Weather takes place within the context of the general circulation of the atmosphere: weather, climate and the atmosphere
Atmosphere and Weather Systems is A Level content and the first of the two compulsory topics in Section A of WGE03, Contested Planet.
It builds on two ideas from World at Risk at AS: that hazards and disasters are different things, and that the climate is warming because of greenhouse gas emissions.
Here you go underneath those ideas to the physical system itself: what the atmosphere is made of, how it moves heat around the planet, and how that movement produces the depressions, anticyclones, tropical cyclones and droughts that the rest of the unit examines.
You need to know definitions of weather and climate; the structure and composition of the atmosphere and the role different gases play in climate.
This first section sets out the vocabulary every later answer depends on.
(for every section of this unit)
- Where A1 sits. Atmosphere and Weather Systems is compulsory. It is Question 1 in Section A of WGE03, Contested Planet, and it also feeds Question 3, the synoptic essay, which joins A1 and A2 to the World at Risk topic you studied at AS.
- Question 1 takes one of two shapes. Either a single 10-mark figure item, or a 10-mark figure item followed by a 15-mark essay. Which shape A1 gets changes from series to series.
- 10 marks: "Using Figure 1, explain ..." or "Using Figure 1, suggest reasons ...". Levels marked (three levels), AO1 4 and AO2 6. Explanation of processes tied to named features or values in the figure; describing the figure earns nothing on its own; no conclusion is needed.
- 15 marks: "Assess ..." in Question 1, "To what extent ..." in the synoptic Question 3. Levels marked (four levels), AO1 5 and AO2 10. Define the key terms of the question in your first lines, use named and located examples with dates and figures, judge as you go and finish with a conclusion in the question's own words.
- Never on this paper: Describe, Define, Calculate or multiple choice. The skills in these notes (reading maps, synoptic charts, track maps and rainfall records) are tested by explaining a figure, not by calculating from it.
- Physical questions are answered worst. Examiners report more weak answers on atmosphere items than on any human topic, mostly because candidates describe what a figure shows instead of explaining the process behind it. Every section of these notes therefore spends time on the chain of causes.
The idea: one hot afternoon is weather, a hot summer every year is climate
On 21 July 2016 the weather station at Mitribah, in the desert of north-west Kuwait, recorded 53.9 °C. After a detailed check of the instrument and the site, the World Meteorological Organization (WMO) accepted it in 2019 as the highest temperature ever reliably measured in Asia.
That reading is weather: the state of the atmosphere at one place at one time.
The fact that summer days in Kuwait are almost always above 40 °C, that rain falls on only a few days a year, and that a shamal wind can raise dust at almost any time from May to August is climate: the long-term pattern of the weather, including how variable it is and how often extremes happen.
The difference matters in every answer.
A single record day cannot prove that a climate is changing, and a single cool summer cannot prove that it is not.
Climate is measured over long periods, normally 30 years; the WMO's current standard period for comparing places is 1991 to 2020.
When you explain a change in a figure, ask first whether it shows weather (hours to days), seasonal variation, year-to-year variability, or a change in the climate itself.
The structure of the atmosphere
The atmosphere is a thin shell of gas held to the Earth by gravity.
Half its mass lies below about 5.5 km, and nine-tenths below about 16 km.
It is divided into layers by the way temperature changes with height (Figure 1).
- Troposphere (from the surface to about 8 km over the poles and 16 to 18 km over the equator). Temperature falls with height, on average by about 6.5 °C per kilometre, because the air is heated from below: sunlight passes through it and warms the ground, and the ground warms the air. Warm air near the surface can rise through cooler air above, so the troposphere is constantly overturning. It holds about three-quarters of the atmosphere's mass and almost all its water vapour, which is why clouds, rain, storms and almost all weather happen here.
- Tropopause. The boundary where cooling stops. Rising air in a thunderstorm spreads out sideways when it reaches it, which is why the top of a cumulonimbus cloud flattens into an anvil.
