Earth's Atmosphere

Composition of the Atmosphere

The atmosphere is the layer of gases surrounding Earth, held in place by gravity. It is a mixture of gases, mainly:

Nitrogen (N₂): 78% — the most abundant gas; relatively unreactive, dilutes oxygen to safe levels
Oxygen (O₂): 21% — essential for respiration in animals and combustion
Argon (Ar): ~0.9% — an inert noble gas
Carbon dioxide (CO₂): ~0.04% — essential for photosynthesis; key greenhouse gas
Other gases: water vapour, neon, helium, methane, ozone (trace amounts)

Easy to remember: N₂ = 78%, O₂ = 21%, rest = 1% (Ar + CO₂ + others)

The Four Main Layers

The atmosphere is divided into layers based on how temperature changes with altitude. Temperature does NOT decrease steadily — it alternately decreases and increases depending on the layer.

Layer	Altitude	Temperature trend	Key features
Troposphere	0–12 km	Decreases with altitude (from ~15°C to -56°C)	All weather occurs here; contains ~75% of atmospheric mass; humans live here
Stratosphere	12–50 km	Increases with altitude (up to ~0°C)	Contains the ozone layer (20–35 km); absorbs UV radiation; jet aircraft fly here
Mesosphere	50–80 km	Decreases with altitude (down to -90°C)	Coldest layer; most meteors burn up here; noctilucent clouds form here
Thermosphere	80–700 km	Increases sharply (up to 2 000°C+)	Very thin air; auroras occur here; International Space Station orbits in lower thermosphere; radio waves reflect off this layer

Memory tip for the layers: The Strato-Mesosphere Thermo — or think of temperature: Down, Up, Down, Up as you rise through Troposphere → Stratosphere → Mesosphere → Thermosphere.

Air Pressure

Air pressure is the weight of the air column above a point. At sea level, air pressure is about 101 325 Pa (1 atmosphere). As altitude increases, there is less air above, so pressure decreases. At 12 km (top of troposphere), pressure is only about 20% of sea level pressure. At 50 km, it is less than 0.1%.

This is why aeroplanes must pressurise their cabins — at cruising altitude the air is too thin to breathe without assistance.

The Ozone Layer

The ozone layer is a region in the stratosphere (about 20–35 km up) where ozone (O₃) is concentrated. Ozone absorbs most of the Sun's harmful ultraviolet (UV) radiation, particularly UV-B and UV-C rays.

Without the ozone layer, UV radiation would damage DNA and cause skin cancer, cataracts, and harm to plants and marine life.
Chemicals called CFCs (chlorofluorocarbons) — once used in refrigerators and aerosol sprays — were destroying the ozone layer, causing an "ozone hole" over Antarctica.
The Montreal Protocol (1987) banned CFCs worldwide; the ozone layer is slowly recovering.

The Greenhouse Effect

The natural greenhouse effect keeps Earth warm enough to support life. Here is how it works:

Short-wave solar radiation passes through the atmosphere and warms Earth's surface.
Earth's surface radiates heat as long-wave infrared (heat) radiation.
Greenhouse gases (CO₂, water vapour, methane, nitrous oxide) absorb this outgoing heat and re-radiate some of it back toward Earth's surface.
This raises the average surface temperature to about +15°C (without any greenhouse effect it would be about -18°C).

The enhanced greenhouse effect is the problem: human activities (burning fossil fuels, deforestation) release extra CO₂ and other greenhouse gases, trapping more heat than normal and causing global temperatures to rise — global warming, which leads to climate change.

Key distinction: The natural greenhouse effect is essential for life. The enhanced greenhouse effect from extra human-produced gases is causing dangerous climate change. Do not confuse the two.

Answer the following questions in your exercise book. Use diagrams where they help explain your answer.

Question 1 · (3 marks)

State the percentage composition of the three most abundant gases in Earth's atmosphere. For each gas, state one role it plays for living organisms or Earth's systems.

Question 2 · (4 marks)

Draw and label a diagram of the four main atmospheric layers. For each layer, indicate: (a) the approximate altitude range, and (b) one key feature or event that occurs there.

Question 3 · (3 marks)

Explain why air pressure decreases with altitude. At what approximate altitude does pressure drop to about 20% of sea level pressure? Why do aeroplane cabins need to be pressurised?

Question 4 · (4 marks)

Describe the role of the ozone layer. (a) Where in the atmosphere is it found? (b) What type of radiation does it absorb and why is this important? (c) What chemicals damaged the ozone layer, and what international agreement was made to address this?

Question 5 · (4 marks)

Explain the natural greenhouse effect step by step. In your answer: (a) name three greenhouse gases, (b) describe how heat is trapped, and (c) state what Earth's average temperature would be without any greenhouse effect.

Question 6 · (3 marks)

Distinguish between the natural greenhouse effect and the enhanced greenhouse effect. Explain how human activities contribute to the enhanced greenhouse effect and give TWO consequences of this for Earth's climate.

Question 7 · (5 marks)

A weather station's high-altitude probe recorded the following readings as it rose from the ground into the mesosphere:

Altitude (km)	0	12	20	35	50	80
Temperature (°C)	15	-56	-44	-22	0	-90

(a) Which two atmospheric layer boundaries lie at 12 km and 50 km? (2 marks)
(b) Describe the temperature trend shown in the table between 12 km and 50 km, and name the layer this trend occurs in. (2 marks)
(c) The temperature falls fairly steadily from 0°C at 50 km to -90°C at 80 km. Using this trend, estimate the temperature at 65 km (halfway between 50 km and 80 km) and show your working. (1 mark)

Composition of the Atmosphere

The Four Main Layers

Air Pressure

The Ozone Layer

The Greenhouse Effect

Atmosphere Layers Explorer