NCERT Solutions for Class 11 Geography Chapter 8: Solar Radiation, Heat Balance and Temperature

These Class 11 Geography Chapter 8 solutions cover Solar Radiation, Heat Balance and Temperature from Fundamentals of Physical Geography, the NCERT textbook continued for the 2026–27 session. The chapter explains how the earth receives energy as insolation, the factors that make insolation vary, how the atmosphere is heated and cooled (conduction, convection, advection, terrestrial radiation), the heat budget that keeps the earth’s temperature constant, the factors controlling temperature distribution, and the phenomenon of inversion of temperature. Below you get verbatim NCERT exercise questions with full step-by-step answers, key terms, extra practice, MCQs, Assertion–Reason and FAQs.

Class: 11 Subject: Geography Book: Fundamentals of Physical Geography Chapter: 8 Topic: Solar Radiation, Heat Balance and Temperature Session: 2026–27
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Class 11 Geography Chapter 8 – Overview

The earth receives almost all of its energy from the sun as short-wave solar radiation; the energy received is called insolation. On average the top of the atmosphere receives about 1.94 calories per sq cm per minute. The amount of insolation reaching the surface varies with the rotation of the earth, the angle of inclination of the sun’s rays (which depends on latitude), the length of the day, the transparency of the atmosphere and the configuration of the land. The earth, after being heated, becomes a radiating body and sends energy back as long-wave terrestrial radiation, heating the atmosphere from below. The atmosphere is warmed and cooled through conduction, convection and advection. Treating incoming insolation as 100 units, the heat budget shows that 35 units are reflected (the albedo), 65 units are absorbed, and exactly 65 units are radiated back — so the earth neither warms nor cools over time. Temperature distribution is controlled by latitude, altitude, distance from the sea, air masses and ocean currents, shown on maps with isotherms. Finally, inversion of temperature reverses the normal lapse rate, causing fog, smog and air drainage in valleys.

Key Terms & Concepts

Insolation: the incoming solar radiation received by the earth, in short wavelengths. The earth intercepts only a very small portion of the sun’s energy and receives on average 1.94 cal/cm²/minute at the top of the atmosphere.

Aphelion & Perihelion: on 4th July the earth is farthest from the sun (152 million km) — aphelion; on 3rd January it is nearest (147 million km) — perihelion. Hence insolation on 3rd January is slightly more than on 4th July.

Factors affecting insolation: (i) rotation of the earth on its axis; (ii) angle of inclination of the sun’s rays; (iii) length of the day; (iv) transparency of the atmosphere; (v) configuration of land. The earth’s axis makes an angle of 66½° with the plane of its orbit, which strongly affects insolation at different latitudes.

Conduction: heating that takes place when two bodies of unequal temperature are in contact — energy flows from the warmer to the cooler body until both attain the same temperature. It heats the lowest layers of the atmosphere.

Convection: vertical heating of the atmosphere as air in contact with the earth rises on heating in the form of currents. It is confined to the troposphere.

Advection: transfer of heat through the horizontal movement of air. In the middle latitudes most diurnal weather variation is caused by advection; the hot local wind ‘loo’ of northern India is an example.

Terrestrial radiation: the long-wave energy radiated by the heated earth back to the atmosphere, absorbed mainly by carbon dioxide and other greenhouse gases, heating the atmosphere from below.

Albedo: the part of insolation reflected back to space (about 35 units out of 100) — the reflectivity of the earth.

Heat budget / heat balance: the balance between insolation received (65 units absorbed) and the energy returned to space (17 + 48 = 65 units), keeping the earth’s temperature constant.

Normal lapse rate: the rate at which temperature normally decreases with height — 6.5°C per 1,000 m.

Isotherms: lines on a map joining places that have equal temperature.

Inversion of temperature: a reversal of the normal lapse rate in which temperature increases with height; common on long, clear, calm winter nights and over polar areas. Related to it are surface inversion (fog and smog) and air drainage in valleys.

NCERT “Exercises” — Full Solutions

All questions below are reproduced verbatim from the NCERT textbook’s end-of-chapter Exercises. Answers are original, written in exam-ready style. (Figure/map-based and project work are answered in words.)

1. Multiple choice questions.

(i) The sun is directly overhead at noon on 21st June at: (a) The equator    (b) 23.5° S    (c) 23.5° N    (d) 66.5° N

ANSWER (c) 23.5° N. On 21st June (summer solstice for the northern hemisphere) the sun’s rays are vertical over the Tropic of Cancer, which lies at 23.5° N. This is why the northern hemisphere then has its longest day and receives maximum insolation.

