Atmosphere is the layer of gases surrounding a planet, held in place by gravity. Earth's atmosphere is what allows us to breathe: it contains the oxygen we need and protects us from the sun's harmful radiation. When you look up at the sky, you're looking through miles of atmosphere, which is one reason the sky appears blue (the atmosphere scatters blue light from the sun).

Scientists divide Earth's atmosphere into different layers based on temperature and altitude. The lowest layer, where weather happens and most planes fly, is called the troposphere. Higher up is the stratosphere, which contains the ozone layer that shields us from ultraviolet radiation. Spacecraft have to punch through the entire atmosphere to reach orbit.

Without an atmosphere, Earth would be almost as lifeless as the Moon: no air to breathe, no clouds or rain, wild temperature swings, and little protection from meteorites. Mars has a very thin atmosphere, which is one reason it's so hard for humans to explore.

The word also describes the mood or feeling of a place. A library might have a quiet, studious atmosphere, while a birthday party has a festive atmosphere. When you walk into a room and sense tension, you're picking up on the atmosphere created by the people there. Just as a planet's atmosphere surrounds everything on it, the atmosphere of a place surrounds everyone in it, shaping how they feel and act.