Why this chapter matters for UPSC: This is Chapter 8, "Solar Radiation, Heat Balance and Temperature", in the current rationalised NCERT Fundamentals of Physical Geography (Reprint 2026-27); it was Chapter 9 in pre-2023 editions. It explains how the earth receives energy from the sun, how that energy heats the air, how the earth returns the same amount to space, and why temperature varies from place to place. Every later climatology chapter (pressure, winds, humidity, world climates) builds on it, and UPSC has asked about temperature inversion and urban heat islands in Mains GS1.

Contemporary hook: NOAA's Mauna Loa record puts the 2025 annual mean CO₂ at 427.35 ppm, up from 424.61 ppm in 2024 (NOAA GML annual-mean file, accessed 1 October 2026). CO₂ is one of the gases that absorbs the earth's long-wave radiation, which is the mechanism this chapter describes under "terrestrial radiation" and the "heat budget".

🧠 First Principles — Read This First

The sun sends energy to the earth mostly as short waves: visible light and some ultraviolet. The energy that reaches the earth is called insolation, short for incoming solar radiation. Think of standing in sunlight on a winter morning: you feel warm even though the air around you is cold, because the light passes through the air and is absorbed by you.

The air is heated in the same indirect way. Sunlight passes through the atmosphere with little absorption and warms the ground. The warm ground then gives off heat as long waves, which carbon dioxide and water vapour absorb. So the atmosphere is heated mainly from below, by the earth, and only a little directly by the sun. The ground also warms the air touching it (conduction), warm air rises (convection), and winds carry heat sideways (advection).

Over a year the earth neither heats up nor cools down overall, so it must send back to space exactly as much energy as it receives. This equality is the heat budget. Some regions receive more than they lose (the tropics) and some lose more than they receive (the poles); winds and ocean currents move the surplus poleward.

Temperature is the measure of how hot a place is. It depends on latitude, altitude, distance from the sea, air masses and ocean currents, and local features such as slope. A place where cold air settles under warmer air shows a temperature inversion.

UPSC tests these mechanisms directly: what heats the atmosphere, why the subtropics receive more insolation than the equator, NCERT's heat-budget figures, and what inversions do to fog and pollution.

PART 1 — Quick Reference

Table 1: Why insolation varies (NCERT's five factors)

Factor	How it works
Rotation of the earth on its axis	Causes the daily cycle of insolation
Angle of inclination of the sun's rays	Depends on latitude; slant rays spread over a larger area and pass through more atmosphere
Length of the day	Longer days add more insolation
Transparency of the atmosphere	Cloud, dust and water vapour reduce what reaches the ground (less influence, NCERT)
Configuration of land (aspect)	The direction a slope faces changes what it receives (less influence, NCERT)

Source: NCERT, Fundamentals of Physical Geography, Class XI, Chapter 8 (Reprint 2026-27), kegy208.pdf, p. 67.

Table 2: How the atmosphere is heated and cooled

Process	Direction	NCERT point to remember
Conduction	Ground to the air in contact with it	Continues until both bodies reach the same temperature or contact breaks; important for heating the lowest layers
Convection	Vertical	Heated air rises in currents; confined only to the troposphere
Advection	Horizontal	More important than vertical movement; causes most day-to-night weather variation in middle latitudes; the loo of northern India in summer is an outcome of advection
Terrestrial radiation	Earth to atmosphere, long wave	The heated earth radiates long waves that CO₂ and other greenhouse gases absorb; the atmosphere is heated indirectly by the earth

Source: NCERT kegy208.pdf, pp. 68–69.

Table 3: NCERT heat budget (100 units of insolation at the top of the atmosphere)

Flow	Units
Reflected to space before reaching the surface (albedo of the earth)	35
of which: from the tops of clouds	27
of which: from snow- and ice-covered areas	2
Absorbed	65
of which: within the atmosphere	14
of which: by the earth's surface	51
Earth's surface radiates back as terrestrial radiation	51
of which: radiated directly to space	17
of which: absorbed by the atmosphere	34 (6 directly, 9 by convection and turbulence, 19 as latent heat of condensation)
Atmosphere radiates to space	48 (14 from insolation + 34 from terrestrial radiation)
Total returned to space	17 + 48 = 65, balancing the 65 absorbed

Source: NCERT kegy208.pdf, p. 69. NCERT names only 27 + 2 of the 35 reflected units; the remaining 6 are not itemised in the text.

