The atmosphere is a dynamic system, shaped by physical processes that move air around and chemical processes that build and break molecules. Both come back to one thing: the Sun does not heat the planet evenly.
📚 What you need to know
The atmosphere’s structure comes from continuous physical and chemical processes.
The enhanced greenhouse effect is the main cause of global warming, and differs from the natural effect because greenhouse gas concentrations have been artificially raised.
Differences in temperature and pressure drive air movement: warm air is less dense and rises, cold air is denser and sinks.
This creates convection currents, which drive winds and weather systems — described by the tricellular model of atmospheric circulation.
Ozone (O3) forms in the stratosphere through reactions involving sunlight and oxygen, and absorbs harmful UV radiation.
Ozone destruction and reformation create a dynamic equilibrium, keeping stratospheric ozone concentration roughly stable.
Physical process 1: the enhanced greenhouse effect
The natural greenhouse effect is a necessary process that traps enough heat to keep the climate habitable. The enhanced greenhouse effect disrupts that balance, because human activity has pushed greenhouse gas concentrations beyond natural levels and intensified the trapping of heat.
Wording that scores: the difference is not the mechanism — it is identical. The difference is the concentration of absorbing gas, and therefore the rate at which the planet loses energy to space.
Physical process 2: air movement
Start from the density rule and everything else follows:
Warm air is less dense, so it rises. Rising air leaves lower pressure at the surface below it.
Cool air is denser, so it sinks. Sinking air creates higher pressure at the surface below it.
Air flows from high pressure to low pressure. That flow is wind.
Rising air cools, water vapour condenses, and you get cloud and rain. Sinking air warms and dries, so you get clear skies.
Differential heating and the tricellular model
The Sun heats Earth unevenly. At the equator sunlight arrives almost overhead, concentrating energy on a small area. Towards the poles the same beam strikes at a shallow angle and spreads over a much larger area, so it delivers less energy per square metre. The equator is warmer; the poles are colder; and the atmosphere spends its time trying to move heat from one to the other.
This is why the world’s great rainforests sit near the equator and its great deserts sit near 30 degrees north and south. The pattern is not a coincidence — it is where air rises and where it sinks.
You met this model in Climate and Biomes. It is the same content, so use it in both places: circulation explains rainfall and temperature by latitude, and those in turn explain where each biome sits and how productive it is.
Chemical process: the ozone cycle
Ozone is a molecule of three oxygen atoms, O3. It forms mainly in the stratosphere through reactions involving sunlight and oxygen, and it protects life by absorbing a large share of the Sun’s harmful ultraviolet radiation.
What matters for the exam is that ozone is being destroyed and rebuilt all the time.
🧩 The cycle in four steps
UV radiation from the Sun strikes an ozone molecule.
The ozone absorbs that energy and breaks apart, giving an oxygen molecule (O2) and a free oxygen atom (O).
The free oxygen atom combines with another oxygen molecule, forming ozone again.
Because destruction and reformation happen at similar rates, a dynamic equilibrium is reached and the concentration stays roughly stable.
“Dynamic” means both reactions keep happening; “equilibrium” means they happen at the same rate, so the amount present does not change. Pollutants cause damage by speeding up destruction relative to reformation.
EXAM PRACTICE
Explain why many of the world’s major deserts are found at around 30 degrees north and south of the equator. [4]
Step 1: start at the equator
Intense, direct sunlight heats the surface strongly, so air warms, becomes less dense and rises.
Step 2: what happens to the rising air
As it rises it cools, water vapour condenses and falls as rain — the equator is wet, with low pressure.
Step 3: where the air goes next
That air moves polewards at altitude, cools further and sinks at about 30 degrees, creating high pressure.
Step 4: why it is dry
Sinking air warms and its capacity to hold water rises, so little condensation occurs.
Persistent descending, drying air at 30 degrees produces desert conditionsThis chain answers half a dozen different questions. Learn it once as a sequence, not as separate facts.
💡 Exam tip
Anchor every circulation answer in density: warm and less dense rises, cool and denser sinks.
Pair pressure with weather: low pressure = rising air = wet, high pressure = sinking air = dry.
Name the three cells if asked about the tricellular model, and say what happens at 0, 30, 60 and 90 degrees.
Define dynamic equilibrium properly — two opposing processes at equal rates, not “nothing is happening”.
Keep ozone depletion separate from the greenhouse effect. Same page of the syllabus, different problem.
⚠️ Common mix-up
Saying the equator is closer to the Sun. It is about the angle of the incoming radiation and how widely the energy is spread.
Thinking dynamic equilibrium means static. Molecules are being destroyed and rebuilt constantly.
Saying ozone reflects UV. It absorbs it, and the absorbed energy is what breaks the molecule apart.
Mixing up cell names and positions. Hadley 0 to 30, Ferrel 30 to 60, Polar 60 to 90.
Assuming enhanced means a different mechanism. The process is identical; the concentration is not.
Up next: How the Atmosphere Changes With Altitude (HL) — gravity, thinning air and the standard lapse rate.
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