(ii) In which one of the following cities, are the days the longest? (a) Tiruvanantpuram    (b) Chandigarh    (c) Hyderabad    (d) Nagpur

ANSWER (b) Chandigarh. The length of the longest day increases as one moves away from the equator towards the poles. Among the cities listed, Chandigarh lies at the highest latitude (in northern India), so during the northern summer it experiences the longest day, while Tiruvananthapuram, being nearest the equator, has the most even day length.

(iii) The atmosphere is mainly heated by the: (a) Short wave solar radiation    (b) Reflected solar radiation    (c) Long wave terrestrial radiation    (d) Scattered solar radiation

ANSWER (c) Long wave terrestrial radiation. The atmosphere is largely transparent to incoming short-wave solar radiation, which passes through to heat the earth’s surface. The heated earth then radiates long-wave terrestrial radiation, which is absorbed by carbon dioxide and other greenhouse gases — so the atmosphere is heated mainly from below by terrestrial radiation.

(iv) Make correct pairs from the following two columns.

ANSWER The correct pairs are given below.
Column IColumn II
(i) Insolation(c) The incoming solar radiation
(ii) Albedo(d) The percentage of visible light reflected by an object
(iii) Isotherm(b) The lines joining the places of equal temperature
(iv) Annual range(a) The difference between the mean temperature of the warmest and the coldest months
So: (i)–(c), (ii)–(d), (iii)–(b), (iv)–(a).

(v) The main reason that the earth experiences highest temperatures in the subtropics in the northern hemisphere rather than at the equator is: (a) Subtropical areas tend to have less cloud cover than equatorial areas. (b) Subtropical areas have longer day hours in the summer than the equatorial. (c) Subtropical areas have an enhanced “green house effect” compared to equatorial areas. (d) Subtropical areas are nearer to the oceanic areas than the equatorial locations.

ANSWER (a) Subtropical areas tend to have less cloud cover than equatorial areas. Maximum insolation is received over the subtropical deserts, where cloudiness is the least, so the sun’s rays reach the surface with little reflection or absorption. The equator, despite the sun being nearly overhead, has heavy cloud cover and rain, which reduce insolation at the surface — hence the highest temperatures occur in the subtropics.

2. Answer the following questions in about 30 words.

(i) How does the unequal distribution of heat over the planet earth in space and time cause variations in weather and climate?

ANSWER Because different parts of the earth receive different amounts of insolation, the air is unevenly heated. This creates pressure differences, setting up winds that transfer heat from surplus to deficit regions, and the resulting variations in temperature, pressure and winds cause differences in weather and climate.

(ii) What are the factors that control temperature distribution on the surface of the earth?

ANSWER The temperature of air at any place is controlled by (i) the latitude of the place, (ii) its altitude, (iii) distance from the sea, (iv) air-mass and ocean currents, and (v) local aspects such as slope and exposure to the sun.

(iii) In India, why is the day temperature maximum in May and why not after the summer solstice?

ANSWER In May the sky over India is mostly clear and dry, so insolation reaching the surface is at its peak. After the summer solstice (21 June) the monsoon arrives; clouds and rain reflect and reduce insolation and lower temperatures, so May, not late June, records the highest day temperatures.

(iv) Why is the annual range of temperature high in the Siberian plains?

ANSWER The Siberian plains lie deep in the interior of the Eurasian continent, far from the moderating influence of the sea. Land heats and cools quickly, so summers are very hot and winters extremely cold — this continentality gives the highest annual range of temperature, over 60°C.

3. Answer the following questions in about 150 words.

(i) How do the latitude and the tilt in the axis of rotation of the earth affect the amount of radiation received at the earth’s surface?

ANSWER Latitude: The amount of insolation received depends mainly on the angle at which the sun’s rays strike the surface, and this angle is determined by latitude. Near the equator the rays fall almost vertically, covering a small area, so the energy is concentrated and intense. As latitude increases, the rays become more slanting and spread over a larger area, so the energy received per unit area decreases. Slant rays also travel through a greater depth of the atmosphere, leading to more absorption, scattering and diffusion, which further weakens the insolation reaching higher latitudes. Tilt of the axis: The earth’s axis makes an angle of 66½° with the plane of its orbit (a tilt of 23½° from the vertical). Because of this tilt, the latitude that receives vertical rays changes through the year — for example, the rays are vertical over the Tropic of Cancer on 21 June and over the Tropic of Capricorn on 22 December. This shifting also changes the length of day and night with the seasons. Together, latitude and the axial tilt decide how much radiation a place receives at any time of year, producing the seasons and the unequal heating of the earth.

