The atmosphere: structure and heat budget

The first principle

The atmosphere is heated mostly from below, not from above. Sunlight passes through the air largely unabsorbed, warms the ground and the ocean, and the warm surface then heats the air above it by infrared radiation, conduction, convection and the release of latent heat when water vapour condenses. That one fact explains why temperature falls with height in the lowest layer, why that layer overturns and produces weather, and why gases that absorb infrared, the greenhouse gases, control how warm the surface is.

The core

Composition

• Dry air is about 78 per cent nitrogen, 21 per cent oxygen and just under 1 per cent argon, with carbon dioxide at about 420 parts per million. Water vapour varies from almost nothing over cold deserts to about 4 per cent over warm oceans, and it is the most important gas for weather because it carries latent heat and forms clouds. Dust and other aerosols, though tiny in mass, give water vapour a surface on which to condense.

Layers and why temperature changes with height

• Troposphere. Heated from the surface below, it cools upward at an average of about 6.5°C per kilometre (the normal lapse rate), because rising air expands and air higher up is farther from the heat source.
• Stratosphere. Above the tropopause, to about 50 km, temperature rises with height because the ozone layer, concentrated at about 15 to 35 km, absorbs ultraviolet radiation. Warm air sitting on cooler air is stable, so there is little vertical mixing, which is why jet aircraft cruise in the lower stratosphere.
• Mesosphere. From about 50 to 85 km there is little ozone, so temperature falls again to about minus 90°C at the mesopause, the coldest part of the atmosphere; most meteors burn up here.
• Thermosphere. Above about 85 km the few molecules absorb X rays and extreme ultraviolet, so their temperature rises to several hundred degrees or more, although the air is so thin that it holds little heat. Its lower part contains the ionosphere, layers of gas ionised by solar radiation that reflect radio waves back to Earth. Above about 500 km the exosphere fades into space.

Why the troposphere determines weather

• It holds the ingredients. About three quarters of the atmosphere's mass and almost all its water vapour and aerosols lie in the troposphere, so clouds, rain, snow, fog and storms can form only there.
• It is heated from below, so it overturns. Warm, light air at the surface rises and cool air sinks; rising air expands, cools at the dry adiabatic rate of about 10°C per kilometre, reaches its dew point, and condenses into cloud. Condensation releases latent heat, which drives thunderstorms and cyclones. The name troposphere means the turning sphere.
• It is capped by the tropopause. Above it temperature stops falling, so rising air loses its buoyancy and spreads out, which is why thunderstorm tops flatten into an anvil. The tropopause is about 8 km high over the poles and about 18 km over the equator, because strong surface heating and convection at the equator lift it higher; tropical storms therefore grow taller.
• It contains the jet streams. Narrow belts of fast westerly wind near the tropopause, such as the subtropical westerly jet over north India in winter, steer western disturbances and influence the onset of the monsoon (see the note on pressure belts and winds).

Dimensions

Insolation and what controls it

• Insolation is the solar energy received at the surface. It depends on the angle of the sun's rays (vertical rays spread over less area and pass through less air), the length of the day, the transparency of the atmosphere (cloud, dust and water vapour), and the distance of the Earth from the sun, which matters little. The tropics therefore receive more than they lose, while high latitudes lose more than they receive, and the atmosphere and oceans redistribute this surplus.

The heat budget

• Of 100 units of sunlight reaching the top of the atmosphere, about 30 are reflected by clouds, ice, snow and the surface; this is the Earth's albedo. About 19 units are absorbed in the atmosphere on the way down, and about 51 by land and ocean.
• The warm surface does not keep this energy. It returns it to the atmosphere as longwave infrared radiation, by convection and conduction, and as latent heat through evaporation. Greenhouse gases, chiefly water vapour, carbon dioxide and methane, absorb much of this infrared and radiate part of it back down, which keeps the average surface temperature at about 15°C instead of about minus 18°C.
• In the long run 70 units leave as infrared from the atmosphere and surface, and 30 are reflected, so outgoing energy equals incoming energy. Adding greenhouse gases upsets this balance: the Intergovernmental Panel on Climate Change (IPCC) reported in 2021 that the Earth is gaining energy, about 90 per cent of which goes into the oceans.

