Solar Radiation, Heat Balance and Temperature, Class 11 Geography

What Solar Radiation, Heat Balance and Temperature is about

Weightage: This chapter is part of Unit IV, Climate, of Fundamentals of Physical Geography, which carries 8 of the 70 theory marks in the school annual exam together with Composition and Structure of Atmosphere, Atmospheric Circulation and Weather Systems and Water in the Atmosphere. Map work carries a further 5 marks for each book.Source: CBSE Geography (029) syllabus 2026-27, Class XI course structure

Key concepts

Insolation and the earth-sun distance

NCERT section Solar Radiation

• The earth receives most of its energy in short wavelengths; this energy is incoming solar radiation, shortened to insolation.
• The earth is a geoid resembling a sphere, so rays fall obliquely at the top of the atmosphere and the earth intercepts only a very small portion of the sun's energy.
• Average received at the top of the atmosphere: 1.94 calories per sq cm per minute.
• Solar output at the top of the atmosphere varies slightly through the year because the earth-sun distance changes:

Position	Date	Distance
Aphelion (farthest)	4 July	152 million km
Perihelion (nearest)	3 January	147 million km

• So the earth receives slightly more on 3 January than on 4 July.
• But this effect is masked by the distribution of land and sea and atmospheric circulation, so it has little effect on daily weather.

Worked example

If the earth is nearest the sun in January, why is January winter in India? (2 marks)

Try it first, then show the solutionSolution

1. Perihelion (3 January, 147 million km) gives only slightly more insolation at the top of the atmosphere.
2. This small difference is masked by other factors; seasons depend mainly on the angle of the sun's rays and day length, which in January are low and short in the northern hemisphere.

Answer: Distance matters little; the slant of the rays and short days make January cold in India.

Variability of insolation at the surface

NCERT section Solar Radiation

Insolation varies during a day, a season and a year because of:
1. Rotation of the earth on its axis
2. Angle of inclination of the sun's rays
3. Length of the day
4. Transparency of the atmosphere
5. Configuration of land (aspect)
The last two have less influence.

Tilt of the axis: the earth's axis makes 66½° with the plane of its orbit, which strongly influences insolation at different latitudes (Figure 8.1, summer solstice).

Angle of the rays: depends on latitude. The higher the latitude, the smaller the angle with the surface, so rays are slant:

• Vertical rays cover less area than slant rays; when energy spreads over more area, energy per unit area falls.
• Slant rays pass through a greater depth of atmosphere, so more absorption, scattering and diffusion.

Passage through the atmosphere: the atmosphere is largely transparent to short-wave solar radiation. Much of the near-infrared part is absorbed in the troposphere by water vapour, ozone and other gases. Tiny suspended particles scatter visible light to space and to the surface: this gives the blue sky and the red rising and setting sun.

Spatial distribution at the surface:

• About 320 W/m² in the tropics to about 70 W/m² at the poles.
• Maximum over subtropical deserts, where cloudiness is least.
• The equator receives less than the tropics (more cloud).
• At the same latitude, more over continents than oceans.
• Middle and high latitudes receive less in winter than in summer.

Worked example

Why does a place at 60°N receive less insolation per square metre than a place at 20°N at noon in summer? Give two reasons. (2 marks)

Try it first, then show the solutionSolution

1. At higher latitude the rays are more slanted, so the same energy spreads over a larger area and energy per unit area falls.
2. Slant rays also travel through a greater depth of atmosphere, losing more energy by absorption, scattering and diffusion.

Answer: Larger area covered and longer path through the atmosphere.

Heating and cooling of the atmosphere

NCERT section Heating and Cooling of Atmosphere

Process	How heat moves	Key point
Conduction	Between two bodies of unequal temperature in contact, from warmer to cooler, until temperatures equalise or contact breaks	Heats the lower layers: air touching the heated land warms slowly, then the layers above
Convection	Heated air near the surface rises vertically in currents, carrying heat upward	Confined to the troposphere
Advection	Horizontal movement of air	More important than vertical movement; most day-night weather variation in middle latitudes; the loo of northern India in summer
Terrestrial radiation	Heated earth radiates long-wave energy to the atmosphere	Absorbed by CO2 and other greenhouse gases; the atmosphere is heated indirectly from below

Sequence: sun → short-wave insolation heats the surface → surface heats the air by conduction, convection, advection and long-wave terrestrial radiation → the atmosphere in turn radiates heat to space. The amount received is finally returned, keeping temperatures at the surface and in the atmosphere constant overall.

