The Atmosphere and the Heat Budget

Prerequisites: Climatology — overview · Earth’s Revolution and the Seasons

The Hook: Why Delhi Chokes in November

Connect: In the News → Concept

Almost every November and December, Delhi and the Indo-Gangetic plain are covered by thick smog, and the Commission for Air Quality Management invokes the stages of the Graded Response Action Plan (GRAP). Emissions from vehicles, industry, construction dust and crop-residue burning are part of the story — but they exist all year round.

What changes in winter is the atmosphere itself. Long, clear, calm nights cool the ground fast, a layer of warm air settles over a layer of cold air, and pollution is trapped near the ground like a lid on a pot. That lid is a temperature inversion — and it makes sense only once we understand how the atmosphere is heated.

The Core Concept

Composition of the atmosphere

Constituent	Share of dry air by volume (approx.)	Role
Nitrogen	78%	Largely inert; nitrogen cycle
Oxygen	21%	Respiration, combustion
Argon	0.93%	Inert gas
Carbon dioxide	About 0.04% — above 420 parts per million in recent years (NOAA, Mauna Loa)	Transparent to incoming sunlight but absorbs outgoing terrestrial radiation — the main long-lived greenhouse gas
Ozone	Trace, concentrated in the stratosphere	Absorbs ultraviolet radiation

Water vapour is variable — up to about 4% by volume in warm, humid tropics, nearly absent in cold, dry air. It decreases with height and from the Equator to the poles. It absorbs heat and is the source of all clouds and precipitation.
Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to form clouds.

Layers of the atmosphere

Layer	Height (approx.)	Temperature trend	Key features
Troposphere	Surface to ~8 km at the poles and ~18 km at the Equator	Falls with height (~6.5 °C per km)	All weather, clouds and almost all water vapour; thickest at the Equator because strong convection lifts air higher
Tropopause	Top of the troposphere	Roughly constant	Boundary; jet streams flow near it
Stratosphere	Up to ~50 km	Rises with height	Ozone layer absorbs UV and warms this layer; stable air — preferred by jet aircraft
Mesosphere	~50 to ~80 km	Falls sharply	Coldest layer (about −100 °C near its top); meteors burn up here
Thermosphere / Ionosphere	~80 to ~400 km	Rises sharply	Electrically charged ions reflect radio waves back to Earth; auroras
Exosphere	Above the thermosphere	—	Very thin air that gradually merges with space

Insolation

Insolation is the incoming solar radiation, received as short waves. The amount at any place depends on:

Angle of incidence — vertical rays concentrate energy; slanting rays spread it.
Length of the day.
Transparency of the atmosphere — clouds, dust and water vapour reflect and absorb it.
Configuration of the land (aspect and slope).

Surprise fact: the maximum insolation at the surface is received over the subtropical deserts, not the Equator, because the Equator is cloudier while the deserts have clear skies (NCERT).

Albedo is the share of incoming radiation a surface reflects. Fresh snow and ice have very high albedo; forests, oceans and dark soils have low albedo. Loss of Arctic sea ice exposes darker ocean, which absorbs more heat — a positive feedback in climate change.

How the atmosphere is heated

The atmosphere is heated mostly from below, by the Earth’s surface — not directly by the Sun.

Process	What happens
Conduction	Heat passes from the warm ground to the air in contact with it
Convection	Heated air rises vertically and carries heat upward — confined to the troposphere
Advection	Horizontal transfer of heat by wind (for example, the loo in north India)
Terrestrial radiation	The heated Earth emits long-wave radiation that greenhouse gases absorb — the main heat source of the lower atmosphere

The heat budget

The Earth neither keeps getting warmer nor cooler in the short run: over a year, the energy it receives equals the energy it loses. NCERT illustrates this by taking incoming insolation as 100 units:

graph TD
    A[100 units of insolation] --> B[35 units reflected to space: albedo]
    A --> C[14 units absorbed by the atmosphere]
    A --> D[51 units absorbed by the Earth's surface]
    D --> E[17 units radiated directly to space]
    D --> F[34 units absorbed by the atmosphere]
    C --> G[Atmosphere radiates 48 units to space]
    F --> G

Reflected (35): 27 from the tops of clouds, 2 from snow and ice, 6 by scattering.
Out to space as long-wave radiation (65): 17 directly from the surface + 48 from the atmosphere (14 + 34).
35 + 65 = 100 — balance.

Across latitudes there is no balance: there is a surplus of energy between about 40° N and 40° S and a deficit towards the poles. Winds and ocean currents carry the surplus poleward — the driver of the general circulation.