Table 4: Factors controlling temperature (NCERT)

Factor	Effect
Latitude	Insolation varies with latitude, so temperature does too
Altitude	The atmosphere is heated from below, so temperature falls with height at the normal lapse rate of 6.5°C per 1,000 m
Distance from the sea	The sea heats and cools slowly, land quickly; places near the sea have a smaller range and are moderated by land and sea breezes
Air masses	Warm air masses raise temperature; cold air masses lower it
Ocean currents	Coasts washed by warm currents are warmer than coasts washed by cold currents
Local aspects	Slope and exposure modify temperature locally

Source: NCERT kegy208.pdf, p. 70.

PART 2 — Concepts & Narrative

Solar radiation and insolation

Insolation is the energy received by the earth from the sun, mostly in short wavelengths. Because the earth is a geoid resembling a sphere, the sun's rays fall obliquely at the top of the atmosphere and the earth intercepts only a very small part of the sun's output. NCERT gives the average received at the top of the atmosphere as 1.94 calories per sq cm per minute.

Beyond NCERT, the same quantity is now expressed as the total solar irradiance (the "solar constant"). NASA gives it as about 1,361 W/m², a value established by its SORCE satellite instrument; it varies by about 0.1% over the 11-year solar cycle, and averaged over the whole globe it corresponds to about 340 W/m² of incoming sunlight (NASA Goddard, Solar Irradiance Science page, accessed 1 October 2026).

The earth–sun distance changes during the year: 147 million km at perihelion (3 January) and 152 million km at aphelion (4 July). That is a difference of only about 3% in distance. Because intensity falls with the square of distance, the earth receives roughly 7% more energy at the top of the atmosphere in early January than in early July (derived: (152/147)² ≈ 1.07). NCERT's point is that this variation is masked by the distribution of land and sea and by atmospheric circulation, so it has little effect on daily weather. Northern Hemisphere summer falls in June–July because of the tilt of the axis, not because of distance.

Why insolation varies with latitude and season

The earth's axis makes an angle of 66½° with the plane of its orbit, and NCERT treats this tilt as having the greater influence on how much insolation each latitude receives. The tilt sets the angle of the sun's rays and the length of the day at each latitude through the year.

Explainer

Angle of the sun's rays: the main control

Hold a torch straight above a table and the patch of light is small and bright. Tilt the torch and the same light spreads over a larger, dimmer patch. Sunlight behaves the same way. At low latitudes the noon sun is high, so a beam of sunlight falls on a small area. At high latitudes the sun is low, so the same beam spreads over a larger area and each square metre receives less energy.

Slant rays lose more energy for a second reason: they pass through a greater depth of atmosphere, so more of the energy is absorbed, scattered and diffused before it reaches the ground (NCERT p. 68).

This is why NCERT's exercise asks where the sun is overhead at noon on 21 June (23.5°N) and which Indian city has the longest days in June (Chandigarh, the most northerly of the options; NCERT p. 73).

Passage of solar radiation through the atmosphere

The atmosphere is largely transparent to short-wave solar radiation. Within the troposphere, water vapour, ozone and other gases absorb much of the near-infrared part. Very small suspended particles scatter the visible spectrum both towards space and towards the ground. NCERT attributes the blue colour of the sky and the red colour of the rising and setting sun to this scattering.

Spatial distribution of insolation at the surface

Surface insolation ranges from about 320 W/m² in the tropics to about 70 W/m² at the poles (NCERT). Two points are often missed:

Maximum insolation is received over the subtropical deserts, where cloudiness is least. The equator receives comparatively less than the tropics because of its cloud cover. NCERT's exercise (MCQ v) asks why the highest temperatures in the northern hemisphere occur in the subtropics rather than at the equator; the answer is that subtropical areas have less cloud cover.
At the same latitude, insolation is generally higher over continents than over oceans. In winter, middle and higher latitudes receive less than in summer.