Two physics notes in the chapter: Planck's law: the hotter a body, the more energy it radiates and the shorter the wavelength (hence the sun gives short waves, the earth long waves). Specific heat: energy needed to raise the temperature of one gram of a substance by one degree Celsius.

Worked example

Why is the atmosphere said to be heated 'from below' rather than directly by the sun? (2 marks)

Try it first, then show the solutionSolution

1. The atmosphere is largely transparent to short-wave solar radiation, so most insolation passes through and heats the surface.
2. The heated earth radiates long-wave terrestrial radiation, which carbon dioxide and other greenhouse gases absorb; so the air is heated indirectly, from the ground up.

Answer: Because it absorbs long-wave radiation from the heated earth, not much short-wave sunlight.

Heat budget of the earth

NCERT section Heating and Cooling of Atmosphere

The earth as a whole neither accumulates nor loses heat; insolation received must equal heat lost by terrestrial radiation. Take insolation at the top of the atmosphere = 100 units (Figure 8.2):

Incoming

• 35 units reflected to space before reaching the surface: 27 from cloud tops, 2 from snow and ice (the rest of the 35, by arithmetic 6, is also returned without being absorbed). This reflected amount is the albedo.
• 65 units absorbed: 14 within the atmosphere, 51 by the surface.

Outgoing from the surface

• The surface radiates 51 units as terrestrial radiation: 17 directly to space, 34 absorbed by the atmosphere: 6 directly, 9 by convection and turbulence, 19 as latent heat of condensation.

Outgoing from the atmosphere

• The atmosphere absorbed 48 units (14 + 34) and radiates them to space.

Balance: 17 + 48 = 65 units returned = 65 units absorbed. This is the heat budget (heat balance), which explains why the earth neither warms up nor cools down despite huge heat transfers.

Latitudinal variation (Figure 8.3): net radiation surplus between 40°N and 40°S, deficit near the poles. The surplus is redistributed poleward, so the tropics do not keep heating and high latitudes do not freeze permanently.

Worked example

The tropics receive more heat than they lose, yet they do not get hotter every year, and the poles do not keep cooling. Explain, using the net radiation balance. (3 marks)

Try it first, then show the solutionSolution

1. There is a surplus of net radiation between about 40°N and 40°S and a deficit near the poles.
2. The surplus heat is redistributed poleward by winds and ocean currents.
3. So the tropics do not progressively heat up and the high latitudes do not freeze permanently, while the global budget stays balanced.

Answer: Poleward transfer of surplus heat from 40°N-40°S to the deficit polar regions keeps both stable.

Temperature and factors controlling it

NCERT section Heating and Cooling of Atmosphere

Heat versus temperature: heat is the molecular movement of particles in a substance; temperature measures, in degrees, how hot or cold a place or object is.

Five factors:
1. Latitude: temperature depends on insolation, which varies with latitude.
2. Altitude: the atmosphere is heated from below, so places near sea level are warmer than places higher up. The rate of fall with height is the normal lapse rate: 6.5°C per 1,000 m.
3. Distance from the sea: the sea heats and cools slowly, land quickly; temperature varies less over the sea. Places near the sea get the moderating influence of the sea and of land and sea breezes.
4. Air masses and ocean currents: warm air masses raise temperature, cold ones lower it; coasts with warm currents are warmer than coasts with cold currents.
5. Local aspects.

Annual range of temperature: the difference between the mean temperature of the warmest and coldest months. (NCERT's worked example for New Delhi: daily mean monthly temperature = (mean daily max + mean daily min) ÷ 2; January 14.2°C, May 32.75°C; range 18.55°C.)

Worked example

Using the normal lapse rate, estimate the temperature at a town 2,200 m above sea level when a coastal town at 200 m records 28°C. (2 marks)

Try it first, then show the solutionSolution

1. Height difference = 2,200 - 200 = 2,000 m; fall = 2 × 6.5 = 13°C.
2. Temperature ≈ 28 - 13 = 15°C.

Answer: About 15°C.

Distribution of temperature in January and July

NCERT section Heating and Cooling of Atmosphere

Shown with isotherms: lines joining places of equal temperature (Figure 8.4).

General pattern: isotherms are broadly parallel to latitudes. Deviations are greater in January than July, especially in the northern hemisphere, which has much more land, so land masses and ocean currents have a strong effect.

January (northern hemisphere):

• Isotherms bend north over oceans and south over continents.
• The North Atlantic is warmed by the Gulf Stream and North Atlantic Drift, so isotherms bend north; over Europe temperature falls sharply and isotherms bend south, most of all in the Siberian plain.
• Mean January temperature along 60°E is -20°C at both 80°N and 50°N.
• Mean January temperatures: over 27°C in equatorial oceans, over 24°C in the tropics, 2°C to 0°C in middle latitudes, -18°C to -48°C in the Eurasian continental interior.