- Stratosphere (to about 50 km). Temperature rises with height because ozone absorbs ultraviolet radiation from the sun and warms the air. Warm air lying above cooler air is very stable, so there is little vertical mixing and almost no weather. The ozone layer here protects surface life from harmful ultraviolet radiation.
- Mesosphere (about 50 to 85 km). There is little ozone to absorb sunlight, so temperature falls again, to about −86 °C near 85 km, the coldest part of the atmosphere.
- Thermosphere (above about 85 km). The very thin gas absorbs high-energy solar radiation, so temperature rises steeply with height, but there are so few molecules that the air holds almost no heat.
Why the tropopause is higher over the equator matters later.
Strong surface heating there drives deep convection that lifts the whole troposphere; over the poles cold, dense air keeps it shallow.
The resulting slope of the tropopause between the tropics and the poles is where the jet streams form (section 8).
The composition of the atmosphere
Gas
Share of dry air (by volume)
Role in climate
Nitrogen (N₂)
78.08%
Almost no direct effect on radiation
Oxygen (O₂)
20.95%
Almost no direct effect on infrared; the source of ozone in the stratosphere
Argon (Ar)
0.93%
Inert, no effect
Carbon dioxide (CO₂)
about 0.043% (427 ppm, 2025)
Greenhouse gas: absorbs and re-emits infrared
Methane (CH₄)
about 0.0002% (about 1.9 ppm)
Greenhouse gas, much stronger per molecule than CO₂
Nitrous oxide (N₂O)
about 0.00003%
Greenhouse gas, long-lived
Ozone (O₃)
traces, most in the stratosphere
Absorbs ultraviolet (heats the stratosphere); a greenhouse gas near the surface
Water vapour (H₂O)
0 to about 4%, not counted in dry air
The largest natural greenhouse effect; source of clouds and precipitation
Two other ingredients matter.
Aerosols are tiny solid or liquid particles: desert dust from Arabia and the Sahara, sea salt, smoke, sulphate from burning fuels.
They scatter and absorb sunlight, and they act as condensation nuclei on which cloud droplets form.
Clouds themselves both reflect sunlight (cooling) and absorb infrared from below (warming).
Why the gases matter: the greenhouse effect
Sunlight arrives mostly as short-wave radiation, which passes through the atmosphere's main gases with little absorption.
The ground absorbs it, warms up and emits long-wave infrared radiation.
Nitrogen and oxygen let infrared pass, but molecules with three or more atoms (water vapour, carbon dioxide, methane, nitrous oxide, ozone) absorb it and re-emit it in all directions, including back towards the surface.
The surface therefore ends up warmer than it would be with sunlight alone: about 15 °C on average, against about −18 °C without the natural greenhouse effect.
Carbon dioxide at Mauna Loa averaged 427.35 ppm in 2025 (NOAA's annual mean), against about 280 ppm before industrialisation, a rise of about 53%.
Each extra molecule strengthens the effect.
Water vapour then amplifies the change: warmer air can hold about 7% more water vapour for every 1 °C of warming, and the extra vapour absorbs more infrared.
That is why water vapour is described as a feedback that responds to warming, while carbon dioxide is a forcing that starts it.
Connect it
The layers and gases explain several later ideas.
The tropopause caps the height of thunderstorms and tropical cyclones (section 13).
Latent heat released when water vapour condenses powers convection and cyclones.
The heating of the tropics compared with the poles, set by how much sunlight each receives and how the gases handle it, drives the general circulation in the next section.
At AS you learnt how rising greenhouse gases warm the climate; here you see the physical mechanism, which is what examiners want when a synoptic essay asks how global warming changes weather hazards.
Meaning of the data for people
The air's composition seems fixed, yet a change of four-hundredths of one per cent in carbon dioxide has measurable effects on Kuwait's summers.
The Gulf is already close to the limits of safe outdoor work in July.
Each degree of warming adds heat and humidity stress for outdoor workers, raises electricity demand for air conditioning (much of it generated by burning gas and oil, adding more carbon dioxide) and lengthens the season of dangerous heat.
Small changes in trace gases become large changes in human wellbeing.
