Layers of the Atmosphere

Introduction

The atmosphere is a thin blanket of gas held around the Earth by gravity. If Earth were the size of an apple, the part that holds almost all the air would be thinner than the apple's skin. Yet this blanket is not uniform: going upwards, the air thins out and its temperature rises and falls in a distinctive pattern. Scientists use that pattern to divide the atmosphere into layers, each with its own character.

Core explanation

Air thins with height. Gravity pulls air molecules towards the ground, so the air is most dense at the surface. About half of all the air lies below roughly 5.5 km, and about 99% lies below about 30 km. At the top of Mount Everest, each breath contains only about a third as many oxygen molecules as at sea level, although oxygen is still about 21% of the air there.

Layers defined by temperature. Moving upwards, the temperature changes from falling to rising and back again. The boundaries where this happens separate four main layers:

Layer Approximate height Temperature trend with height --- --- --- Troposphere 0 to about 12 km (8 km at the poles, 16 km at the equator) falls, to about −55 °C at the top Stratosphere about 12 to 50 km rises, to near 0 °C at the top Mesosphere about 50 to 85 km falls, to below −90 °C Thermosphere about 85 to 600 km rises steeply, to several hundred °C or more

Beyond the thermosphere, the exosphere fades gradually into space.

The troposphere. This is the layer we live in. It contains about three-quarters of the mass of the atmosphere and nearly all its water vapour, so almost all clouds, rain and weather happen here. It is warmed mainly from below by the ground, which is why it gets colder with height — roughly 6.5 °C colder for every kilometre you climb. Warm air rising and cool air sinking keep the troposphere well mixed.

The stratosphere. Here the temperature rises with height because ozone (O₃) absorbs ultraviolet radiation from the Sun and converts it into heat. The region of highest ozone concentration, roughly 15 to 35 km up, is called the ozone layer . It protects living things by absorbing most of the harmful UV-B and UV-C radiation. Because warmer air sits above cooler air, the stratosphere is very stable, with little vertical mixing; long-haul aircraft often cruise near its lower boundary to fly above most turbulence and weather.

The mesosphere. Temperature falls again because there is little ozone to absorb energy. Most meteors burn up here as they hit the thin air at high speed.

The thermosphere. The very few gas particles here absorb high-energy radiation from the Sun and move extremely fast, giving very high temperatures. The air is so thin, however, that it would not feel hot. The aurorae (northern and southern lights) glow in this layer, and the International Space Station orbits within it at about 400 km.

Composition. Up to about 80 km the air is well mixed, so the proportions of nitrogen, oxygen and argon are nearly the same as at the ground. Only the total amount of air falls.

Step-by-step reasoning

To identify a layer from a temperature profile:

1. Read the height of each point where the graph changes direction. 2. Temperature falling from the ground: troposphere. 3. Temperature then rising: stratosphere (ozone absorbing UV). 4. Falling again: mesosphere; rising steeply after that: thermosphere.

Visual explanation

Sketch a graph with temperature across the bottom and height up the side. The line leans left from the ground to about 12 km, bends to lean right up to 50 km, leans left again to 85 km, then swings sharply right. The zig-zag shape, with its three turning points, marks the boundaries between the layers.

Real-world analogy

The atmosphere is like a tall block of flats in which each floor is heated differently. The ground floor is heated by the warm floor below it (the Earth's surface); a middle floor has its own heaters (ozone absorbing sunlight); the floors in between cool down because nothing heats them.

Real-world example

Climbers on very high mountains feel the cold and the thin air of the upper troposphere. Many use bottled oxygen above about 8000 m because each breath carries too few oxygen molecules, even though the percentage of oxygen in the air is the same as at sea level.

Why?

Why does almost all weather happen in the troposphere? It holds nearly all the water vapour needed for clouds and rain, and because it is heated from below, warm air near the ground keeps rising and cooling. This constant rising and sinking drives clouds, storms and winds. The stratosphere above is too dry and too stable for most weather.

Common misconception

"There is less oxygen at the top of a mountain because the percentage of oxygen is lower." The percentage is still about 21%. What falls is the total amount of air in each cubic metre, because the air is less dense, so each breath contains fewer oxygen molecules.

Worked example

Question: The ground temperature is 15 °C. Using a cooling rate of 6.5 °C per km, estimate the air temperature at 8 km in the troposphere.

Reasoning: Fall in temperature = 6.5 × 8 = 52 °C. Temperature = 15 − 52 = −37 °C.

Answer: About −37 °C.

Quick check

1. In which layer of the atmosphere is the ozone layer found? Answer: The stratosphere.

Exam focus

Learn the order troposphere, stratosphere, mesosphere, thermosphere, and the approximate heights of the troposphere and stratosphere. Explain the temperature rise in the stratosphere by ozone absorbing ultraviolet radiation, and state that nearly all weather happens in the troposphere.

Advanced insight

The boundaries between the layers are named with "-pause": the tropopause, stratopause and mesopause. The tropopause is higher over the equator than over the poles because strong heating at the equator makes air rise further before it stops. The thermosphere's high temperature refers to particle speed, not to how hot it would feel.

Summary

The atmosphere is divided into the troposphere, stratosphere, mesosphere and thermosphere according to how temperature changes with height. The troposphere contains most of the air and nearly all weather and gets colder with height. The stratosphere warms with height because its ozone absorbs ultraviolet radiation. Air becomes thinner with height, but its composition stays nearly constant up to about 80 km.

Practice questions

1. Name the four main layers of the atmosphere, starting from the ground. Answer: Troposphere, stratosphere, mesosphere, thermosphere. 2. Why does the temperature rise with height in the stratosphere? Answer: Ozone in the stratosphere absorbs ultraviolet radiation from the Sun and converts it into heat. 3. Why is the troposphere colder at the top than at the bottom? Answer: It is heated mainly from below by the Earth's surface, so air further from the ground is cooler. 4. Explain why the ozone layer is important to living things. Answer: It absorbs most of the Sun's harmful ultraviolet radiation, which can damage cells and cause sunburn and skin cancer.