Definition · Plain-language
Atmosphere
The atmosphere is the blanket of gases that surrounds the Earth, held in place by gravity.
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The air around the Earth
The atmosphere is the envelope of gases surrounding our planet, kept from drifting into space by Earth’s gravity. We usually call it “the air”. Although it extends hundreds of kilometres upward, it has no sharp outer edge — it simply thins until it merges with space. By far the greatest density of gas, and almost all the weather, sits in the lowest few kilometres. The atmosphere is one of Earth’s key systems, interacting closely with the oceans, land and living things, and it is what makes our planet habitable.
What it is made of
Dry air is a remarkably consistent mixture: about 78% nitrogen and 21% oxygen make up almost the whole of it, with argon accounting for most of the remaining 1%. Beyond these, tiny but vital amounts of other gases are present, including carbon dioxide, neon, helium and methane, along with a variable amount of water vapour. Although carbon dioxide and water vapour are minor by quantity, they have an outsized effect, because they are greenhouse gases that trap heat and help keep the planet warm. The ozone layer, high up, absorbs harmful ultraviolet radiation from the Sun.
The layers of the atmosphere
The atmosphere is divided into layers by how temperature changes with height. The troposphere, the lowest layer, holds most of the air and nearly all weather, and cools with altitude. Above it, the stratosphere contains the ozone layer and warms with height. The mesosphere above that is where most meteors burn up, and it is the coldest layer. Higher still, the thermosphere becomes extremely hot and hosts the auroras and many satellites. The outermost exosphere gradually fades into the vacuum of space. Each layer has distinct conditions that shape what happens within it.
Key facts
At a glance
- Definition: the layer of gases surrounding Earth, held by gravity
- Composition: ~78% nitrogen, ~21% oxygen, ~1% argon and trace gases
- Key trace gases: carbon dioxide and water vapour (greenhouse gases)
- Layers: troposphere, stratosphere, mesosphere, thermosphere, exosphere
- Weather in: the troposphere, the lowest layer
- Why it matters: provides oxygen, traps warmth and shields the surface
Common misconceptions
What people often get wrong
Often heard: The atmosphere is mostly oxygen.
Actually: Air is about 78% nitrogen and only around 21% oxygen. Nitrogen, not oxygen, is by far the most abundant gas in the atmosphere.
Often heard: The atmosphere ends at a clear, sharp boundary with space.
Actually: The atmosphere simply thins with height until it fades into space, with no hard edge. Any boundary line, such as the Kármán line, is a convenient convention rather than a physical wall.
Often heard: Every layer of the atmosphere gets colder as you go up.
Actually: Temperature rises and falls between layers. The troposphere and mesosphere cool with height, but the stratosphere and thermosphere warm with height, because of ozone and solar heating.
