Solar Radiation, Heat Balance and Temperature

Every weather event you will study next — winds, pressure belts, monsoons, cyclones, rain — starts with uneven heating. Places that get more heat have lighter, rising air; places that get less have heavier, sinking air. Those differences in pressure make air move, and moving air carries heat and moisture around the globe. So this chapter is the engine room of the whole Climate unit.

• Comes after Chapter 7, which described the atmosphere's gases and layers — especially carbon dioxide and water vapour, which let sunlight in but trap the earth's heat.
• Leads into Chapter 9 (pressure and winds, which are caused by the uneven heating explained here), Chapter 10 (water vapour, condensation, fog) and Chapter 11 (climates and global warming).
• Links to Chapter 13: warm and cold ocean currents, which bend isotherms and move heat polewards.
• UPSC: GS1 Geography (physical geography of the world, climatology) and GS3 Environment / Disaster management (heat waves, urban heat islands, smog and fog).

• Insolation is short-wave; the earth gives heat back as long-wave (terrestrial) radiation.
• The atmosphere is heated mainly from below — by the earth, not directly by the sun.
• Heat budget: 100 in → 35 reflected (albedo) + 65 absorbed → 17 + 48 = 65 out.
• Net surplus between 40°N and 40°S, deficit towards the poles → heat moves polewards.
• Temperature falls 6.5°C per 1,000 m (normal lapse rate); an inversion reverses it.
• In January, NH isotherms bend north over oceans, south over continents; largest annual range > 60°C in NE Eurasia.

The energy the earth gets from the sun arrives mostly in short wavelengths. It is called insolation. Because the earth is round (a geoid, nearly a sphere), the rays meet the top of the atmosphere at a slant over most of the globe. On average the top of the atmosphere receives 1.94 calories per sq cm per minute.

This amount changes slightly through the year because the earth's distance from the sun changes. The earth is farthest from the sun — 152 million km — on 4 July (aphelion), and nearest — 147 million km — on 3 January (perihelion). So the earth receives a little more insolation on 3 January than on 4 July. But the effect is hidden by bigger factors such as how land and sea are spread out and how the atmosphere circulates, so it barely affects day-to-day weather.

Insolation changes through the day, through the seasons and through the year. NCERT lists five causes:

• Rotation of the earth on its axis (day and night).
• Angle of the sun's rays, which depends on latitude.
• Length of the day.
• Transparency of the atmosphere (clouds, dust, water vapour) — less important.
• Configuration of the land — its aspect (which way a slope faces) — less important.

The earth's axis makes an angle of 66½° with the plane of its orbit (it leans 23½° from the upright). This tilt has the biggest influence on how much insolation each latitude receives, because it changes both the angle of the rays and the length of the day through the year.

The angle of the rays depends on latitude. The higher the latitude, the lower the sun stands in the sky and the more slanting its rays. Slanting rays have two disadvantages: (1) they cover a larger area, so the energy per square metre is less; (2) they pass through a thicker layer of atmosphere, so more of their energy is absorbed, scattered and diffused on the way.

The atmosphere is largely transparent to short-wave solar radiation, so most of it reaches the ground. On the way, in the troposphere, water vapour, ozone and other gases absorb much of the near-infrared. Very small particles scatter the visible light both back to space and down towards the ground. This scattering colours the sky: the blue sky and the red rising and setting sun are both caused by scattering.

How insolation is spread over the surface. It ranges from about 320 W/m² in the tropics to about 70 W/m² at the poles. The maximum is over the subtropical deserts, because they have the least cloud. The equator gets less than the tropics because it is cloudy most afternoons. At the same latitude, continents receive more than oceans. In winter, middle and high latitudes receive less than in summer.

Finally, the atmosphere itself radiates heat out to space. In the end, the same amount of heat that came from the sun goes back to space, so the temperature of the surface and the air stays steady over time. The next section counts exactly how.

The heat budget balances for the earth as a whole, but not at every latitude. Between 40°N and 40°S the earth–atmosphere system takes in more radiation than it loses — a surplus. Near the poles it loses more than it gains — a deficit. The surplus heat is carried polewards. That is why the tropics do not keep getting hotter and the high latitudes do not freeze for good.

When insolation interacts with the atmosphere and the ground, it produces heat, which we measure as temperature. Heat is the molecular movement of particles in a substance; temperature is the measurement, in degrees, of how hot or cold a thing or place is.

The world pattern is easiest to see on maps of January and July, the coldest and warmest months in each hemisphere. In general isotherms run parallel to the lines of latitude. They bend away from this more in January than in July, and more in the northern hemisphere, because it has far more land.

• January, northern hemisphere: isotherms bend north over the oceans and south over the continents. The Gulf Stream and North Atlantic Drift (warm currents) make the North Atlantic warmer, so the isotherms bend north there; over Europe and above all the Siberian plain, the land is very cold and they bend south.
• Along 60°E, mean January temperature is −20°C at both 80°N and 50°N — the cold interior is as cold as the far north.
• January means: over 27°C in equatorial oceans, over 24°C in the tropics, 2°C to 0°C in the middle latitudes, −18°C to −48°C in the Eurasian interior.
• Southern hemisphere: the ocean dominates, so isotherms are nearly parallel to latitude and temperature changes gradually: 20°C along 35°S, 10°C along 45°S, 0°C along 60°S.

