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Hot weather can be uncomfortable—but humid days can feel downright oppressive! Humidity is part of the water cycle, the movement of water around Earth. As water moves between oceans, land, and air, it also changes its physical state: liquid, solid, or gas. When molecules of liquid water move fast enough to escape into the air, they turn into water vapor. That's evaporation. When those molecules lose energy, vapor turns back into liquid. That's condensation. When it happens high up in the air, condensation forms clouds; down on Earth's surface, it forms dew. The rate of condensation versus evaporation determines how humid or dry the air feels. Temperature plays a role in this ratio—the warmer the air, the more water can evaporate. That's why hot summer days can get so muggy, with relative humidity approaching 100 percent, and dew points well north of 70 degrees. There's more to learn, but don't sweat it—just press "play" to get started!