In simple terms
A friendly intro before the formal notes — no formulas yet.
Earth's Atmospheric Blanket
The atmosphere acts like a giant, complex blanket for our planet. It lets in the sun's light to warm us up, but traps some of the outgoing heat to keep the temperature just right for life.
Think of a car parked in the sun. The sun's light (short-wave radiation) passes easily through the car's windows, heating the seats and dashboard. These surfaces then radiate heat (long-wave radiation), but this heat can't escape through the windows as easily. The trapped heat makes the inside of the car much warmer than the air outside – this is a simple model of the greenhouse effect.
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The sun emits short-wave radiation, including visible light, which travels towards Earth.
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Some of this incoming solar radiation (insolation) is reflected by clouds and the Earth's surface (albedo), but most is absorbed by the ground.
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The warmed Earth's surface re-radiates energy as long-wave infrared radiation (heat).
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Greenhouse gases in the atmosphere absorb this outgoing long-wave radiation, trapping heat and warming the lower atmosphere.
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Key formulas
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Full topic notes
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The Structure and Composition of the Atmosphere
The atmosphere is a thin envelope of gases surrounding the Earth, divided into several distinct layers based on temperature changes. For ESS, we are primarily concerned with the two lowest layers. The troposphere extends from the surface up to about 12 km. It contains 75% of the atmosphere's mass, is where all weather occurs, and its temperature decreases with altitude. Above this lies the stratosphere (from ~12 to 50 km), where temperature increases with altitude due to the absorption of UV radiation by the ozone layer.
Composition: The atmosphere is approximately 78% nitrogen (N₂), 21% oxygen (O₂), 0.9% argon (Ar), and a small fraction of other gases, including the greenhouse gases.
Troposphere: The lowest layer, containing most of the atmosphere's water vapour, clouds, and pollution. Temperature falls with increasing altitude.
Stratosphere: The second layer, containing the vital ozone layer which protects life from harmful UV-B radiation. Temperature rises with increasing altitude.
The Greenhouse Effect: Earth's Natural Thermostat
The greenhouse effect is a natural and essential process. Without it, Earth would be too cold to support life as we know it. The process involves a delicate energy exchange, governed by the properties of different types of electromagnetic radiation and atmospheric gases.
Step 1: Insolation: Short-wave radiation from the Sun passes through the atmosphere.
Step 2: Absorption: Some radiation is reflected by clouds and the surface, but a significant portion is absorbed by the Earth's surface, warming it.
Step 3: Re-radiation: The warmed Earth's surface radiates energy back out as long-wave infrared radiation (heat).
Step 4: Trapping: Greenhouse gases (like CO₂, CH₄, H₂O) in the troposphere are effective at absorbing this outgoing long-wave radiation. They then re-radiate this energy in all directions, including back towards the Earth's surface, trapping heat in the lower atmosphere.
Be precise in your exam answers. Do not say greenhouse gases 'reflect' heat. They 'absorb and re-radiate' long-wave radiation. Also, always distinguish between incoming 'short-wave' solar radiation and outgoing 'long-wave' terrestrial radiation. These details are crucial for securing top marks.
Earth's Energy Balance and Albedo
For Earth's global temperature to remain stable over long periods, the energy entering the system must equal the energy leaving it. This is known as the Earth's energy balance. A key factor influencing this balance is albedo, which is the proportion of incoming solar radiation that is reflected by a surface. Light-coloured, shiny surfaces like ice and snow have a high albedo, while dark, matt surfaces like forests and oceans have a low albedo.
Albedo =
Worked examples
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A patch of open ocean receives an average of 200 W m⁻² of insolation. The ocean water has an albedo of 0.06. Calculate the amount of energy absorbed by the ocean per square metre. Show your working.
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Calculate the reflected energy:
An area of land has a net radiation budget. The incoming solar radiation absorbed by the surface is 160 W m⁻². The surface radiates 390 W m⁻² upwards. The atmosphere radiates 330 W m⁻² back down to the surface (the greenhouse effect). Calculate the net radiation at the surface and state whether this leads to warming or cooling.
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Identify all energy inputs to the surface:
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What is the atmosphere?
A dynamic mixture of gases that forms a layer around the Earth, held in place by gravity. It is essential for life, regulating temperature and protecting from harmful radiation.
Key takeaways
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Composition: The atmosphere is approximately 78% nitrogen (N₂), 21% oxygen (O₂), 0.9% argon (Ar), and a small fraction of other gases, including the greenhouse gases.
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Troposphere: The lowest layer, containing most of the atmosphere's water vapour, clouds, and pollution. Temperature falls with increasing altitude.
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Stratosphere: The second layer, containing the vital ozone layer which protects life from harmful UV-B radiation. Temperature rises with increasing altitude.
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Test Your Knowledge on Atmospheric Systems
Test Your Knowledge on Atmospheric Systems
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