Notes · Environmental ScienceUK · A-Levels

The atmosphere

This chapter examines the atmosphere as a dynamic system of energy stores and flows. It covers the composition and vertical layered structure of the atmosphere and how temperature changes with altitude, the Earth's energy budget and the natural greenhouse effect, the global pattern of atmospheric circulation that shapes climate and biomes, and the natural functioning of the stratospheric ozone layer that shields the surface from ultraviolet radiation.

4 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 1

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Exam profile

AO1 · Describe the composition and structure of the atmosphere, the energy budget, circulation and the ozone layerAO2 · Apply the energy-budget and greenhouse concepts to radiation data and unfamiliar contextsAO3 · Interpret temperature-altitude and radiation data and analyse the natural balance of the atmosphere

Operators:describeexplainapplyanalyseinterpretcalculate

basic level

AS-Level expects you to describe the layers of the atmosphere, the natural greenhouse effect, the broad pattern of circulation and the role of the ozone layer.

higher level

The full A-Level requires you to interpret radiation and temperature data, explain circulation in terms of differential heating, and analyse the ozone balance through the Chapman reactions.

Depth

Text

Contents · 4 sections

The atmosphere

Composition and vertical structure of the atmosphere#

FoundationAQA 7447 3.2.1

The layers of the atmosphere

Fig. 1The atmosphere's layers, defined by whether temperature rises or falls with height (not to scale).

Dry air is a mixture dominated by nitrogen (about 78%) and oxygen (about 21%), with argon (about 1%) and small but critically important amounts of carbon dioxide and other trace gases; water vapour is highly variable. Although carbon dioxide, methane and water vapour are present only in traces, they exert an influence out of all proportion to their abundance through the greenhouse effect, which is why small changes in their concentration matter so much. This composition is itself partly a product of life, most obviously the oxygen produced by photosynthesis over geological time.

The atmosphere is layered vertically, and the layers are defined by how temperature changes with height. The troposphere, from the surface up to about 12 km, is where weather occurs and where temperature falls with altitude; almost all water vapour and the bulk of the mass are here. Above it the stratosphere, up to about 50 km, is where temperature rises with altitude because ozone there absorbs ultraviolet radiation and warms the air; this temperature inversion makes the stratosphere stable and is why aircraft fly in its lower reaches. Higher still, the mesosphere sees temperature fall again, and the thermosphere sees it rise as the thin gas absorbs very short-wave solar radiation.

The alternating rises and falls of temperature between the layers are the key to telling them apart, and the boundaries between them (the tropopause, stratopause and mesopause) mark where the trend reverses. A common examination task is to read a temperature-against-altitude graph and identify the layers from the direction of the temperature change, so it is worth being able to sketch the profile from memory: down, up, down, up.

The layering matters environmentally because it controls how pollutants and heat behave. Because the troposphere is unstable, with warm air below and cooler air above, pollutants released at the surface are usually mixed and dispersed; but a temperature inversion, in which a layer of warm air traps cooler air below it, halts this mixing and lets pollutants accumulate, producing smog episodes. The stable stratosphere, by contrast, allows substances that reach it, such as ozone-depleting gases, to persist and spread globally.

Worked example

Reading a temperature profile

A weather balloon records temperature falling from 15 degrees Celsius at the surface to -55 degrees Celsius at 11 km, then rising to about -3 degrees Celsius at 45 km. Identify the layers and explain the reversal.

First layer
Temperature falls from the surface to 11 km, so this is the troposphere; the tropopause is at about 11 km where the trend reverses.
Second layer
Temperature then rises to 45 km, so this is the stratosphere.
Explain the reversal
In the stratosphere, ozone absorbs ultraviolet radiation and warms the air, so temperature rises with height rather than falling.

Result: The profile shows the troposphere (cooling) then the stratosphere (warming), the reversal caused by ozone absorbing UV.

Typical mistakes

Assuming temperature always falls with altitude; it rises in the stratosphere and thermosphere.
Confusing the ozone layer (stratosphere) with tropospheric ozone (a pollutant near the surface).

Active revision

Describe how temperature changes with altitude through the troposphere and stratosphere, and explain the difference in terms of where solar energy is absorbed.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

The Earth's energy budget and the natural greenhouse effect#

StandardAQA 7447 3.2.1

The Earth's energy budget and the greenhouse effect

Fig. 2Greenhouse gases absorb outgoing long-wave radiation and re-emit part of it back to the surface, warming it.

