Humid describes air that contains a lot of moisture or water vapor. On a humid day, the air feels heavy and damp, almost like you could wring it out like a wet towel. Your skin might feel sticky, and you'll probably sweat more than usual because your body has trouble cooling itself when the air is already full of moisture.

Different places have different levels of humidity. Rainforests and coastal areas tend to be very humid, while deserts are dry with low humidity. In the summer, many American cities become humid as warm air holds more water vapor. That's why a 90-degree day in Arizona feels different from a 90-degree day in Georgia: the dry heat versus the humid heat.

You might hear someone say “it's not the heat, it's the humidity” when complaining about uncomfortable weather. They mean that high humidity makes hot temperatures feel even more oppressive. A humid basement might develop mold, and humid conditions can make your hair frizzy or cause glasses to fog up when you walk outside from an air-conditioned building.