- Weather: the state of the atmosphere at a particular place and time (temperature, precipitation, wind, cloud, humidity and pressure), over hours to days.
- Climate: the long-term pattern of weather at a place, usually averaged over 30 years, including its variability and the frequency of extremes.
- Troposphere: the lowest layer of the atmosphere, in which temperature normally falls with height and almost all weather takes place.
- Environmental lapse rate: the rate at which air temperature falls with height in the troposphere, on average about 6.5 °C per kilometre.
- Greenhouse gas: a gas that absorbs long-wave infrared radiation emitted by the Earth's surface and re-emits it, some of it back towards the surface.
Using Figure 1, find the average rate of temperature change in the troposphere, and explain what it shows.
1. Read the values: 15 °C at 0 km and −56.5 °C at 11 km.
2. Temperature change: 15 − (−56.5) = 71.5 °C over 11 km.
3. Rate: 71.5 ÷ 11 = 6.5 °C per km.
Answer: temperature falls on average by 6.5 °C for every kilometre of height.
This shows the troposphere is heated from below, by the ground, not by direct sunlight; air further from the heat source is colder.
It also explains why the summit plateau of Jebel Shams in Oman, at about 3,000 m, is often roughly 20 °C cooler than Muscat on the coast on the same day.
Using Figure 1, explain the changes in temperature with height in the atmosphere.
(10)
Top answer (Level 3). In the troposphere temperature falls from 15 °C at the surface to −56.5 °C at 11 km, about 6.5 °C per kilometre, because the air is heated from below: short-wave sunlight passes through the air, heats the ground, and the ground warms the air above it by long-wave radiation and convection [AO1, tied to values: AO2].
Above the tropopause, temperature is steady and then rises to about −2.5 °C at 47 km, because ozone in the stratosphere absorbs ultraviolet radiation and warms the air [AO1 + AO2]; this warm layer over cooler air is stable, which is why weather stays below about 11 km here.
In the mesosphere temperature falls again to about −86 °C at 85 km because there is little ozone to absorb sunlight [AO2].
In the thermosphere it rises again as the thin gas absorbs high-energy solar radiation, though so few molecules are present that the air holds little heat [AO1].
Why Level 3: every change is explained by a heating process, every explanation is tied to a value or layer in the figure, and the whole profile is covered.
Weaker answer (Level 1). "Temperature goes down to −56.5 °C, then stays the same, then goes up to −2.5 °C, then goes down to −86 °C, then goes up."
What it lacks: it describes the line only.
No process is given, so it reaches Level 1 at best.
A news report says: "Kuwait had its coolest July in a decade, so the Gulf's climate is not warming."
Correct the claim.
1. One month is weather, or at most seasonal variability, not climate.
2. A climate trend is judged over decades, against a 30-year average such as 1991-2020.
3. Natural variability (a persistent northerly wind pattern, more cloud or dust) can make one month cool even while the long-term average rises.
Answer: the claim confuses weather with climate.
A single cool month says nothing about the trend; only a long record, compared with a 30-year average, can show whether the climate is warming.
When a figure shows a vertical profile or a layered system, explain each change by its process (heating from below, ozone absorbing ultraviolet, no absorber) and quote the value where the change happens.
A 10-mark item rewards the chain from process to pattern; the numbers alone earn nothing.
- Saying the troposphere is heated directly by the sun. Sunlight mostly passes through the air; the ground absorbs it and heats the air from below, which is why temperature falls with height.
- Writing that greenhouse gases "trap heat" or "reflect heat back". They absorb long-wave infrared and re-emit it in all directions; some of that re-emitted energy returns to the surface.
- Linking the ozone hole to global warming. Ozone loss lets more ultraviolet reach the surface (a health risk), but it is not the cause of global warming; that is the rise in greenhouse gases such as carbon dioxide.
- Treating a record hot day as proof of climate change, or a cold one as disproof. Climate is a long-term pattern; individual events must be read against a 30-year record.