The Earth's temperature is set by a balance between incoming energy from the Sun and outgoing energy from the Earth. Incoming solar radiation is mostly short-wave (visible and near-infrared) because the Sun is hot. Of this, some is reflected straight back to space by clouds, aerosols and bright surfaces such as ice and desert; the fraction reflected is the albedo. The rest is absorbed, mainly by the surface, which warms and re-emits energy as long-wave (infrared) radiation because the Earth is much cooler than the Sun. Over the long term, incoming absorbed energy and outgoing radiation are equal, and the planet is in radiative balance.

The natural greenhouse effect arises because greenhouse gases in the atmosphere, chiefly water vapour, carbon dioxide, methane and nitrous oxide, are largely transparent to incoming short-wave radiation but absorb outgoing long-wave radiation. The gases absorb the infrared and re-emit it in all directions, including back down to the surface, so the lower atmosphere and surface are warmer than they would be if the energy escaped directly to space. This is a wholly natural and essential process that keeps the average surface temperature around

+15\,^{\circ}\text{C}

rather than about

-18\,^{\circ}\text{C}

; the environmental problem, examined in the next chapter, is its human enhancement.

The key to the greenhouse effect is the difference in wavelength between the incoming and outgoing radiation. Because the Sun is hot it radiates at short wavelengths that pass through the greenhouse gases; because the Earth is cool it radiates at long wavelengths that those same gases absorb. A frequent misconception is that greenhouse gases trap heat like a blanket by blocking convection; in fact they intercept and re-radiate infrared radiation, which is a radiative rather than an insulating mechanism, and this distinction is worth stating precisely.

Anything that changes the albedo or the concentration of greenhouse gases changes the balance and therefore the temperature. A rise in albedo, for example from more ice or reflective aerosols, cools the planet; a rise in greenhouse gases warms it. Because the surface temperature is the outcome of this budget, the same framework is used to understand ice ages, volcanic cooling and human-caused warming, making the energy budget one of the most important ideas in the whole subject.

Worked example

Reasoning about albedo

A large area of reflective sea ice melts and is replaced by dark open ocean. Predict the effect on the local energy budget and temperature.

Change in albedo
Ice has a high albedo and reflects much incoming radiation; dark ocean has a low albedo and absorbs much more.
Effect on the budget
More short-wave radiation is now absorbed rather than reflected, so more energy enters the system locally.
Effect on temperature
The extra absorbed energy warms the surface, which can melt more ice, an example of positive feedback.

Result: Lower albedo means more radiation is absorbed, warming the surface and driving further ice loss (positive feedback).

Typical mistakes

Describing greenhouse gases as trapping heat like a physical blanket rather than absorbing and re-emitting long-wave radiation.
Saying the greenhouse effect is entirely bad; the natural effect is essential and only its enhancement is the problem.

Active revision

Explain why greenhouse gases allow incoming solar radiation to pass but absorb the radiation emitted by the Earth's surface.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Atmospheric circulation and its climatic effects#

StandardAQA 7447 3.2.1

The Hadley cell

Fig. 3A convection cell driven by differential heating: rising, wet air at the equator; sinking, dry air near 30 degrees.

The Sun heats the Earth unevenly: the equator receives far more energy per unit area than the poles, because there the Sun is high and its energy is concentrated, whereas near the poles the same energy is spread over a larger, angled surface and passes through more atmosphere. This differential heating is the engine of atmospheric circulation. Warm, less dense air rises at the equator, creating a belt of low pressure, spreads towards the poles high in the atmosphere, cools and sinks at about 30 degrees latitude, creating a belt of high pressure, and returns towards the equator at the surface. This loop is the Hadley cell.

Two further cells, the Ferrel and Polar cells, complete a three-cell model in each hemisphere, producing alternating belts of low and high pressure with latitude. Rising air at low-pressure belts cools and its water vapour condenses, giving heavy rainfall, which is why the equatorial belt supports rainforest; sinking air at high-pressure belts warms and dries, which is why the world's great hot deserts lie near 30 degrees latitude. The circulation therefore directly explains the global distribution of precipitation and, with it, the pattern of biomes met in the first chapter.

The rotation of the Earth deflects these moving air masses, an effect called the Coriolis effect, so that surface winds do not blow straight along the pressure gradient but are turned, producing the prevailing winds such as the trade winds and the westerlies. The oceans redistribute heat as well, through surface currents driven partly by these winds, so atmosphere and ocean together transport energy from the warm tropics towards the cold poles and moderate the climate. This poleward transport of heat is a fundamental response to the equator-to-pole energy imbalance.

Because circulation is driven by the energy budget, changes in the budget can shift the circulation. A warming climate is expected to alter the position and strength of the pressure belts and jet streams, changing rainfall patterns and the frequency of extreme weather in ways that are an active area of research. Understanding the basic three-cell model is enough at this level to explain why deserts and rainforests occur where they do and to reason qualitatively about how circulation links the atmosphere to climate and life.

