atmosphere
/ AT-muhs-feer /
An atmosphere is the blanket of gases that a planet's gravity holds snug against its surface — invisible, but very much there. On Earth it's mostly nitrogen and oxygen, plus a shifting amount of water vapour, thinning out the higher you go until it fades into space. You feel it every time the wind pushes against you or your ears pop in a plane: that's the weight and pressure of the air you live at the bottom of.
This thin skin of air does an astonishing amount of work. It filters out the Sun's most dangerous radiation, burns up incoming meteors, and gives us oxygen to breathe. Its trace gases also trap heat — the greenhouse effect — keeping Earth's average temperature about 33°C warmer than it would otherwise be, enough to stop the whole planet from freezing over. Meanwhile the sheer bulk of the air stores and carries heat around, so days don't scorch and nights don't plunge to lethal cold. Without all this, the surface would swing wildly, and nothing could breathe.
A common mistake is to picture the atmosphere as huge and deep. It isn't. About three-quarters of it sits within roughly 11 kilometres of the ground — the cruising height of an airliner — and 99% lies below about 30 kilometres. Compared to the size of the Earth, it's no thicker than the skin on an apple, which is exactly why it's worth protecting.
Roughly 78% nitrogen, 21% oxygen, and a 1% mix of argon, carbon dioxide, water vapour and trace gases — that last sliver does much of the climate's heavy lifting.
Earth's air, by volume of dry air.
The word comes from the Greek atmos (vapour) and sphaira (sphere) — literally a "ball of vapour." Scientists slice the atmosphere into layers by temperature: the troposphere where weather happens, then the stratosphere (home of the ozone layer), the mesosphere, and the thermosphere.