(ii) Discuss the processes through which the earth-atmosphere system maintains heat balance.

ANSWER The earth as a whole neither accumulates nor loses heat — the insolation received equals the heat lost by terrestrial radiation. Taking the insolation at the top of the atmosphere as 100 units, the balance works as follows:
ProcessUnits
Reflected back to space (albedo)35 (27 from cloud tops + 2 from snow/ice + 6 reflected elsewhere)
Absorbed within the atmosphere14
Absorbed by the earth’s surface51
The 51 units absorbed by the earth are radiated back as terrestrial radiation: 17 units go directly to space, and 34 units are absorbed by the atmosphere (6 directly, 9 by convection and turbulence, 19 by latent heat of condensation). The atmosphere thus holds 48 units (14 from insolation + 34 from terrestrial radiation), all of which are radiated back to space. Balance: Total returning to space = 17 (direct from earth) + 48 (from atmosphere) = 65 units, which exactly balances the 65 units absorbed from the sun. This equilibrium — the heat budget — explains why the earth neither warms up nor cools down despite the huge transfer of heat.

(iii) Compare the global distribution of temperature in January over the northern and the southern hemisphere of the earth.

ANSWER In January it is winter in the northern hemisphere and summer in the southern hemisphere, and the two hemispheres behave very differently because the northern hemisphere has far more land. Northern hemisphere: Because of the large land mass, the effects of continents and ocean currents are strongly felt, so the isotherms are very irregular. They deviate to the north over the oceans and to the south over the continents. Over the North Atlantic the warm Gulf Stream and North Atlantic Drift bend the isotherms northward, while over the cold land of Europe and especially the Siberian plain the isotherms bend sharply south — the mean January temperature reaches about −20°C along 60°E, and the Eurasian interior records −18°C to −48°C. Southern hemisphere: Here the ocean dominates, so the isotherms are more or less parallel to the latitudes and temperature changes gradually. For example, the 20°C, 10°C and 0°C isotherms run parallel to 35°S, 45°S and 60°S respectively. Thus, the southern hemisphere shows a smooth, regular temperature distribution, while the northern hemisphere shows large deviations caused by the unequal distribution of land and sea.

Project Work: Select a meteorological observatory located in your city or near your town. Tabulate the temperature data as given in the climatological table of observatories: (i) Note the altitude, latitude of the observatory and the period for which the mean is calculated. (ii) Define the terms related to temperature as given in the table. (iii) Calculate the daily mean monthly temperature. (iv) Draw a graph to show the daily mean maximum, the daily mean minimum and the mean temperature. (v) Calculate the annual range of temperature. (vi) Find out in which months the daily range of temperature is the highest and the lowest. (vii) List out the factors that determine the temperature of the place and explain the possible causes for temperature variation in the months of January, May, July and October.

ANSWER (method & worked example) This is a field/data project, so use the actual data of an observatory near you. The textbook gives a worked example for New Delhi (Safdarjung): latitude 28°35′ N, altitude 216 m, based on observations of 1951–1980. Daily mean monthly temperature = (Mean of Daily Max. + Mean of Daily Min.) ÷ 2.
MonthMean Daily Max. (°C)Mean Daily Min. (°C)Daily mean monthly temp. (°C)
January21.17.3(21.1 + 7.3) ÷ 2 = 14.2
May39.625.9(39.6 + 25.9) ÷ 2 = 32.75
Annual range of temperature = mean temperature of the warmest month − mean temperature of the coldest month = 32.75°C − 14.2°C = 18.55°C. Defining the terms: Mean daily maximum = the average of the highest temperatures recorded each day in a month; mean daily minimum = the average of the lowest daily temperatures; highest/lowest recorded = the single highest/lowest reading ever noted in that month; daily range = mean daily max. − mean daily min. For the graph, plot the months on the x-axis and the three temperature curves on the y-axis. The temperature of a place is determined by its latitude, altitude, distance from the sea, air masses and ocean currents, and local aspect; in India January is cool (winter, low sun angle), May is hottest (clear dry skies, high insolation), July is moderated by the monsoon clouds and rain, and October is warm and pleasant as the monsoon retreats.

