What you'll learn
This revision guide covers the atmosphere's structure, composition, and role in weather systems—essential content for CIE IGCSE Environmental Management. You'll learn how atmospheric gases influence environmental processes, understand weather patterns and their formation, and grasp how human activities alter atmospheric composition. This knowledge underpins climate change, pollution, and ecosystem management topics.
Key terms and definitions
Atmosphere — the layer of gases surrounding Earth, held in place by gravity and essential for life
Troposphere — the lowest atmospheric layer (0-12 km) where weather occurs and temperature decreases with altitude
Stratosphere — the atmospheric layer above the troposphere (12-50 km) containing the ozone layer, where temperature increases with altitude
Greenhouse gases — atmospheric gases (carbon dioxide, methane, water vapour, nitrous oxide) that absorb and re-emit infrared radiation, warming the Earth's surface
Albedo — the proportion of incoming solar radiation reflected by a surface, expressed as a percentage or decimal (ice has high albedo; dark soil has low albedo)
Air pressure — the force exerted by the weight of air molecules above a surface, measured in millibars (mb) or hectopascals (hPa)
Precipitation — any form of water (rain, snow, sleet, hail) that falls from clouds to the Earth's surface
Convection current — the circular movement of air caused by heating and cooling, driving wind patterns and weather systems
Core concepts
Composition of the atmosphere
The atmosphere consists of a mixture of gases, with proportions that have remained relatively stable over recent geological time, though human activities are now altering this balance.
Main atmospheric gases:
- Nitrogen (N₂): 78%
- Oxygen (O₂): 21%
- Argon (Ar): 0.93%
- Carbon dioxide (CO₂): 0.04% (approximately 420 ppm in 2024, increasing)
- Water vapour (H₂O): 0-4% (variable, depending on temperature and location)
- Trace gases: methane, ozone, nitrous oxide, helium, neon
Key functions of atmospheric gases:
Nitrogen is relatively inert but essential for protein formation in living organisms through the nitrogen cycle. Oxygen enables aerobic respiration in organisms and combustion processes. Carbon dioxide is absorbed by plants during photosynthesis and acts as a greenhouse gas. Water vapour transports thermal energy, forms clouds and precipitation, and is the most significant greenhouse gas by volume.
Changes in atmospheric composition:
Pre-industrial CO₂ concentration was approximately 280 ppm; current levels exceed 420 ppm due to fossil fuel combustion and deforestation. Methane concentrations have more than doubled since 1750 due to agriculture (rice paddies, livestock) and fossil fuel extraction. These changes enhance the natural greenhouse effect, contributing to global warming.
Structure of the atmosphere
The atmosphere is divided into distinct layers based on temperature variation with altitude.
Troposphere:
- Extends from Earth's surface to approximately 12 km (varies with latitude)
- Contains approximately 80% of atmospheric mass
- Temperature decreases with altitude at approximately 6.5°C per km (environmental lapse rate)
- All weather phenomena occur here
- Contains virtually all water vapour
- Mixing of gases is thorough due to convection currents
Stratosphere:
- Extends from 12 km to approximately 50 km
- Contains the ozone layer (15-35 km altitude)
- Temperature increases with altitude due to UV absorption by ozone
- Very stable air with little vertical mixing
- Commercial aircraft often cruise in lower stratosphere for fuel efficiency
- Protects life by absorbing harmful ultraviolet radiation
Higher layers (mesosphere, thermosphere, exosphere) are not extensively covered at IGCSE level but students should know they exist above 50 km and contain progressively less mass.
Solar radiation and the energy budget
The Earth receives energy from the Sun primarily as short-wave radiation (visible light and UV). This energy drives all atmospheric processes and weather systems.
Energy transfers:
Approximately 30% of incoming solar radiation is reflected back to space (by clouds, ice, snow, and other reflective surfaces). This is the Earth's average albedo. Different surfaces have different albedo values: fresh snow (80-90%), clouds (40-90%), forests (10-20%), oceans (5-10%), dark soil (5-15%).