Southern hemisphere: the ocean's effect is strong; isotherms are more or less parallel to latitudes and change gradually: 20°C, 10°C and 0°C run along 35°S, 45°S and 60°S.

July: isotherms generally parallel to latitudes; equatorial oceans over 27°C; over land, over 30°C in the subtropical continental region of Asia along 30°N; the 10°C isotherm runs along 40°N, and along 40°S the temperature is 10°C.

Annual range (Figure 8.5): highest, over 60°C, in north-eastern Eurasia due to continentality; least, 3°C, between 20°S and 15°N.

Worked example

Two places lie near 60°N: one on the coast of Norway, warmed by the North Atlantic Drift, and one deep in the Siberian plain. Which will have the larger annual range of temperature, and why? (2 marks)

Try it first, then show the solutionSolution

1. The Siberian place is far from the sea in the continental interior; land heats and cools quickly, so January means fall to between -18°C and -48°C while summers are warm.
2. The Norwegian coast is moderated by the sea and the warm current, so winters stay mild; continentality gives north-eastern Eurasia an annual range of over 60°C.

Answer: The Siberian place: continentality, far from the moderating sea, gives extreme winters and a very large annual range.

Inversion of temperature and air drainage

NCERT section Inversion of Temperature

Normal lapse rate: temperature falls with height. When this is reversed (temperature rises with height near the ground), it is an inversion of temperature.

Conditions: usually short-lived but quite common. Ideal: a long winter night with clear skies and still air. The day's heat radiates away at night, so by early morning the ground is cooler than the air above. Over polar areas, inversion is normal throughout the year.

Effects of surface inversion:

• Promotes stability in the lower atmosphere.
• Smoke and dust collect beneath the inversion layer and spread horizontally.
• Dense morning fogs, especially in winter.
• Lasts a few hours until the sun warms the earth.

Air drainage (hills and mountains): cold, heavy, dense air produced at night flows downslope under gravity, almost like water, and piles up in pockets and valley bottoms with warm air above. This is air drainage, a form of inversion; it protects plants on the slopes from frost damage.

Worked example

Why do fruit growers in hilly areas prefer slopes to valley floors? (2 marks)

Try it first, then show the solutionSolution

1. At night cold, dense air drains down the slopes and collects in valley bottoms, with warmer air above (air drainage, an inversion).
2. So valley floors are colder and frost-prone, while slopes stay warmer; air drainage protects plants on the slopes from frost damage.

Answer: Cold air settles in valleys at night; slopes stay warmer and escape frost.

Solar Radiation, Heat Balance and Temperature in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Factors Controlling Temperature, World Distribution and Inversion of Temperature
• Insolation, Heating of the Atmosphere and the Heat Budget of the Earth

Solar Radiation, Heat Balance and Temperature: key facts

Solar constant (NCERT value)

1.94 calories per sq cm per minute at the top of the atmosphere

Average received.

Earth-sun distance

Aphelion 4 July, 152 million km; perihelion 3 January, 147 million km

Effect masked by land-sea distribution and circulation.

Insolation at the surface

About 320 W/m² (tropics) → about 70 W/m² (poles); maximum over subtropical deserts

Equator gets less than the tropics.

Heat budget

$100=35reflected+14(atmosphere)+51(surface);returned=17+48=65$

34 to atmosphere = 6 direct + 9 convection/turbulence + 19 latent heat.

Normal lapse rate

$Fallintemperature(°C)=height(m)\times 6.5÷1,000$

Chapter 7 gives 1°C per 165 m, a slightly different average.

Daily mean monthly temperature

(mean daily maximum + mean daily minimum) ÷ 2

From NCERT's project example.

Annual range

Mean temperature of warmest month - mean temperature of coldest month

Highest over 60°C (NE Eurasia), least 3°C (20°S-15°N).

Net radiation

Surplus between 40°N and 40°S; deficit near the poles

Surplus moves poleward.

Southern hemisphere January isotherms

$20°C\approx 35°S,10°C\approx 45°S,0°C\approx 60°S$

Parallel to latitudes because of oceans.

Solar Radiation, Heat Balance and Temperature: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Solar Radiation, Heat Balance and Temperature

The question patterns to expect, and the method that works for each.

Tricks to remember Solar Radiation, Heat Balance and Temperature

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Aphelion and perihelion

  'A for Away in July (152); P for Proximity in January (147)'.