Extra Practice Questions

Short Answer Type Questions

Q1. Define insolation and state the average amount received at the top of the atmosphere.

ANSWERInsolation is the incoming solar radiation received by the earth in short wavelengths. On average the earth receives about 1.94 calories per square centimetre per minute at the top of its atmosphere, intercepting only a tiny portion of the sun’s total energy.

Q2. Distinguish between aphelion and perihelion.

ANSWERAphelion is the position when the earth is farthest from the sun (152 million km), occurring on 4th July. Perihelion is when the earth is nearest the sun (147 million km), occurring on 3rd January. Hence insolation on 3rd January is slightly more than on 4th July.

Q3. What is the normal lapse rate?

ANSWERThe normal lapse rate is the rate at which temperature normally decreases with increasing height in the atmosphere. Its value is 6.5°C for every 1,000 metres of ascent, because the atmosphere is heated from below by terrestrial radiation.

Q4. What are isotherms?

ANSWERIsotherms are lines drawn on a map that join places having equal temperature. They are used to show the distribution of temperature over the earth’s surface; in general they run roughly parallel to the latitudes, deviating where land, sea and ocean currents disturb the pattern.

Q5. What is the albedo of the earth?

ANSWERAlbedo is the part of incoming insolation that is reflected back to space without heating the earth. Out of 100 units of insolation, about 35 units are reflected (27 from cloud tops, 2 from snow and ice, and 6 from elsewhere); this reflected fraction is the earth’s albedo.

Long Answer Type Questions

Q1. Explain the different processes of heating and cooling of the atmosphere.

ANSWERThe atmosphere is heated and cooled through four main processes. Conduction occurs when two bodies of unequal temperature are in contact — air in contact with the warm land gets heated, and heat flows from the warmer to the cooler layer until they are equal; it heats only the lowest layers. Convection is vertical heating: warmed air rises as currents and carries heat upward, a process confined to the troposphere. Advection is the horizontal transfer of heat by moving air; in the middle latitudes most diurnal weather change is due to advection, and India’s hot ‘loo’ is an example. Terrestrial radiation is the long-wave energy radiated by the heated earth, absorbed by carbon dioxide and other greenhouse gases, which heats the atmosphere from below. Together these processes distribute the sun’s energy through the atmosphere.

Q2. Describe the factors that control the distribution of temperature on the earth’s surface.

ANSWERFive chief factors control temperature distribution. Latitude: insolation, and hence temperature, decreases from the equator to the poles as the sun’s rays become more slanting. Altitude: since the atmosphere is heated from below, temperature falls with height at the normal lapse rate of 6.5°C per 1,000 m, so highlands are cooler than nearby plains. Distance from the sea: the sea heats and cools slowly while land does so quickly, so coastal places have a small temperature range (maritime), and interiors a large range (continental). Air masses and ocean currents: warm air masses and warm currents raise temperature, while cold ones lower it — coasts washed by the Gulf Stream are warmer than those with cold currents. Local aspect: the slope and direction a place faces, and how exposed it is to the sun, also modify its temperature.

Q3. What is inversion of temperature? Explain its types and effects.

ANSWERInversion of temperature is the reversal of the normal lapse rate, in which temperature increases with height instead of decreasing. It usually lasts a short while but is common. Ground/surface inversion occurs on a long winter night with clear skies and still air: the day’s heat is radiated off, and by early morning the earth becomes cooler than the air above it. Such inversion promotes stability; smoke and dust collect under the inversion layer, dense fogs form on winter mornings, and these usually clear once the sun warms the ground. Over polar areas this inversion is normal throughout the year. Inversion due to air drainage takes place in hills and mountains: cold, dense air produced at night flows downslope under gravity and collects in valley bottoms with warmer air above. This air drainage protects plants from frost damage. However, inversions can also trap pollutants and cause smog over cities.

