Layers of the atmosphere
Big idea: The atmosphere is like a layered cake — each layer has different characteristics and plays a different role in supporting life on Earth.
The four main layers
The atmosphere extends about 100 km above Earth's surface and is divided into four main layers based on temperature changes with altitude.
- Troposphere (0–12 km): Where we live and weather happens. Temperature decreases with altitude (~6.5°C per km). Contains 75% of atmospheric mass and almost all water vapour.
- Stratosphere (12–50 km): Contains the ozone layer. Temperature increases with altitude due to UV absorption by ozone. Very stable — no weather here!
- Mesosphere (50–80 km): Temperature decreases again. Meteors burn up here (shooting stars!).
- Thermosphere (80–700 km): Temperature increases dramatically. Where the aurora occurs and satellites orbit.
Exam tip: You mainly need to know the troposphere (weather, life, greenhouse effect) and stratosphere (ozone layer). These are the most commonly tested!
Atmospheric composition
The atmosphere is a mixture of gases, with some playing crucial roles despite being present in tiny amounts.
Major gases
- Nitrogen (N₂): ~78%
- Oxygen (O₂): ~21%
- Argon (Ar): ~0.9%
Trace gases (but important!)
- Carbon dioxide (CO₂): ~0.04%
- Water vapour (H₂O): 0–4%
- Methane (CH₄): trace
- Ozone (O₃): trace
The trace gases (especially CO₂, H₂O, and CH₄) are the greenhouse gases — tiny amounts, but huge impact on climate!
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Why the atmosphere matters for life
Big idea: Without the atmosphere, Earth would be a frozen, lifeless rock. The atmosphere maintains temperatures suitable for life and protects us from harmful radiation.
Life-supporting functions
- Temperature regulation: The greenhouse effect keeps Earth's average temperature at ~15°C instead of -18°C
- UV protection: The ozone layer absorbs harmful ultraviolet radiation
- Oxygen supply: Provides O₂ for respiration and CO₂ for photosynthesis
- Water cycling: Holds water vapour for the water cycle and precipitation
- Pressure: Maintains atmospheric pressure needed for liquid water to exist
The troposphere — where life happens
Almost all living things exist in the troposphere. This layer is characterised by:
- Decreasing temperature with altitude (why mountains are cold!)
- Convection currents that mix air and create weather
- Contains almost all atmospheric water vapour
- Where the greenhouse effect operates
Exam tip: When explaining how the atmosphere supports life, always link specific functions to specific outcomes — e.g., 'The greenhouse effect maintains temperatures suitable for liquid water, which is essential for life.'
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IB-style question — layers of the atmosphere [3]
A weather balloon rises from the ground to about 30 km altitude. Name the two atmospheric layers it passes through in order, and identify which layer contains most of the weather and water vapour. [3]
How to answer it, step by step
- Name the layers in order
• First layer from the ground: troposphere (0–about 12 km).
• Next layer up: stratosphere (about 12–50 km, holds the ozone layer). - Point to where weather happens
• Almost all weather and water vapour sit in the troposphere.
• This is the lowest, densest layer where we live and clouds form.
Final answer
Use the exact layer names (troposphere, stratosphere) and remember the troposphere is the bottom 'weather' layer — vague terms like 'lower air' score nothing.