Distribution of temperature

Temperature generally decreases from the Equator towards the poles.
Isotherms bend sharply where they cross from land to sea: in January they bend towards the Equator over northern continents (land is colder) and towards the pole over oceans (sea is warmer).
The highest annual range of temperature is found in the interiors of large continents (north-eastern Eurasia); the lowest over the oceans of the Southern Hemisphere.
The normal lapse rate — the fall of temperature with height in the troposphere — is about 6.5 °C per 1,000 m.

Temperature inversion

A temperature inversion is a reversal of the normal lapse rate: temperature rises with height over a layer near the ground.

Ideal conditions: long winter nights; clear skies (so the ground loses heat rapidly by radiation); calm, still air; dry air; snow-covered ground.

Type	How it forms	Example
Surface (radiation) inversion	Ground cools fast at night; air touching it becomes colder than the air above	Winter nights over the north Indian plains
Valley (air drainage) inversion	Cold, dense air drains downslope at night and collects in valley bottoms	Frost in valley floors; in hilly areas, orchards and settlements prefer the warmer upper slopes
Advection inversion	Warm air moves over a cold surface (cold sea or snow)	Coastal fog
Frontal inversion	Warm air mass overrides a cold air mass along a front	Temperate cyclones

Consequences: stable air that does not rise; fog, smog and the trapping of pollutants; frost damage to crops; poor visibility for aviation and road transport.

The ozone layer

Ozone in the stratosphere absorbs most of the Sun’s harmful ultraviolet radiation.
Chlorofluorocarbons (CFCs) and other ozone-depleting substances caused the Antarctic ozone hole, first reported in the mid-1980s.
The Montreal Protocol (1987) phased out these substances; its Kigali Amendment (2016) added the phase-down of hydrofluorocarbons (HFCs), which are potent greenhouse gases.
The UNEP/WMO Scientific Assessment of Ozone Depletion: 2022 (released January 2023) projected that the ozone layer is on track to recover to 1980 values by around 2066 over the Antarctic, around 2045 over the Arctic and around 2040 elsewhere, if current policies stay in place.

Memory Anchors

Layers in order: “The Strong Man Takes Exercise” → Troposphere · Stratosphere · Mesosphere · Thermosphere · Exosphere.

Temperature trend zig-zag: Fall – Rise – Fall – Rise going up (tropo ↓, strato ↑, meso ↓, thermo ↑). The two “rises” have heaters: ozone (stratosphere) and solar radiation absorbed by thin gases (thermosphere).

Heat budget numbers: “35 bounce, 14 + 51 absorbed; 17 + 48 go home.”

Inversion recipe: “Long Clear Calm Dry nights” — LCCD.

For Prelims

Facts worth a mark

The troposphere is thickest at the Equator (~18 km) and thinnest at the poles (~8 km).
Temperature rises with height in the stratosphere (ozone absorbs UV) and in the thermosphere.
The mesosphere is the coldest layer; meteors burn up in it.
The ionosphere reflects radio waves.
The atmosphere is heated mainly by terrestrial (long-wave) radiation from below.
Maximum insolation is received over subtropical deserts, not the Equator.
Of 100 units of insolation, 35 are reflected (the Earth’s albedo); 65 are absorbed and later re-radiated.
Temperature inversion favours fog, smog and frost; it is common on long, clear, calm winter nights.
Montreal Protocol, 1987; Kigali Amendment, 2016 (HFC phase-down).

For Mains

Dimensions: physical basis (radiation, conduction, convection, advection); energy balance and latitudinal imbalance; inversion and air quality; ozone depletion and recovery as a model of international cooperation; greenhouse effect and climate change.
Keywords: insolation, albedo, terrestrial radiation, greenhouse effect, heat budget, lapse rate, atmospheric stability, mixing height, temperature inversion, ozone-depleting substances.
Value-addition: in north India, winter inversion and low wind speed lower the mixing height, so the same emissions give much higher pollution concentrations than in summer — an argument for year-round emission control rather than episodic emergency action. The Montreal Protocol’s success is often cited as a template for climate agreements.

Common Traps

Trap #1: 'The Sun heats the atmosphere directly'

Most short-wave sunlight passes through the atmosphere. The atmosphere is heated mainly from below, by long-wave radiation, conduction and convection from the Earth’s surface — which is why temperature normally falls with height in the troposphere.

Trap #2: 'The Equator receives the maximum insolation at the surface'

The Equator receives the most at the top of the atmosphere over the year, but persistent cloud cuts what reaches the ground. Subtropical deserts receive the most at the surface.

Trap #3: 'Temperature inversion happens on cloudy, windy nights'

Clouds act like a blanket and wind mixes the air — both prevent inversion. Inversion needs clear skies and calm air.

Practice

Prelims

UPSC Practice · Prelims GS I Qahb-1

Consider the following statements:

The troposphere is thicker over the Equator than over the poles.
Temperature decreases with height throughout the stratosphere.
The ionosphere reflects radio waves back to the Earth.