MCQs & Assertion–Reason

1. The energy received by the earth in short wavelengths is called:

(a) terrestrial radiation    (b) insolation    (c) albedo    (d) advection

2. The earth is at perihelion (nearest the sun) on:

(a) 4th July    (b) 21st June    (c) 3rd January    (d) 22nd December

3. The average insolation received at the top of the atmosphere is about:

(a) 0.94 cal/cm²/min    (b) 1.94 cal/cm²/min    (c) 2.94 cal/cm²/min    (d) 3.94 cal/cm²/min

4. The vertical heating of the atmosphere by rising air currents is called:

(a) conduction    (b) convection    (c) advection    (d) radiation

5. The hot local wind ‘loo’ of northern India is an outcome of:

(a) conduction    (b) convection    (c) advection    (d) terrestrial radiation

6. Out of 100 units of insolation, the amount reflected back to space (albedo) is about:

(a) 14 units    (b) 35 units    (c) 51 units    (d) 65 units

7. The normal lapse rate is:

(a) 1°C per 1,000 m    (b) 6.5°C per 1,000 m    (c) 10°C per 1,000 m    (d) 6.5°C per 100 m

8. Lines joining places of equal temperature on a map are called:

(a) isobars    (b) isohyets    (c) isotherms    (d) contours

9. The highest annual range of temperature (over 60°C) is found in the north-eastern part of which continent?

(a) North America    (b) Eurasia    (c) Africa    (d) Australia

10. Over polar areas, temperature inversion is:

(a) never seen    (b) seen only in summer    (c) normal throughout the year    (d) seen only during eclipses

Answer key: 1-(b), 2-(c), 3-(b), 4-(b), 5-(c), 6-(b), 7-(b), 8-(c), 9-(b), 10-(c).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: The atmosphere is heated mainly from below.

Reason: The atmosphere is largely transparent to short-wave solar radiation but absorbs long-wave terrestrial radiation.

A-R 2. Assertion: Insolation received on 3rd January is slightly more than on 4th July.

Reason: On 3rd January the earth is at perihelion, nearest the sun.

A-R 3. Assertion: Coastal places have a smaller annual range of temperature than interior places.

Reason: The sea heats and cools more slowly than land, so it moderates the temperature of nearby places.

A-R 4. Assertion: In May the day temperature in India is higher than after the summer solstice.

Reason: After the summer solstice the days become shorter, so less insolation is received.

A-R 5. Assertion: During temperature inversion, fog and smog form near the ground on winter mornings.

Reason: Surface inversion promotes stability, trapping smoke and dust beneath the inversion layer.

Answer key: 1-(A), 2-(A), 3-(A), 4-(C), 5-(A).

Exam Tips & Common Mistakes

How to score full marks in this chapter

Memorise the key numbers — insolation 1.94 cal/cm²/min, axial inclination 66½°, aphelion (4 July, 152 million km) and perihelion (3 January, 147 million km), normal lapse rate 6.5°C/1,000 m, and the heat-budget figures (35 reflected, 65 absorbed, 17 + 48 = 65 returned). For the heat balance question, present the figures as a table or flow so the examiner can follow the 100 → 65 + 65 logic. Clearly distinguish conduction, convection and advection with one example each, and always answer map/figure questions (isotherms in January vs July) in well-organised words, naming the Gulf Stream, North Atlantic Drift and the Siberian plain.

Common mistakes to avoid

  • Saying the atmosphere is heated directly by short-wave solar radiation — it is heated mainly by long-wave terrestrial radiation.
  • Confusing aphelion (farthest, 4 July) with perihelion (nearest, 3 January).
  • Mixing up conduction (contact), convection (vertical) and advection (horizontal).
  • Writing the lapse rate as 6.5°C per 100 m instead of per 1,000 m.
  • Forgetting that the heat budget must balance — 65 units received equal 65 units returned (17 + 48).
  • Saying the equator always has the highest temperature — the subtropical deserts get more insolation because of less cloud cover.

Frequently Asked Questions

What is Chapter 8 of Class 11 Geography (Fundamentals of Physical Geography) about?

Chapter 8, Solar Radiation, Heat Balance and Temperature, explains how the earth receives insolation, the factors that make it vary, how the atmosphere is heated and cooled (conduction, convection, advection, terrestrial radiation), the heat budget that keeps the earth’s temperature constant, the factors controlling temperature distribution, and the inversion of temperature.

What is the heat budget of the earth?

The heat budget is the balance between energy received and lost. Of 100 units of insolation, 35 are reflected (albedo) and 65 are absorbed; the earth and atmosphere then radiate 65 units back to space (17 directly from the earth and 48 from the atmosphere). Because received equals returned, the earth neither warms nor cools over time.

What is inversion of temperature?

Inversion of temperature is a reversal of the normal lapse rate in which temperature increases with height instead of decreasing. It is common on long, clear, calm winter nights and over polar areas, and causes surface fog and smog as well as air drainage in mountain valleys.

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