Temperature inversion

• Normally temperature falls with height; in an inversion a layer of warmer air lies above colder air. It forms on long, clear, calm winter nights when the ground radiates heat away quickly and chills the air touching it, and in valleys where cold dense air drains down at night. An inversion makes the air stable, so smoke and pollutants are trapped near the ground; this is one reason for Delhi's winter smog and fog over the Indo Gangetic plain.

In practice

Auroras: the northern and southern lights

• What they are. Auroras are curtains and arcs of coloured light in the upper atmosphere, mostly between about 100 and 300 km, usually seen in high latitudes. The aurora borealis appears around the north magnetic pole and the aurora australis around the south magnetic pole, often at the same time and roughly as mirror images.
• The trigger. The sun constantly emits a solar wind of charged particles, and in coronal mass ejections (CMEs) it hurls out billions of tonnes of magnetised plasma. The Earth's magnetic field forms a shield, the magnetosphere, which deflects most of this. When a CME's magnetic field points opposite to the Earth's, the two connect, energy is stored in the stretched magnetotail on the night side, and when it snaps back, electrons and protons are driven down along field lines into two rings around the magnetic poles called the auroral ovals.
• Why it glows. These fast particles collide with oxygen and nitrogen atoms and molecules and excite them; as they return to their normal state they emit light of particular colours. Oxygen at about 100 to 250 km gives the common green, oxygen higher up gives red, and nitrogen gives blue and purple.
• Geomagnetic storms. A strong CME causes a geomagnetic storm, which pushes the auroral ovals towards the equator. In May 2024 an extreme storm, the strongest in about two decades, made auroras visible far from the poles, including red auroras photographed from Hanle in Ladakh by the Indian Astronomical Observatory. Such storms also disturb satellites, Global Positioning System (GPS) signals, radio communication and power grids, which is why the Space Weather Prediction Center of the National Oceanic and Atmospheric Administration (NOAA) and India's Aditya L1 solar observatory (launched 2023) watch the sun.

Issues and way forward

• A changing heat budget. Rising greenhouse gases trap more outgoing infrared, warming the troposphere while the stratosphere cools, a fingerprint the IPCC (2021) uses to attribute warming to human activity. Cutting emissions under the Paris Agreement (2015) is the only lasting remedy.
• Ozone and the stratosphere. Chlorofluorocarbons thinned the ozone layer, letting more ultraviolet reach the surface; the Montreal Protocol (1987) has put it on a path to recovery around the middle of this century, according to the scientific assessment of the World Meteorological Organization and the United Nations Environment Programme (UNEP) in 2022.
• Winter inversions and pollution. Inversions do not cause pollution but trap it, so the answer lies in cutting emissions from vehicles, industry and crop residue burning, as under the National Clean Air Programme (Ministry of Environment, Forest and Climate Change, 2019).
• Space weather risk. Society now depends on satellites and grids that geomagnetic storms can damage, so better forecasting and hardened infrastructure are needed.

Common confusions

• Heat and temperature in the thermosphere. Its molecules move very fast, so its temperature is high, but there are so few of them that a body there would feel cold.
• Environmental and adiabatic lapse rate. The environmental lapse rate (about 6.5°C per km) describes still air around us; adiabatic rates describe a rising parcel of air cooling by expansion.
• Greenhouse effect and global warming. The greenhouse effect is natural and makes life possible; global warming is its enhancement by human emissions.
• Ozone hole and global warming. They are different problems with different gases, although some ozone depleting gases are also greenhouse gases.

For the exam

• Keyword bank: troposphere, tropopause, lapse rate, adiabatic cooling, latent heat, stratosphere, ozone, ionosphere, insolation, albedo, heat budget, greenhouse effect, inversion, magnetosphere, solar wind, coronal mass ejection, geomagnetic storm, auroral oval.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): The troposphere holds about three quarters of the atmosphere's mass and nearly all its water vapour, though it is only about 8 to 18 km thick.
  • Introduction (event): In May 2024 the strongest geomagnetic storm in about two decades made auroras visible from Ladakh, showing how solar activity reaches deep into our skies.
  • Conclusion: Because the atmosphere is heated from below, the troposphere overturns and makes weather, while the layers above act as shields against ultraviolet and charged particles.
  • Conclusion: The heat budget is in balance only as long as greenhouse gases are stable, which is why climate policy is, at its root, about restoring that balance.