use them to keep plants healthy. Even some musical instruments need careful humidity control, so a concert hall might use hygrometers to make sure expensive violins don't crack or warp.