Which of the statements given above is/are correct?

A 1 only
B 2 only
C 1 and 3 only
D 1, 2 and 3

Solution

Statement 1 — correct. Strong convection at the Equator lifts the tropopause to about 18 km, compared with about 8 km at the poles.
Statement 2 — wrong. Temperature rises with height in the stratosphere because ozone absorbs ultraviolet radiation.
Statement 3 — correct. Charged layers of the ionosphere reflect radio waves, enabling long-distance radio communication.

Answer: C

UPSC-pattern practice question

UPSC Practice · Prelims GS I Qahb-2

Which one of the following combinations of conditions is most favourable for a surface temperature inversion?

A Long nights, clear skies and calm air
B Short nights, cloudy skies and strong winds
C Long nights, cloudy skies and strong winds
D Short nights, clear skies and strong winds

Solution

A surface inversion forms when the ground loses heat rapidly by radiation and the air above is not mixed. Long nights give more time for cooling, clear skies let heat escape (clouds would radiate it back), and calm air prevents mixing. Cloud and wind in the other options work against inversion.

Answer: A

UPSC-pattern practice question

UPSC Practice · Prelims GS I Qahb-3

Consider the following statements about the Earth’s heat budget:

About one-third of incoming solar radiation is reflected back to space without heating the Earth.
The atmosphere is heated more by incoming short-wave radiation than by outgoing terrestrial radiation.
There is a net surplus of radiation in the low latitudes and a net deficit in the high latitudes.
Snow and ice have a lower albedo than forests.

How many of the above statements are correct?

A Only one
B Only two
C Only three
D All four

Solution

Statement 1 — correct. About 35 of every 100 units are reflected — the Earth’s albedo.
Statement 2 — wrong. The atmosphere absorbs much more of the outgoing long-wave terrestrial radiation; it is largely transparent to incoming short waves.
Statement 3 — correct. Roughly between 40° N and 40° S there is a surplus; poleward there is a deficit. Winds and ocean currents transfer the surplus.
Statement 4 — wrong. Snow and ice have a very high albedo.

So two statements are correct.

Answer: B

UPSC-pattern practice question

Mains

UPSC Practice · Mains GS I 10 marks · 150 words Qahb-m1

What is temperature inversion? Explain how it contributes to the winter air-pollution crisis in north India.

Solution

Introduction: Normally temperature falls with height in the troposphere (about 6.5 °C per km). A temperature inversion is a reversal of this lapse rate, with warmer air lying above cooler air near the ground.

Body:

Formation in north India: long winter nights, clear skies and calm conditions let the ground lose heat rapidly; cold, dense air settles near the surface under warmer air; the land-locked Indo-Gangetic plain, bounded by the Himalaya, limits ventilation.
Effect on pollution: the inversion makes the air stable and lowers the mixing height; pollutants from vehicles, industry, construction dust, biomass and crop-residue burning cannot disperse vertically; moisture helps form fog that mixes with smoke into smog.
Compounding factors: low wind speeds, festive-season emissions, and seasonal crop-residue burning in the north-west.
Implications: health emergencies, school closures, aviation and road disruption, economic loss.
Responses: GRAP stages, the National Clean Air Programme, cleaner fuels and transport, crop-residue management, and regional airshed-level planning.

Conclusion: Inversion does not create the pollution but amplifies it. Lasting improvement depends on cutting emissions year-round, not just during winter emergencies.

UPSC-pattern practice question

Quick Revision

Point	One-liner
Composition	N₂ 78%, O₂ 21%, Ar 0.93%, CO₂ ~0.04%
Water vapour	Up to ~4%; decreases with height and latitude
Troposphere	~8 km (poles) to ~18 km (Equator); weather layer
Stratosphere	Ozone; temperature rises with height
Mesosphere	Coldest; meteors burn
Ionosphere	Reflects radio waves
Heat budget	35 reflected; 51 surface + 14 atmosphere absorbed; 17 + 48 re-radiated
Max insolation	Subtropical deserts
Normal lapse rate	~6.5 °C per 1,000 m
Inversion	Long, clear, calm nights; fog, smog, frost

Key Takeaways

The atmosphere is heated from below by long-wave terrestrial radiation — hence the normal fall of temperature with height.
Globally the heat budget balances; across latitudes it does not — and that imbalance drives winds and ocean currents.
Temperature inversion turns ordinary emissions into a pollution crisis by trapping them near the ground.

The Hook: Why Delhi Chokes in November

The Core Concept

Composition of the atmosphere

Layers of the atmosphere

Insolation

How the atmosphere is heated

The heat budget

Distribution of temperature

Temperature inversion

The ozone layer

Memory Anchors

For Prelims

For Mains

Common Traps

Practice

Prelims

Mains

Quick Revision

Key Takeaways

Related Topics

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