Heating and cooling of the atmosphere

NCERT describes four ways in which heat moves into and through the atmosphere.

Conduction. After the earth is heated by insolation, the air in contact with the ground is heated slowly, and the layers above it are heated in turn by contact. Heat flows from the warmer body to the cooler one until both reach the same temperature or contact is broken. Conduction matters mainly for the lowest layers of air.

Convection. Air heated at the surface rises vertically in currents and carries heat upward. NCERT stresses that convective transfer is confined to the troposphere.

Advection. Heat carried by the horizontal movement of air is advection. NCERT says horizontal movement is relatively more important than vertical movement. In the middle latitudes most of the day-and-night variation in weather is caused by advection alone. In the tropics, especially northern India in summer, the hot local wind called loo is an outcome of advection.

Terrestrial radiation. Once heated, the earth itself becomes a radiating body and gives off energy in long-wave form. Carbon dioxide and the other greenhouse gases absorb this long-wave radiation, so the atmosphere is heated indirectly, from below. The atmosphere in turn radiates heat to space. NCERT's exercise MCQ (iii) asks what mainly heats the atmosphere; the answer is long-wave terrestrial radiation, not short-wave sunlight.

Explainer

Why the air is warmest near the ground

If the sun heated the air directly, the air nearest the sun would be warmest. In fact the troposphere is warmest at the bottom and cools with height. The reason is that the air absorbs little of the incoming short-wave sunlight but absorbs much of the long-wave radiation coming up from the ground. The ground is the heater; the air is heated from below by terrestrial radiation, conduction and convection. This single fact explains the normal lapse rate (6.5°C per 1,000 m), why mountain tops are colder than valleys at the same latitude, and why convection stops at the top of the troposphere.

Heat budget of the planet earth

The earth as a whole does not accumulate or lose heat over time. This is possible only if the insolation it absorbs equals the energy it sends back to space. NCERT works the budget in units, taking insolation at the top of the atmosphere as 100 (Table 3). Read the table as three steps. First, 35 units are reflected straight back to space (NCERT itemises 27 from cloud tops and 2 from snow- and ice-covered areas); this reflected share is the albedo of the earth. Second, the 65 units that are absorbed are shared between the atmosphere (14) and the surface (51). Third, the surface passes on all 51 units: a small part (17) escapes straight to space, and the larger part (34) goes into the atmosphere, mostly as latent heat released when water vapour condenses (19), then by convection and turbulence (9), and least by direct absorption of radiation (6). The atmosphere ends up holding 48 units and radiates them to space, so the 65 units absorbed are matched by 65 returned.

The budget explains why the earth neither warms up nor cools down despite the large transfers of heat taking place.

Beyond the Book

NCERT's budget and a current satellite-based budget

NASA Earth Observatory's energy-budget explainer gives a different split of incoming sunlight: about 29% reflected to space by clouds, particles and bright surfaces, about 23% absorbed by the atmosphere (water vapour, dust, ozone, clouds) and about 48% absorbed by the surface, with about 340 W/m² arriving at the top of the atmosphere on average (NASA Earth Observatory, "Climate and Earth's Energy Budget"). NASA's Earth fact sheet lists the earth's Bond albedo as 0.294.

The two budgets are built from different data and do not need to be reconciled for the exam. In a UPSC answer based on NCERT, use NCERT's figures: 35 reflected; 65 absorbed (14 atmosphere + 51 surface); 17 + 48 returned. If you quote the satellite figures, name the source.

Variation in the net heat budget

The balance holds for the earth as a whole, not for every latitude. NCERT's Figure 8.3 shows a surplus of net radiation between 40°N and 40°S and a deficit towards the poles. The surplus is carried poleward by the atmosphere and oceans. Without this redistribution, the tropics would keep heating up and the high latitudes would stay permanently frozen. Pressure belts, planetary winds and ocean currents (the next chapters) are the means of this transfer.