Worked example

Explaining a climatic pattern

Use the three-cell circulation model to explain why a region on the equator has heavy year-round rainfall while a region at 30 degrees latitude is arid.

At the equator
Intense heating makes air rise; as it rises it cools, water vapour condenses and heavy rain falls, so the equatorial belt is wet.
At 30 degrees
The air that rose at the equator has lost its moisture and descends at about 30 degrees; as it sinks it warms and dries, so little rain falls.
Conclude
Rising air at the equatorial low gives rainforest; sinking air at the subtropical high gives desert.

Result: Rising, cooling air at the equator brings rain; sinking, warming air at 30 degrees brings aridity.

Typical mistakes

Saying air sinks at the equator; air rises at the equatorial low-pressure belt and sinks at about 30 degrees.
Forgetting that rising air causes rainfall (rainforest) whereas sinking air causes aridity (desert).

Active revision

Explain why the world's major hot deserts are found at around 30 degrees north and south of the equator.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

The natural ozone layer and ultraviolet absorption#

AdvancedAQA 7447 3.2.1

The Chapman cycle of stratospheric ozone

Fig. 4Ozone is continually formed and broken down; both steps absorb ultraviolet radiation, shielding the surface.

The ozone layer is a region of the stratosphere, roughly 15 to 35 km up, where ozone (

\text{O}_3

) is relatively concentrated. Even there it is a trace gas, but it performs the vital function of absorbing most of the Sun's harmful ultraviolet (UV) radiation, especially the shorter-wavelength UV-B and UV-C, before it reaches the surface. This shielding protects living organisms from the damage UV causes to DNA and proteins, which in humans is linked to skin cancer, cataracts and a weakened immune system, and which harms phytoplankton at the base of marine food webs.

Ozone is continually formed and destroyed in a natural balance described by the Chapman cycle. High-energy ultraviolet radiation splits an oxygen molecule into two oxygen atoms; each atom combines with another oxygen molecule to form ozone; ozone in turn absorbs ultraviolet and splits back into an oxygen molecule and an atom; and oxygen atoms and ozone can recombine to oxygen. The formation and breakdown steps both absorb ultraviolet radiation, which is exactly why the layer both shields the surface and warms the stratosphere. In an undisturbed atmosphere these reactions reach a dynamic equilibrium so that the amount of ozone stays roughly constant.

It is important to keep two roles of ozone completely separate. In the stratosphere, ozone is beneficial because it absorbs ultraviolet radiation; in the troposphere at ground level, ozone is a harmful secondary pollutant formed in photochemical smog that damages lungs and vegetation. The phrase good up high, bad nearby captures the distinction. Confusing the two, or confusing ozone depletion with the greenhouse effect, is one of the most common and heavily penalised errors in this part of the course.

The natural equilibrium of the Chapman cycle can be disturbed by substances that provide additional routes to destroy ozone faster than it is formed, most importantly the chlorine released from CFCs. That human disruption, the ozone hole and the successful international response are examined in the next chapter on climate change and ozone depletion; here the point is simply that the layer exists in a natural, dynamic balance that shields the surface and can be pushed out of balance.

Worked example

Explaining the UV shield

Explain how the continual formation and breakdown of ozone in the stratosphere reduces the ultraviolet radiation reaching the Earth's surface.

Formation absorbs UV
Ultraviolet radiation splits oxygen molecules into atoms, which combine with oxygen to form ozone; this step removes UV energy.
Breakdown absorbs UV
Ozone then absorbs further ultraviolet radiation and splits back into oxygen, again removing UV energy.
Net effect
Because both steps absorb ultraviolet radiation, much less reaches the surface, and the absorbed energy warms the stratosphere.

Result: The formation and breakdown of ozone each absorb ultraviolet radiation, so the layer shields the surface and warms the stratosphere.

Typical mistakes

Confusing ozone depletion with the greenhouse effect; they are separate problems with separate causes.
Treating ground-level ozone and stratospheric ozone as the same thing; one is a pollutant, the other a shield.

Active revision

Explain why the stratospheric ozone layer both protects life at the surface and warms the stratosphere.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

References & sources

Sources

AQA

AQA AS and A-level Environmental Science (7447) specification

Department for Education

GCE AS and A level subject content

The atmosphere

4 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 1

The atmosphere

Key points

Reading a temperature profile

Key points

Reasoning about albedo

Key points

Explaining a climatic pattern

Key points

Explaining the UV shield

Sources

The atmosphere

Key points

Reading a temperature profile

Key points

Reasoning about albedo

Key points

Explaining a climatic pattern

Key points

Explaining the UV shield

Sources