The composition and layered structure of the atmosphere, the natural greenhouse effect, how the atmosphere distributes heat and drives climate, and the importance of the ozone layer.

What this dot point is asking

AQA wants you to describe the composition and layered structure of the atmosphere, explain the natural greenhouse effect, describe how the atmosphere distributes heat and influences climate, and explain the importance of the ozone layer. A frequent discriminator is keeping the greenhouse effect (infrared, troposphere) and ozone depletion (ultraviolet, stratosphere) clearly separate.

Composition of the atmosphere

Nitrogen is largely inert and cycles slowly; oxygen supports respiration and combustion and was produced over geological time by photosynthesis. Carbon dioxide and water vapour are the main natural greenhouse gases, and water vapour also drives weather through evaporation and condensation. The relative proportions of the major gases have been roughly constant over human history, but the trace gas concentrations (especially carbon dioxide and methane) have changed sharply through human activity.

Structure of the atmosphere

The atmosphere is layered by how temperature changes with height:

• Troposphere (surface to about 12 km): contains most of the mass and all weather; temperature falls with height because it is heated from the warm surface below.
• Stratosphere (about 12 to 50 km): contains the ozone layer; temperature rises with height because ozone absorbs ultraviolet radiation and releases heat. This temperature inversion makes the layer stable and free of weather, which is why aircraft cruise here.
• Mesosphere (about 50 to 85 km): temperature falls again; meteors burn up here.
• Thermosphere (above about 85 km): very thin air absorbs high-energy solar radiation, so temperature rises steeply.

The natural greenhouse effect

The key is the change in wavelength. Incoming solar radiation is short-wave (visible and ultraviolet) and passes through the atmosphere largely unabsorbed. The surface absorbs it, warms, and re-emits energy as long-wave infrared. Greenhouse gases are transparent to short-wave but absorb specific infrared wavelengths, trapping that energy near the surface. Without this natural effect the average surface temperature would be about minus 18 degrees Celsius instead of about plus 15 degrees Celsius, so the planet would be frozen. Human emissions strengthen it, producing the enhanced greenhouse effect and global warming.

Heat distribution and climate

Solar radiation strikes the equator nearly vertically (concentrated) but the poles obliquely (spread out), so the tropics receive far more energy than the poles. This imbalance drives heat redistribution: warm tropical air rises and moves polewards, setting up convection cells (such as the Hadley cell) and the global wind belts, while ocean currents carry warm and cold water between latitudes. This atmospheric and oceanic circulation creates climate zones, prevailing winds and weather patterns, and prevents the tropics overheating and the poles freezing further.

The ozone layer

The ozone layer in the stratosphere absorbs most of the Sun's harmful ultraviolet (UV) radiation, particularly the damaging UV-B. Ozone ($O_3$) is continually formed and destroyed in a natural cycle that absorbs UV energy. By filtering out UV, the layer protects living things from DNA damage, skin cancer, cataracts and harm to plankton and crops. Its formation early in Earth's history allowed complex life to colonise the land. Human-made chlorofluorocarbons release chlorine that catalytically destroys ozone, which is why the Montreal Protocol phased them out.

Try this

Q1. State the two most abundant gases in dry air and their approximate percentages. [2 marks]

• Cue. Nitrogen about 78 percent and oxygen about 21 percent.

Q2. Explain why the temperature rises with height in the stratosphere but falls with height in the troposphere. [3 marks]

• Cue. The stratosphere is heated from within by ozone absorbing ultraviolet; the troposphere is heated from the warm surface below, so it cools with distance from the ground.

Q3. Explain why the natural greenhouse effect is important for life, including the approximate temperatures involved. [3 marks]

• Cue. Greenhouse gases absorb outgoing infrared and warm the surface; without it the Earth would be about minus 18 degrees Celsius rather than about plus 15 degrees Celsius, too cold for liquid water and life.