The layers of the atmosphere, from the weather to the edge of space
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Drive 12 km along a motorway and you have hardly gone anywhere. Point the same 12 km straight up and you pass through the layer that holds about three quarters of all the air on Earth and nearly all of its weather. The atmosphere is deep enough to protect us but, compared with the size of the planet, remarkably thin.
Worked example: a globe with air
A classroom globe is 30 cm across; the real Earth is about 12,700 km across. Scaling 12 km of troposphere by the same factor:
- 12 ÷ 12,700 ≈ 0.00094.
- 0.00094 × 30 cm ≈ 0.028 cm, or about 0.3 mm.
On that globe, the layer containing all our weather would be roughly as thick as three sheets of printer paper.
What is air made of?
| Gas | Share of dry air (approx.) |
|---|---|
| Nitrogen | 78% |
| Oxygen | 21% |
| Argon | 0.9% |
| Carbon dioxide | about 0.04% |
| Others | tiny amounts |
Real air also contains water vapour, from almost none in cold, dry air to a few per cent in hot, humid places. Notice that most of what you breathe is nitrogen, not oxygen.
Five layers
Scientists divide the atmosphere by what the temperature does as you climb. The heights below are averages and change with latitude and season.
1. Troposphere: ground to about 12 km
Nearly all weather happens here: cloud, rain, snow and wind. The troposphere holds most of the atmosphere's mass and almost all of its water vapour. It is deeper over the Equator (around 16 to 18 km) than over the poles (around 8 km).
Temperature falls as you climb, by about 6.5 °C per kilometre on average. Airliners cruise near the top, 10 to 12 km up, where the air outside is often below −50 °C.
2. Stratosphere: about 12 to 50 km
Here the temperature stops falling and starts to rise with height. The cause is the ozone layer, a region mostly between about 15 and 35 km where ozone is more concentrated. Ozone absorbs ultraviolet light from the Sun, warming the air and shielding living things below from much of that radiation. The stratosphere is dry and calm, with very little cloud.
3. Mesosphere: about 50 to 85 km
Temperature falls again. The top of the mesosphere is the coldest part of the atmosphere, often around −90 °C. Most meteors, the "shooting stars" of a clear night, burn up here as specks of space dust slam into the air.
4. Thermosphere: about 85 km to several hundred km
The air is extremely thin. The few particles there absorb energy from the Sun and can reach over 1,000 °C, yet it would not feel hot, because there are too few particles to pass the heat on. The aurora glows in this layer when charged particles from the Sun strike the air. The International Space Station orbits within the thermosphere, at about 400 km.
5. Exosphere: the outermost layer
The exosphere fades into space. Particles are so far apart that some escape the Earth's gravity altogether.
A memory sentence for the order: The Silly Monkey Threw Eggs (troposphere, stratosphere, mesosphere, thermosphere, exosphere).
Where does space begin?
There is no sharp edge, only air that thins out with height. A widely used boundary is the Kármán line, 100 km above sea level. Some organisations use a lower figure of about 80 km. Either way, space starts closer than most people expect: 100 km is only a little further than the drive from London to Brighton.
Air pressure
Air has weight, and the air above you presses on everything around you. Normal sea-level pressure is about 1,013 hectopascals (hPa). A useful rule of thumb is that pressure roughly halves for every 5.5 km you climb.
Worked example: pressure with height
Start with a rounded 1,000 hPa at sea level.
- At 5.5 km: 1,000 ÷ 2 = 500 hPa.
- At 11 km: 500 ÷ 2 = 250 hPa.
The summit of Mount Everest, 8,849 m high, lies between these heights. The pressure there is only about a third of sea-level pressure, so each breath holds about a third as much oxygen, which is why most climbers carry bottled oxygen.
Worked example: temperature on a mountain
It is 18 °C at sea level. Estimate the temperature at the top of a 3 km mountain.
- Drop = 3 × 6.5 = 19.5 °C.
- 18 − 19.5 = −1.5 °C.
Real weather varies, so this is only an estimate, but it explains snow on summits above warm valleys.
The ozone hole
In 1985 scientists from the British Antarctic Survey reported that the ozone layer over Antarctica was thinning sharply each spring. The cause was a group of gases called CFCs, then used in fridges and aerosol cans. In 1987 countries agreed the Montreal Protocol to phase them out. The ozone layer is now slowly recovering, though full recovery is expected to take decades.
The ozone hole and global warming are separate problems. The hole lets more ultraviolet light reach the ground; global warming is driven mainly by extra greenhouse gases such as carbon dioxide.
Activity: label and test
Part A. Draw a tall rectangle for 0 to 100 km of atmosphere, using the same scale all the way up. Mark the layer boundaries, then add: clouds, an airliner, the ozone layer, a meteor, the aurora and the Kármán line. If you prefer a ready-made page to draw on, the EduPrintables worksheets include blank diagram layouts.
Part B. Show air pressure in action, over a sink and with an adult.
- Fill a glass to the brim with water.
- Slide a piece of stiff card over the top so that no air bubble is trapped.
- Hold the card in place and turn the glass upside down.
- Carefully take your hand away.
The card stays put, because air pressure pushing up on it is greater than the water pressing down.
Other planets have very different atmospheres: thick carbon dioxide on Venus, a thin one on Mars, and giant planets that are mostly atmosphere. Our guide to the planets compares them.
Sources and further reading
- Layers of Earth's atmosphere – UCAR Center for Science Education – each layer described, with heights and temperatures.
- Layers of the atmosphere – NOAA JetStream – the US weather service's guide to the five layers.
- Ozone assessments – NOAA Chemical Sciences Laboratory – the international scientific reports on ozone depletion and recovery under the Montreal Protocol.