The remaining 70% is absorbed by the Earth's surface and atmosphere, warming the planet. The Earth re-radiates this energy as long-wave infrared radiation. Greenhouse gases absorb this infrared radiation and re-emit it in all directions, including back toward Earth's surface, creating the natural greenhouse effect.
Without the natural greenhouse effect, Earth's average temperature would be approximately -18°C rather than the current +15°C, making complex life impossible.
Formation of weather systems
Weather results from the uneven heating of Earth's surface, which creates pressure differences and drives air movement.
Convection and air circulation:
When air is heated, it expands, becomes less dense, and rises, creating an area of low pressure at the surface. As air rises, it cools (through expansion), and water vapour may condense to form clouds. Cool air is denser and sinks, creating areas of high pressure.
This process creates convection currents that redistribute heat from equatorial regions (which receive more direct solar radiation) toward the poles.
Pressure systems:
Low-pressure systems (depressions or cyclones) are associated with:
- Rising air
- Cloud formation
- Precipitation
- Unsettled, often windy weather
- Common in tropical and temperate regions
High-pressure systems (anticyclones) are associated with:
- Descending air
- Clear skies (clouds evaporate as air descends and warms)
- Dry conditions
- Settled weather
- Can bring extreme temperatures (hot in summer, cold in winter)
Wind: Air moves from high-pressure to low-pressure areas, creating wind. Wind speed depends on the pressure gradient (difference in pressure over distance). The steeper the gradient, the stronger the wind.
Cloud formation and precipitation
Clouds form when warm, moist air rises and cools. As air rises, it expands (due to lower atmospheric pressure at altitude) and cools. When air temperature drops to the dew point, water vapour condenses onto tiny particles (condensation nuclei) such as dust, salt, or smoke, forming water droplets or ice crystals.
Types of cloud formation:
Convectional — intense surface heating causes air to rise rapidly, forming cumulonimbus clouds and potentially heavy showers or thunderstorms. Common in tropical regions and during summer afternoons.
Frontal — warm air is forced to rise over cooler, denser air at weather fronts, creating layered clouds (stratus) and prolonged, steady rainfall. Common in temperate regions like the UK.
Orographic (relief) — air is forced to rise over mountains or hills, cools, and forms clouds on the windward side. The leeward side receives less rainfall (rain shadow effect). Examples include the western sides of the UK's upland areas receiving higher rainfall than eastern lowlands.
Precipitation types:
- Rain: water droplets large enough to fall (>0.5 mm diameter)
- Snow: ice crystals formed when temperatures throughout the cloud and to the ground are below 0°C
- Sleet: partially melted snowflakes or refrozen raindrops
- Hail: ice pellets formed in cumulonimbus clouds through repeated rising and falling in updrafts
Factors affecting local weather and climate
Latitude: Areas closer to the equator receive more direct solar radiation throughout the year, resulting in higher temperatures. Polar regions receive less direct (more oblique) radiation and experience lower temperatures.
Altitude: Temperature decreases approximately 6.5°C per 1000 m increase in altitude in the troposphere. Mountain areas are cooler than lowlands at the same latitude. Highland areas also receive more precipitation due to orographic uplift.
Distance from sea (continentality): Oceans heat and cool more slowly than land due to water's high specific heat capacity and mixing. Coastal areas experience:
- Smaller temperature ranges (cooler summers, milder winters)
- More moderate climates
- Higher humidity and cloud cover
- More precipitation
Continental interiors experience:
- Greater temperature extremes
- Lower humidity
- Less cloud cover and precipitation
Prevailing winds: Winds blowing from oceans bring moisture; those from continents are drier. In the UK, prevailing south-westerly winds from the Atlantic bring mild, wet weather. North-easterly winds from continental Europe bring cold, dry conditions in winter.
Ocean currents: Warm currents (like the North Atlantic Drift affecting the UK) raise coastal temperatures. Cold currents (like the Benguela Current off Namibia) lower coastal temperatures and reduce rainfall.