Heat and temperature

NCERT distinguishes the two. Heat represents the molecular movement of the particles of a substance. Temperature is the measurement in degrees of how hot or cold a thing or place is. Two further definitions appear in the chapter's margin:

Key Term

Planck's law, specific heat and albedo (NCERT definitions)

Planck's law: the hotter a body, the more energy it radiates and the shorter the wavelength of that radiation. This is why the very hot sun emits short waves and the much cooler earth emits long waves.
Specific heat: the energy needed to raise the temperature of one gram of a substance by one degree Celsius. Water's high specific heat is why the sea heats and cools slowly.
Albedo: NCERT's exercise defines it as the percentage of visible light reflected by an object. The reflected 35 units in the heat budget are the earth's albedo.

Factors controlling temperature

NCERT lists five controls (Table 4). Applied examples:

Altitude. Leh, at about 3,500 m in Ladakh, is cold for its latitude. IMD's climatological table for Leh (2011–2024) gives a mean daily minimum of −13.4°C in January and a mean daily maximum of 27.8°C in July and August; the all-time lowest recorded is −28.3°C (11 January 1899) and the all-time highest 34.8°C (29 June 1978) (IMD Leh climatological data, accessed 1 October 2026).
Distance from the sea. NCERT's own project example for New Delhi (Safdarjung, 216 m, 1951–1980 means) gives a January mean of 14.2°C and a May mean of 32.75°C, an annual range of 18.55°C, with a highest recorded 47.2°C in May and a lowest recorded 0.6°C in January (NCERT p. 75). An inland plain station has a large range because land heats and cools quickly.
Ocean currents. NCERT's example: the Gulf Stream and North Atlantic Drift make the North Atlantic warmer, so January isotherms bend northward over the ocean.

Distribution of temperature: January and July

Temperature is mapped with isotherms, lines joining places of equal temperature. Because latitude is the main control, isotherms run roughly parallel to the lines of latitude. NCERT's detailed points:

Deviation is greater in January than in July, especially in the northern hemisphere, which has much more land.
January, northern hemisphere: isotherms bend northward over the oceans and southward over the continents. Over the North Atlantic the warm Gulf Stream and North Atlantic Drift push them north; over Europe and especially the Siberian plain they bend south. Along 60°E the mean January temperature is −20°C at both 80°N and 50°N.
January mean values: over 27°C in equatorial oceans, over 24°C in the tropics, 2°C to 0°C in the middle latitudes, and −18°C to −48°C in the Eurasian continental interior.
Southern hemisphere: the ocean's effect is strong, isotherms are nearly parallel to the latitudes and change gradually. The 20°C, 10°C and 0°C isotherms run along 35°S, 45°S and 60°S respectively.
July: isotherms generally run parallel to the latitudes. Equatorial oceans exceed 27°C. Over land, more than 30°C is found in the subtropical continental region of Asia along 30°N. The 10°C isotherm runs along 40°N, and 40°S also has 10°C.
Annual range: more than 60°C over north-eastern Eurasia because of continentality; the least, 3°C, between 20°S and 15°N.

Inversion of temperature

Normally temperature falls with height (the normal lapse rate). When the situation is reversed and temperature rises with height near the ground, it is called an inversion of temperature. NCERT's points:

An inversion is usually short-lived but common. The ideal setting is a long winter night with clear skies and still air: the day's heat is radiated away and by early morning the ground is cooler than the air above it.
Over polar areas, temperature inversion is normal throughout the year.
A surface inversion makes the lower air stable. Smoke and dust collect beneath the inversion layer and spread horizontally. Dense morning fog is common, especially in winter. The inversion usually lasts a few hours until the sun warms the ground.
Air drainage: in hills and mountains, cold air formed at night flows downslope under gravity, like water, and piles up in pockets and valley bottoms with warmer air above. NCERT says air drainage protects plants from frost damage. The protection is on the slopes, which the cold air leaves; the valley bottoms where the cold air collects are the coldest spots.

UPSC Connect

Inversion, fog and air pollution in north India

NCERT's description of a surface inversion (stable lower air, smoke and dust trapped beneath, dense winter fog) is the basis for answering why winter fog and poor air quality are common across the north Indian plains: long, clear, calm winter nights produce surface inversions that limit vertical mixing. Mains GS1 2013 asked exactly this mechanism (gs1-pyq-2013-20b, quoted under Practice Questions).