• July: isotherms run more nearly parallel to latitude. Equatorial oceans are warmer than 27°C.
• Over land, more than 30°C is found in the subtropical continental region of Asia, along 30°N.
• 10°C runs along 40°S (see the inconsistency below about 40°N).

The annual range of temperature is the difference between the mean temperatures of the warmest and the coldest months. The largest range — more than 60°C — is over north-eastern Eurasia, because of continentality: it is far from any sea's moderating influence. The smallest range, about 3°C, is between 20°S and 15°N, mostly over the oceans near the equator, where the sun stays high all year.

Normally temperature falls with height (the normal lapse rate). When the situation is reversed and temperature rises with height, it is called an inversion of temperature. Inversions are usually short-lived but common. The ideal conditions are a long winter night with clear skies and still air: the day's heat radiates away through the night, and by early morning the ground is colder than the air above it. Over the polar areas, inversion is normal throughout the year.

• Stability: a surface inversion makes the lowest layer of air stable — the cold, heavy air does not rise.
• Smoke and dust collect beneath the inversion layer and spread sideways, filling the lowest part of the atmosphere.
• Dense morning fog is common, especially in winter.
• It usually lasts a few hours, until the sun rises and warms the ground.

Air drainage. In hills and mountains, air cooled on the slopes at night is heavy and dense. Under gravity it flows down the slopes almost like water and piles up in hollows and valley bottoms, with warmer air above. This is an inversion caused by air drainage. NCERT notes that it protects plants from frost damage.

Try each question yourself first, then open the outline to compare. The multiple-choice questions are in the self-check below.

Numbers

One-liners

• Sun → short waves; earth → long waves (Planck's law: hotter body, shorter waves).
• The atmosphere is heated mainly from below, by terrestrial radiation.
• Convection is vertical and stays in the troposphere; advection is horizontal and matters more.
• Maximum insolation is over subtropical deserts, not at the equator — fewer clouds.
• Isotherms run roughly parallel to latitude; bends show land, sea and currents at work.
• Albedo = the share of sunlight reflected without heating anything.
• Inversion = warmer air above colder air; normal over the poles all year.
• The loo is an example of advection.

Traps to avoid

• The earth is nearest the sun in January — distance does not cause seasons; the tilt does.
• On 21 June the sun is overhead at 23½°N, not at the equator.
• Albedo: 27 + 2 is only 29 — the other 6 are scattered to space (from the figure). Total 35.
• The text's ‘Figure 9.2’ means Figure 8.2.
• Gulf Stream and North Atlantic Drift are warm currents — they bend January isotherms north.
• July 10°C isotherm: along 40°S, not 40°N (the July map shows 20°C+ at 40°N).
• Normal lapse rate 6.5°C/1,000 m ≠ dry adiabatic lapse rate ~9.8°C/1,000 m (NCERT glossary: 0.98° per 100 m).
• Land–sea temperature contrast is greater in winter, not summer.
• High clouds warm, low clouds cool — not the other way round.

Prelims — what UPSC asks

• NCERT lines are lifted almost verbatim: the January isotherm statements (2024, 2025) and ‘the atmosphere is heated mainly by terrestrial radiation’ (2024, statement form; also 2012). Read the isotherm paragraph and the heating paragraph word for word.
• Sun, tilt and day length is a favourite: where the sun is overhead (1983, 2019), longest day and more than 12 hours of daylight on 21 June (2022, 2024), most sunlight in December (1979), seasons and the tilted axis (1981, 1991, 2013), summer vs winter noon (1993).
• Radiation basics: insolation and how the sun's heat arrives (1989), infrared and water vapour (2023), CO₂ absorbs infrared (2012), cloudy nights and clouds' warming/cooling roles (2001, 2022), albedo of surfaces (2002, 2010).
• Temperature controls: why temperature falls with height (2012), inversion (1986), annual range — continents vs coasts, hemispheres and oceans (2007, 2013), specific heat and land–sea contrast (2023). The Statement-I / Statement-II format (2023–2025) rewards knowing the reason, not just the fact.
• Expect traps built on one wrong word — ‘cold’ for warm currents, ‘summer’ for winter, ‘high’ for low clouds.

Mains — where it appears and how to answer

• GS1 (Geography): temperature inversion and its effects on weather and people (2013); urban heat islands (2013); the loo compared with the Chinook and Foehn (2026).
• GS3 links: heat waves, winter smog and fog over north India, and greenhouse-gas forcing of the heat budget — use this chapter's mechanisms in environment and disaster answers.
• Answer frame: define → mechanism (insolation → ground → long-wave heating; or the inversion sequence) → a sketch (heat-budget boxes, isotherm bend, inversion profile) → world example → India example → impacts and remedies.
• Quote the numbers: 35/65 units, 17 + 48 = 65, 40°N–40°S surplus, 6.5°C per 1,000 m, >60°C range in NE Siberia. Numbers make a 150-word answer look expert.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

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