Humidity is the amount of water vapor in the air. Even when you can't see it, the air around you contains tiny invisible water molecules that have evaporated from oceans, lakes, rivers, and even puddles. On a humid day, the air holds a lot of this moisture, which is why your skin might feel sticky or damp, and why a glass of cold lemonade quickly gets covered in droplets.

You've probably heard weather forecasters talk about “relative humidity.” This measures how much water vapor the air is holding compared to how much it could hold at that temperature. When relative humidity reaches 100 percent, the air is completely saturated and can't hold any more water vapor. That's when you get fog, dew on the grass, or rain.

Humidity affects how hot or cold you feel. On a humid summer day, sweat doesn't evaporate easily from your skin, so you feel hotter and stickier than the actual temperature would suggest. That's why 85 degrees with high humidity can feel much worse than 95 degrees in the desert, where the air is dry. In winter, low humidity can make cold air feel even more bitter and can dry out your skin and throat. Some homes use humidifiers to add moisture to dry indoor air, while others use dehumidifiers to remove excess moisture and prevent mold growth.