PART 3 — UPSC Integration

PYQ pattern

Mains GS1 2013, 5 marks (gs1-pyq-2013-20b): temperature inversion, its effect on weather and on inhabitants. A direct NCERT topic.
Mains GS1 2013, 5 marks (gs1-pyq-2013-20a): causes of urban heat islands. NCERT does not use the term; the answer applies this chapter's ideas (absorption of insolation by surfaces, terrestrial radiation, reduced cooling at night).
Closest adjacent: Mains GS1 2013 (gs1-pyq-2013-21) on why major hot deserts lie between 20° and 30°N on the western sides of continents. It draws on NCERT's point that insolation is maximum over the subtropical deserts, but its main answer lies in pressure belts and cold currents (later chapters).
No Prelims question on this chapter's content is in the site's 2026 Prelims bank.

Mains frameworks

Heat-budget questions ("Discuss the processes through which the earth-atmosphere system maintains heat balance" is NCERT's own exercise 3-ii): insolation in → reflection (35) → absorption (14 + 51) → terrestrial radiation (17 to space, 34 to the atmosphere through radiation, convection–turbulence and latent heat) → atmospheric radiation (48) → balance (65 = 65) → latitudinal surplus and deficit → poleward transfer by winds and currents.
Temperature-distribution questions: latitude → altitude → land–sea contrast → air masses and ocean currents → local aspect, with January isotherm bending over the North Atlantic and Siberia as the illustration.
Inversion questions: conditions (long, clear, calm winter nights; polar regions all year; air drainage in valleys) → effects (stable air, fog, trapped smoke and dust, frost in valley bottoms) → human impact (visibility, transport, health).

UPSC Connect

Cross-paper relevance

GS1 (Geography): insolation, heat budget, temperature controls, inversion.
GS3 (Environment): the greenhouse gases that absorb terrestrial radiation are the subject of climate-change questions, for example gs3-pyq-2022-18 on global warming and the Kyoto Protocol. This chapter supplies the mechanism; the policy content belongs to GS3.

Exam Strategy

Prelims fact-traps from this chapter:

The atmosphere is heated mainly by long-wave terrestrial radiation, not by incoming short-wave sunlight (NCERT exercise MCQ iii).
Perihelion is 3 January (147 million km); aphelion is 4 July (152 million km). The earth is nearest the sun during the northern winter.
Maximum insolation at the surface is over the subtropical deserts, not the equator; the equator gets less because of clouds.
NCERT's heat budget is 35 reflected / 65 absorbed (14 + 51) / 17 + 48 returned. Do not mix in the satellite-based 29/23/48 split unless the question asks for it.
Convection is confined to the troposphere. The loo is an example of advection, not convection.
The surplus belt is 40°N–40°S in NCERT.
Inversion is normal throughout the year over polar areas. Air drainage protects slope vegetation from frost.
Isotherms deviate more in January than in July, and more in the northern than the southern hemisphere.
The normal lapse rate (6.5°C per 1,000 m) is an average fall of temperature with height; it is not by itself a test of whether air is stable.

Mains question patterns: explain a process (inversion, heat balance), explain a distribution (why the subtropics are hotter than the equator; why Siberia has the largest annual range), or apply the controls to a named place.

Practice Questions

UPSC Mains GS1 2013 (5 marks; bank ID gs1-pyq-2013-20b): "What do you understand by the phenomenon of temperature inversion in meteorology? How does it affect the weather and the inhabitants of the place?"
UPSC Mains GS1 2013 (5 marks; bank ID gs1-pyq-2013-20a): "Bring out the causes for the formation of heat islands in the urban habitat of the world."
Practice (UPSC-pattern, not a past paper): Consider the following statements: (1) Convective transfer of heat is confined to the troposphere. (2) The loo of northern India is an outcome of advection. (3) The atmosphere is heated mainly by short-wave solar radiation. Which of the statements given above are correct? (a) 1 and 2 only (b) 2 and 3 only (c) 1 and 3 only (d) 1, 2 and 3. Answer: (a). Statement 3 is wrong; NCERT says the atmosphere is heated mainly by long-wave terrestrial radiation.
Practice (UPSC-pattern, not a past paper): According to NCERT's heat budget, out of 100 units of insolation, how many units are absorbed by the earth's surface? (a) 14 (b) 35 (c) 51 (d) 65. Answer: (c).
Practice (UPSC-pattern, not a past paper, Mains style, 150 words): "The equator receives less insolation at the surface than the subtropics." Explain, and discuss how the surplus of the low latitudes is redistributed.

