Layers of the Atmosphere

The atmosphere is not the same all the way up. As you go higher, the air gets thinner, the temperature changes, and conditions become very different from what we experience at ground level. Scientists divide the atmosphere into five main layers, each named after how the temperature changes with altitude. From bottom to top they are the troposphere, stratosphere, mesosphere, thermosphere and exosphere. Each layer has its own weather, its own visitors and its own role to play.

  • Five layersTroposphere, stratosphere, mesosphere, thermosphere, exosphere
  • Troposphere topApprox. 12 kmWhere almost all weather happens
  • Stratosphere topApprox. 50 kmContains the ozone layer
  • Mesosphere topApprox. 85 kmColdest part of the atmosphere
  • Thermosphere topApprox. 600 kmISS orbits here, plus the auroras
  • Exosphere topApprox. 10,000 kmFades into the vacuum of space

The five layers

1. Troposphere (0 to 12 km)

The layer we live in. Approx. 75% of the air is in the troposphere, plus almost all the water vapour and almost all the weather. The troposphere is warmest at the bottom (heated by the ground) and gets colder as you go up: by about 6.5 °C per kilometre. The top of the troposphere is around -55 °C. Mountains, clouds, hurricanes, rain and most aircraft all stay within this layer.

2. Stratosphere (12 to 50 km)

Above the troposphere. This is where the famous ozone layer sits, between about 20 and 30 km up. Ozone absorbs harmful ultraviolet light from the Sun, warming the upper stratosphere. So unlike the troposphere, the stratosphere actually gets warmer as you go up. Long-distance jets fly in the lower stratosphere because the air is calm and predictable here, well above the weather. The stratosphere is also where the highest weather balloons reach.

3. Mesosphere (50 to 85 km)

Above the stratosphere. The mesosphere is the coldest layer of the atmosphere: temperatures drop to around -85 °C at the top. This is where most meteors from space burn up as bright streaks across the night sky. The mesosphere is too high for aircraft or weather balloons but too low for satellites, making it the least-studied part of the atmosphere.

4. Thermosphere (85 to 600 km)

Above the mesosphere. The thermosphere is extremely thin (the air pressure here is less than a billionth of sea-level pressure), but the individual gas molecules are very energetic. Temperatures can reach 2,000 °C at the top, although a thermometer would not feel hot because there are so few molecules to transfer heat. The International Space Station orbits in the thermosphere, at around 400 km. The spectacular auroras (Northern and Southern Lights) also happen here, where energetic particles from the Sun smash into atmospheric atoms.

5. Exosphere (600 km and above)

The outermost layer. So thin it is barely an atmosphere at all. Gas molecules are so far apart they can travel for hundreds of kilometres without bumping into each other. The exosphere gradually fades into the vacuum of space; there is no clear boundary. Many satellites orbit in the exosphere. Some atoms here have enough speed to escape Earth's gravity completely and drift away into space, slowly thinning the atmosphere over billions of years.

Fact The boundary between Earth's atmosphere and "space" is officially set at 100 km, called the Karman line. Below the line you can still fly with wings, above it you need rockets. Anyone who crosses the Karman line is officially counted as an astronaut by most space agencies. The line is named after Hungarian-American engineer Theodore von Karman, who calculated where it should be.

Why temperature goes up and down

The temperature changes in unexpected ways as you go up.

  • Troposphere: cools with altitude. Heated from below by the ground.
  • Stratosphere: warms with altitude. Heated by ozone absorbing UV.
  • Mesosphere: cools with altitude. Almost no heat source.
  • Thermosphere: warms with altitude. Heated by direct radiation from the Sun.
  • Exosphere: very gradually warms then loses meaning (so few molecules that "temperature" is hard to define).
Did you know? The mesosphere occasionally hosts beautiful glowing clouds called noctilucent clouds ("night-shining" clouds). They form around 80 km up out of tiny ice crystals around meteoric dust, and they only show up around midsummer when the Sun lights them from below the horizon. They were first reported in 1885 (probably triggered by dust from the Krakatoa eruption) and have been getting brighter and more common in recent years.
Deeper dive: how the ISS keeps from falling down

The International Space Station orbits in the upper thermosphere, at around 400 km. Even at that height the atmosphere is not completely empty: there is still a trace of gas molecules. Over time, the ISS bumps into these molecules and slowly loses energy, dropping by roughly 2 km per month.

If nothing was done, the ISS would gradually fall lower until it eventually re-entered the atmosphere properly and burned up. To stop this, the station is regularly reboosted back up. Supply ships docked to the ISS fire their engines a few times a year, pushing the whole station up to a higher orbit again.

Other satellites have it easier or harder. Very low satellites need frequent boosting and rarely last more than a few years. Satellites in geostationary orbit (35,786 km up) are well above any atmosphere and can stay in orbit essentially forever, only limited by their internal fuel for occasional positioning adjustments. The choice of orbit altitude is always a trade-off between cost, lifespan and what the satellite is for.

For more, see the ozone layer and auroras.