The Atmosphere

Composition of the Atmosphere

Earth's atmosphere is a thin layer of gases surrounding the planet, held in place by gravity. Without it, there would be no weather, no protection from the Sun's radiation, and no liquid water.

The atmosphere is a mixture of gases:

Gas	Approximate %	Role
Nitrogen (N2)	78%	Dilutes oxygen; used by plants via nitrogen fixation
Oxygen (O2)	21%	Essential for aerobic respiration and combustion
Argon (Ar)	0.93%	Inert noble gas; no major biological role
Carbon dioxide (CO2)	0.04%	Greenhouse gas; used in photosynthesis
Water vapour	Variable (0–4%)	Greenhouse gas; drives the water cycle
Other gases	Trace	Includes ozone, neon, helium

The composition has changed over billions of years. Early Earth had almost no oxygen — it was mainly methane and carbon dioxide. Photosynthesis by ancient bacteria gradually added oxygen to the atmosphere.

Layers of the Atmosphere

The atmosphere is divided into distinct layers based on how temperature changes with altitude.

Troposphere (0–12 km)

• The lowest layer, containing about 75% of the atmosphere's mass
• All weather (rain, wind, clouds, storms) occurs here
• Temperature decreases with altitude (about 6.5°C per km)
• The boundary at the top is called the tropopause

Stratosphere (12–50 km)

• Contains the ozone layer (mainly between 20–35 km)
• Temperature increases with altitude (ozone absorbs UV radiation and warms the layer)
• Very dry — virtually no water vapour, so no weather
• Commercial aircraft often fly in the lower stratosphere to avoid turbulence

Mesosphere (50–80 km)

• Temperature decreases with altitude — the coldest part of the atmosphere (down to -90°C)
• Where most meteorites and space debris burn up due to friction with air particles
• Noctilucent clouds (rare, electric-blue clouds) form here

Thermosphere (80–700 km)

• Very thin air; temperature can reach over 2000°C (though it would not feel hot — so few particles carry so little heat energy)
• Where the International Space Station orbits (~400 km)
• Auroras (Northern and Southern Lights) occur here — caused by solar particles interacting with atmospheric gases

Exosphere (700 km and beyond)

• The outermost layer — gradually merges with outer space
• Atoms and molecules can escape into space

The Ozone Layer

The ozone layer is a region in the stratosphere with a high concentration of ozone (O3 — three oxygen atoms bonded together). Its main function is to absorb harmful ultraviolet (UV) radiation from the Sun.

Without the ozone layer:

• UV radiation would reach Earth's surface at much higher levels
• Increased rates of skin cancer, cataracts, and sunburn in humans
• Damage to ecosystems, especially phytoplankton in oceans

Ozone Depletion

CFCs (chlorofluorocarbons) — chemicals formerly used in aerosol sprays, refrigerants, and foam packaging — break down ozone molecules in the stratosphere. This created the "ozone hole" over Antarctica (discovered in 1985).

The Montreal Protocol (1987) was an international agreement to phase out CFCs. Thanks to this, ozone levels are slowly recovering.

Greenhouse Gases and the Greenhouse Effect

The greenhouse effect is a natural process that keeps Earth warm enough for life:

1. Sunlight passes through the atmosphere and warms Earth's surface
2. Earth re-emits heat as infrared radiation
3. Greenhouse gases (CO2, methane, water vapour) absorb some of this infrared radiation
4. This trapped heat warms the atmosphere

Main greenhouse gases:

• Carbon dioxide (CO2) — from burning fossil fuels, deforestation
• Methane (CH4) — from livestock, rice paddies, landfills
• Water vapour — the most abundant, but naturally regulated
• Nitrous oxide (N2O) — from fertilisers and agriculture

Human activities are increasing concentrations of greenhouse gases, enhancing the greenhouse effect and causing global warming and climate change.

Atmospheric Pressure

Air has mass, so the atmosphere exerts pressure on Earth's surface. This is called atmospheric pressure (about 101,325 Pa at sea level).

Pressure decreases with altitude — there are fewer air molecules above you as you climb higher. This is why:

• Aircraft cabins are pressurised
• Climbers on Everest need supplemental oxygen
• Cooking at altitude takes longer (water boils at lower temperatures)

Key Terms

Term	Definition
Troposphere	Lowest atmospheric layer; where all weather occurs
Stratosphere	Layer above troposphere; contains the ozone layer
Ozone layer	Region of O3 in stratosphere that absorbs UV radiation
Greenhouse gas	Gas that absorbs and re-emits infrared radiation, trapping heat
CFCs	Chlorofluorocarbons; chemicals that damage the ozone layer
Greenhouse effect	Natural process where gases in the atmosphere trap heat
Atmospheric pressure	Force per unit area exerted by the weight of air above

Worked Example

Q: Explain why the stratosphere gets warmer as altitude increases, but the troposphere gets cooler.

In the troposphere, heat comes from the Earth's surface, which absorbs sunlight. The further from the surface, the less heat reaches you, so temperature drops with altitude.

In the stratosphere, the ozone layer absorbs UV radiation from the Sun. This absorption releases heat, so the higher layers (where ozone is denser) are warmer. Temperature therefore increases with altitude.

Common Mistakes

• Confusing the greenhouse effect with ozone depletion — they are different problems. Greenhouse gases trap heat; CFCs destroy ozone.
• Thinking CFCs cause global warming — CFCs damage the ozone layer; CO2 and methane are the main drivers of climate change.
• Saying oxygen is the most abundant gas — nitrogen (78%) is far more abundant than oxygen (21%).
• Mixing up atmospheric layers — remember the order: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere (mnemonic: Tiny Snakes Make Terrifying Enemies).

Tips and Tricks

• Mnemonic for layers: T-S-M-T-E (Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere) from ground up.
• Temperature pattern in alternating layers: decreases, increases, decreases, increases — alternates each layer.
• The ozone layer is in the stratosphere — think "strat = stratified protection."
• Exam questions often ask you to distinguish the greenhouse effect (beneficial natural process) from the enhanced greenhouse effect (human-caused problem).