📦 Revision Capsule

Revision Capsule

Hard Facts

Top of atmosphere: 1.94 cal/cm²/min (NCERT); about 1,361 W/m² total solar irradiance (NASA).
Perihelion 3 January, 147 million km; aphelion 4 July, 152 million km; distance differs by ~3%, top-of-atmosphere energy by ~7% (derived).
Surface insolation: ~320 W/m² (tropics) to ~70 W/m² (poles); maximum over subtropical deserts; equator less than tropics.
Heat budget: 35 reflected (27 clouds, 2 snow/ice); 65 absorbed (14 atmosphere, 51 surface); surface returns 51 = 17 to space + 34 to atmosphere (6 + 9 + 19); atmosphere radiates 48; 17 + 48 = 65.
Net radiation surplus between 40°N and 40°S; deficit towards the poles.
Normal lapse rate 6.5°C per 1,000 m.
Convection confined to the troposphere; loo = advection.
January isotherms: 20°C, 10°C, 0°C along 35°S, 45°S, 60°S; along 60°E, −20°C at both 80°N and 50°N.
Annual range: >60°C over north-eastern Eurasia; least 3°C between 20°S and 15°N.
Inversion normal all year over polar areas; air drainage protects plants from frost.

Core Concepts

The atmosphere is heated mainly from below, by long-wave terrestrial radiation, conduction and convection.
The earth returns to space what it absorbs; the heat budget balances for the planet but not for each latitude.
The angle of the sun's rays, set by latitude and the 66½° tilt, is the main control of insolation; cloudiness decides where the surface maximum lies.
Isotherms follow latitude except where land–sea contrast and ocean currents bend them, most strongly in the northern hemisphere in January.
A surface inversion makes the lowest air stable and traps smoke, dust and fog.

Confused Pairs

Insolation (incoming short wave) vs terrestrial radiation (outgoing long wave)
Convection (vertical, troposphere only) vs advection (horizontal; loo)
NCERT budget (35 / 14 + 51) vs NASA satellite budget (29 / 23 + 48)
Heat (molecular motion) vs temperature (degree of hotness)

PYQ Pattern

Mains GS1 2013: temperature inversion (gs1-pyq-2013-20b) and urban heat islands (gs1-pyq-2013-20a). Adjacent: hot deserts at 20–30°N (gs1-pyq-2013-21).

Sources

Sources

NCERT, Fundamentals of Physical Geography, Textbook for Class XI, Chapter 8 "Solar Radiation, Heat Balance and Temperature", Reprint 2026-27: kegy208.pdf.
NASA Goddard Space Flight Center, "Solar Irradiance Science" (total solar irradiance 1,361 W/m²; 340 W/m² global average): earth.gsfc.nasa.gov, accessed 1 October 2026.
NASA Earth Observatory, "Climate and Earth's Energy Budget" (29% reflected, 23% absorbed by atmosphere, 48% by surface): earthobservatory.nasa.gov, accessed 1 October 2026.
NASA NSSDC, "Earth Fact Sheet" (Bond albedo 0.294): nssdc.gsfc.nasa.gov, accessed 1 October 2026.
India Meteorological Department, Leh extreme-weather records and climatological table 2011–2024: mausam.imd.gov.in, accessed 1 October 2026.
NOAA Global Monitoring Laboratory, Mauna Loa annual mean CO₂: co2_annmean_mlo.txt, accessed 1 October 2026.
UPSC Mains questions: site bank src/_data/mains/pyq/gs1.json (IDs as cited).

Solar Radiation, Heat Balance and Temperature