Worked examples
Example 1: Calculating percentage composition
Question: A 100 cm³ sample of dry air contains 78 cm³ nitrogen, 21 cm³ oxygen, and 1 cm³ other gases. Calculate the percentage of oxygen in the sample. [2 marks]
Answer: Percentage = (volume of oxygen / total volume) × 100 Percentage = (21 / 100) × 100 = 21%
Mark scheme guidance:
- 1 mark for correct calculation method
- 1 mark for correct answer with unit (%)
Example 2: Explaining weather patterns
Question: Explain why coastal areas generally experience less extreme temperatures than inland areas at the same latitude. [4 marks]
Answer: Water has a high specific heat capacity, meaning it requires more energy to change temperature compared to land [1 mark]. The ocean heats up slowly in summer, keeping coastal areas cooler [1 mark]. The ocean cools down slowly in winter, keeping coastal areas warmer [1 mark]. Inland areas heat and cool rapidly, resulting in hotter summers and colder winters [1 mark].
Mark scheme guidance:
- Reference to specific heat capacity or water's thermal properties
- Explanation of summer cooling effect
- Explanation of winter warming effect
- Comparison with inland/continental areas
Example 3: Analysing cloud formation
Question: A diagram shows warm, moist air moving from the sea toward a mountain range. Describe and explain what will happen to this air as it rises over the mountain. [5 marks]
Answer: As the air rises up the windward side of the mountain, it will cool [1 mark]. This is because air expands and loses temperature as atmospheric pressure decreases with altitude [1 mark]. When the air reaches its dew point, water vapour will condense [1 mark], forming clouds and likely causing precipitation on the windward side [1 mark]. As the air descends the leeward side, it will warm and dry out, creating a rain shadow effect with less precipitation [1 mark].
Mark scheme guidance:
- Process of cooling with altitude
- Reason for cooling (expansion/pressure change)
- Condensation at dew point
- Precipitation formation
- Rain shadow/leeward effect
Common mistakes and how to avoid them
Confusing weather and climate. Weather refers to short-term atmospheric conditions (hours to days). Climate refers to average weather patterns over at least 30 years. Always use the correct term in context.
Misunderstanding greenhouse gas function. Greenhouse gases don't trap heat like a blanket; they absorb and re-emit infrared radiation. Use precise language: "absorb long-wave radiation and re-radiate it in all directions."
Confusing atmospheric layers. Remember that temperature decreases in the troposphere but increases in the stratosphere (due to ozone absorption of UV). Don't assume temperature always decreases with altitude.
Reversing pressure and weather conditions. Low pressure brings unsettled, wet weather (rising air, clouds, rain). High pressure brings settled, dry weather (sinking air, clear skies). Don't mix these up.
Incomplete explanations of processes. When explaining cloud formation, include: rising air, expansion, cooling, reaching dew point, and condensation. Missing steps lose marks.
Ignoring command words. "Describe" requires what happens; "explain" requires why it happens (causes and mechanisms). "Assess" or "evaluate" require weighing up different factors or viewpoints.
Exam technique for "The Atmosphere: Composition and Weather"
Command word precision: "State" requires brief facts (1 mark each). "Explain" requires reasoning with connectives like "because," "therefore," "this causes" (typically 2+ marks). "Describe" requires characteristics or sequences without necessarily explaining why.
Use data when provided: Questions including graphs, diagrams, or tables expect you to quote specific figures. Write "CO₂ increased from 280 ppm to 420 ppm" rather than "CO₂ increased significantly."
Structure longer answers: For 4-6 mark questions, use one distinct point per mark. Start each point on a new line or use clear paragraphing. Examiners can only award marks for points they can clearly identify.
Link processes together: Atmosphere questions often test your understanding of chains of events (e.g., heating → rising air → cooling → condensation → precipitation). Show these connections clearly using appropriate sequencing language.
Quick revision summary
The atmosphere consists mainly of nitrogen (78%) and oxygen (21%), with variable water vapour and increasing CO₂ (now over 420 ppm). The troposphere (0-12 km) contains weather systems driven by uneven solar heating. Rising warm air creates low pressure, clouds, and precipitation; sinking cool air creates high pressure and clear skies. Greenhouse gases absorb infrared radiation, maintaining Earth's temperature. Local weather depends on latitude, altitude, distance from sea, prevailing winds, and ocean currents. Cloud formation requires rising air, cooling to dew point, and condensation onto nuclei.