Check your understanding
1. Using Figure 1, suggest reasons why almost all weather happens in the lowest 11 km of the atmosphere. (10) Total [10]
2. Assess the importance of feedbacks, such as water vapour, compared with forcings, such as rising carbon dioxide, in explaining recent climate change. (15) Total [15]
3. A student writes: "The hole in the ozone layer lets in more heat from the sun, which is the main cause of global warming." Explain why this is wrong. (5) Total [5]
4. Using Figure 1, explain why the tropopause is a boundary for clouds and storms. (10) Total [10]
5. Assess the importance of gases other than carbon dioxide in controlling the Earth's climate. (15) Total [15]
Answers
1. Level 3 (8-10) needs processes tied to the figure. Indicative content: temperature falls with height in the troposphere (15 °C to −56.5 °C), so warm surface air can rise through cooler air, cool, condense and form cloud; almost all water vapour is in the troposphere because it evaporates from the surface and condenses as it rises; in the stratosphere temperature rises with height (to −2.5 °C at 47 km), so air is stable and rising air cannot continue; the tropopause at about 11 km (higher over the equator) caps convection. Level 2: correct ideas with little use of the figure. Level 1: describes the line.
2. Plan. AO1: define forcing (a change imposed on the climate from outside, such as carbon dioxide from burning fossil fuels, which starts warming) and feedback (a response to warming that amplifies or damps it); carbon dioxide at Mauna Loa about 427 ppm in 2025 against about 280 ppm before industrialisation; warmer air holds about 7% more water vapour per 1 °C; other feedbacks (melting ice lowering albedo, clouds). AO2: the forcing starts the change, so without rising carbon dioxide there would be no recent warming for water vapour to amplify; but the water vapour feedback roughly doubles the warming from carbon dioxide alone, so in size it is very important; water vapour cannot drive long-term change by itself because extra vapour rains out within days unless the air stays warmer; ice-albedo feedback matters most at high latitudes, cloud feedbacks are the least certain. Model judgement: forcings are more important in explaining why the climate is changing, because they start the warming and set its direction, but feedbacks are as important in explaining how much it warms; the two cannot be separated, since the feedbacks only respond to the forcing. Not enough: defining the two terms without weighing which explains more of the change.
3. Ozone absorbs ultraviolet radiation in the stratosphere (1), so ozone loss lets more ultraviolet reach the surface, a health risk (+1), but it adds little heat to the lower atmosphere (1). Global warming is caused by rising greenhouse gases (carbon dioxide, methane, nitrous oxide) absorbing and re-emitting long-wave infrared (1), released by burning fossil fuels and farming (+1). The two problems have different causes and different solutions (1). Maximum 5.
4. Level 3 (8-10): temperature in Figure 1 reaches about −56.5 °C at 11 km and then stops falling; above the tropopause it is steady and then rises, so rising air from the troposphere becomes cooler and denser than its surroundings and cannot rise further; thunderstorm tops spread out as anvils at this level; the stratosphere's warm air over cool air is stable with little mixing; almost no water vapour reaches the stratosphere, so clouds cannot form there; the tropopause is higher over the equator (16-18 km) because strong heating lifts the troposphere, so storms there are taller. Level 2 lacks explicit use of Figure 1's values; Level 1 is descriptive.
5. Plan. AO1: define climate and greenhouse effect; water vapour (largest natural greenhouse effect, a feedback), methane and nitrous oxide (strong per molecule, long-lived N₂O), ozone (ultraviolet absorber that sets the stratosphere's structure), aerosols and clouds (cooling and warming). AO2: water vapour gives more of the natural greenhouse effect than carbon dioxide, but it responds to temperature, so carbon dioxide controls the level about which water vapour adjusts; methane matters most over decades because it is strong but short-lived; ozone matters for the structure of the atmosphere and for health more than for warming; dust and sulphate aerosols over Arabia and South Asia cool some regions and change rainfall. Model judgement: other gases are very important for the size of the greenhouse effect, especially water vapour, but carbon dioxide is the most important control on long-term change because water vapour only follows temperature and methane breaks down within about a decade. Not enough: a list of gases with their percentages and no judgement of how much each matters, or for what.