The layers of the atmosphere

How does temperature change all the way up to space?

Temperature with height

The key diagram of the whole chapter, built one layer at a time. Heights to scale.

1. Troposphere: colder as you climb Heated from below, the air cools about 6.5 °C per km (the lapse rate). It is generally unstable and well stirred. Here are all the weather, nearly all the water vapour and dust, about 75% of the air, and most flying. Lower troposphere: surface to 2.1 km. Middle: 2.1 to 7.6 km. Upper: 7.6 km to the tropopause.
2. Tropopause: the cooling stops At the top of the troposphere the lapse rate drops to 1-2 °C per km or less: the temperature stops falling. In the ISA this is at 11 km, at −56.5 °C.
3. Stratosphere: warmer as you climb For the first 8-10 km above the tropopause the temperature stays about the same (isothermal). Then it rises, slowly and then sharply, up to about 0 °C at 50 km.
4. Why: the ozone The inversion is caused by ozone absorbing the sun's ultraviolet light, most of it at 20-25 km. Warm air above cold air is very stable: the stratosphere is dry, calm and has no weather.
5. Mesosphere: colder again Above the stratopause at 50 km there is little to absorb sunlight and little mixing, so the temperature falls again, to the coldest level of the atmosphere at the mesopause, about 80 km (about −90 to −100 °C, 173 K).
6. Thermosphere: very hot, very thin Above the mesopause the few molecules left are heated strongly by the sun, so the temperature climbs with height. Up to about 80 km the gases are evenly mixed (homosphere); above, they separate by weight (heterosphere). Mesopause (80 km) is the coldest level: about −100 °C ≈ 173 K.

Memory hook: Down, up, down, up: tropo, strato, meso, thermo.

Takeaway: Temperature falls through the troposphere, stays at −56.5 °C then rises through the stratosphere (ozone), falls through the mesosphere to the coldest level (about −90 to −100 °C at 80 km), then rises in the thermosphere.

What do the layers look like wrapped round the Earth?

The layers in 3D

A real NASA picture of the Earth with a slice cut out, so you can see the layers like the layers of a cake. They are drawn much thicker than real; the labels give the real heights. Drag to turn it.

• The troposphere: a thin skin All our weather, and almost all flying, happens in this lowest layer. It is 16-18 km deep over the equator and only 8-10 km over the poles.
• It bulges at the equator Look at the orange band in the cut: it is thicker over the equator than over the poles, because warm tropical air rises higher before it stops cooling.
• The stratosphere above it Above the troposphere lies the stratosphere, up to about 50 km. Ozone in it soaks up sunlight, so it gets warmer as you climb. Warm air on top of cold air is very stable, so this layer is calm and dry, with almost no cloud and no weather. Its winds are strong and steady, which is why long-range aircraft like to cruise near its base.
• The mesosphere From 50 to 80 km the air gets colder with height, down to the coldest air of all. Above it, the thin, hot thermosphere fades into space.

Takeaway: The layers are shells round the whole Earth. The lowest, the troposphere, is a thin skin that is thicker over the equator. The stratosphere, mesosphere and thermosphere sit on top of it.

What changes as you climb from the runway to space?

The climb

Drag the altitude, tap a stop, or press Climb. Watch the sky, the temperature and the pressure change.

• Troposphere
• From the ground to the tropopause: 16-18 km at the equator, 8-10 km at the poles (11 km in the ISA). Three quarters of the air and nearly all the water vapour. All weather happens here.
• Stratosphere
• Up to about 50 km. First steady in temperature, then warming as ozone absorbs ultraviolet light. Dry, stable, no weather.
• Mesosphere
• From 50 to 80 km. Temperature falls again; its top, the mesopause, is the coldest place in the atmosphere.
• Thermosphere
• Above 80 km. A few molecules heated to hundreds of degrees by the sun. Contains the ionosphere, which bends radio waves, and the aurora.
• Runway
• Sea level in the standard atmosphere: 15 °C and 1013.25 hPa. The air is densest here, about 1.225 kg in every cubic metre.
• Fair-weather clouds
• Warm air rising off the ground cools until its water vapour condenses. Puffy cumulus mark the tops of those thermals.
• 10 000 ft
• Above about 10 000 ft the air is thin enough that pilots need supplementary oxygen.
• Half the air is below you
• Air is squeezed by its own weight, so it is packed low down. By about 6 km half the atmosphere is beneath you, and pressure has halved.
• Everest
• 8.8 km. Clouds pile against the Indian side of the Himalayas while Tibet stays dry. Over the poles the troposphere would already be ending here.
• Airliners cruise
• FL360 is about 11 km: the tropopause of the standard atmosphere. Outside it is −56.5 °C. Jet streams and clear-air turbulence sit just below the tropopause.
• Tropopause over India
• Over India the tropopause is about 16 km. A thunderstorm cannot rise through it, so its top flattens into an anvil, as in this photo from the space station.
• Into the stratosphere
• NASA's ER-2 flies up here, about 20 km. The sky overhead is almost navy blue. The air is very dry and calm: there is no weather to fly through.
• Ozone layer
• Ozone is spread from 10 to 50 km, densest at 20-25 km. It absorbs the sun's ultraviolet, which shields us and warms this layer. The map shows the seasonal hole over Antarctica.
• Weather balloons burst
• Radiosonde balloons swell as the pressure drops and usually burst around 30-35 km. Ninety-nine per cent of the air is now below you.
• Stratopause
• The top of the stratosphere, about 50 km. The warming from ozone peaks here, close to 0 °C.
• Meteors burn up
• The mesopause, about 80 km: around −100 °C (173 K), the coldest place in the atmosphere. Grains from space burn up here as shooting stars.
• Noctilucent clouds
• The highest clouds of all, made of ice at 80-85 km. Seen in polar summer just after sunset, lit by a sun that has already set below.
• Aurora
• Particles from the sun make the thin gas glow green and red, from about 100 to 300 km. Near the poles: northern and southern lights.
• Space station
• About 400 km. Everything you climbed through is that thin blue line on the horizon. The air thins out into the exosphere and, in the end, into space.

Memory hook: Cold, warm, cold, hot: the temperature zig-zags as you climb.

Takeaway: Troposphere (weather, cooling) → tropopause → stratosphere (ozone, warming) → mesosphere (coldest) → thermosphere (aurora, hot but empty).

Can you put each fact in its layer?

Which layer?

Tap a fact, then the layer it belongs to.

• Almost all clouds and rain
• Cools about 6.5 °C per km
• Holds about 75% of the air's mass
• Ozone warms it
• Mother-of-pearl (nacreous) clouds
• Very dry and stable, no weather
• Coldest place, about −100 °C
• Meteors burn up
• Noctilucent clouds
• Aurora
• Ionosphere bends radio waves

Takeaway: Weather and cooling: troposphere. Ozone and calm: stratosphere. Coldest and meteors: mesosphere. Aurora and radio: thermosphere.

Can you actually see the layers?

Layers from space

A real sunset photographed from orbit. Tap each band of colour.

• Troposphere
• The orange band: the lowest, densest layer. Its dust and water vapour scatter the setting sun's light into reds and oranges.
• Stratosphere
• The bright white band above: thin, dry air with very fine particles that scatter sunlight.
• Mesosphere and above
• Air so thin that the blue fades into the black of space.
• The sun
• The sun, setting behind cloud tops in the troposphere. Its light crosses hundreds of kilometres of air to reach the camera.

Takeaway: At sunset, light passes sideways through the air. The orange band is the dense troposphere; above it the stratosphere glows white and blue.