  Use it when: Dates and distances.

• Insolation factors

  'RAD TA': Rotation, Angle of rays, Day length, Transparency, Aspect (last two minor).

  Use it when: Variability of insolation.

• Heat budget

  '35-14-51 in; 17-48 out; 65 = 65'.

  Use it when: Heat budget numbers.

• Heating processes

  'Conduction touches, Convection climbs, Advection across, Radiation from below'.

  Use it when: Heating of atmosphere.

• Temperature factors

  'LADAL': Latitude, Altitude, Distance from sea, Air masses and currents, Local aspects.

  Use it when: Factors controlling temperature.

Solar Radiation, Heat Balance and Temperature: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Solar Radiation, Heat Balance and Temperature

The slips examiners see most often, and what to write instead.

• ✗Mistake: Linking seasons to the earth-sun distance

  It seems logical that nearer means hotter.

  Wrong working

  It is hot in summer because the earth is nearest the sun.

  Right working

  The earth is nearest on 3 January; the effect is masked. Seasons depend on the angle of rays and day length due to the 66½° tilt.

  ✓Do this: Perihelion in January, winter in India.

  Usually costs: 1-2 marks.

• ✗Mistake: Getting heat budget numbers wrong

  Many numbers to remember.

  Wrong working

  The surface sends 34 units to space.

  Right working

  Surface: 17 to space, 34 to the atmosphere. Atmosphere: 48 to space.

  ✓Do this: Memorise '35-14-51' in and '17-48' out.

  Usually costs: 1-2 marks in a 5-mark answer.

• ✗Mistake: Saying the equator gets the most insolation

  The equator is assumed hottest.

  Wrong working

  Insolation is maximum at the equator.

  Right working

  Maximum over subtropical deserts, where cloudiness is least; the equator gets less than the tropics.

  ✓Do this: Clouds cut equatorial insolation.

  Usually costs: 1 mark per MCQ.

• ✗Mistake: Confusing convection and advection

  Both involve moving air.

  Wrong working

  The loo is caused by convection.

  Right working

  The loo results from advection (horizontal transfer); convection is vertical.

  ✓Do this: Advection = across; convection = up.

  Usually costs: 1 mark.

• ✗Mistake: Defining albedo as absorbed radiation

  Mixing up the two parts of the budget.

  Wrong working

  Albedo is the radiation absorbed by the earth.

  Right working

  Albedo is the reflected amount of radiation (35 units).

  ✓Do this: Albedo = bounced back.

  Usually costs: 1 mark.

NCERT exercise guide: Solar Radiation, Heat Balance and Temperature

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Solar Radiation, Heat Balance and Temperature

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   On 21 June, which place has the longest day?
   (a) Kanniyakumari
   (b) Srinagar
   (c) Chennai
   (d) Bhopal

   Hint

   In the northern summer, days lengthen towards the north.

   Answer

   (b) Srinagar, the most northerly of the four.

2. Q2 · 1 mark · MCQExam level

   On a clear day you stand barefoot on a sunny road and feel the ground hotter than the air above it. This is because:
   (a) the air absorbs most incoming short-wave radiation
   (b) the ground absorbs insolation and then heats the air from below by long-wave radiation, conduction and convection
   (c) the sun heats the air first and the air heats the ground
   (d) reflected radiation heats the ground

   Hint

   The atmosphere is largely transparent to short waves.

   Answer

   (b) The surface is heated first and then heats the air from below.

3. Q3 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): Advection is more important than convection in causing day-to-day weather changes in middle latitudes.
   Reason (R): Convective transfer of energy is confined to the troposphere.
   Choose (a)-(d).

   Hint

   Does R explain A?

   Answer

   (b). Both are true, but R (where convection works) does not explain why horizontal transfer dominates mid-latitude weather.

4. Q4 · 2 marks · Short answerExam level

   Explain why the sky is blue and the rising sun looks red. (2 marks)

   Hint

   Scattering.

   Answer

   Very small particles in the troposphere scatter the visible spectrum towards space and the surface; this scattering of light within the atmosphere gives the blue colour of the sky and the red colour of the rising and setting sun.

5. Q5 · 5 marks · Long answerHarder exam level

   How do latitude and the tilt of the earth's axis affect insolation? (5 marks)

   Hint

   Angle of rays, area, path length, 66½°, day length.

   Answer

   The axis is inclined at 66½° to the orbital plane, so the sun's height and day length change with latitude and season. At higher latitudes rays are slanter: they spread over a larger area and pass through more atmosphere, losing energy to absorption, scattering and diffusion. So insolation falls from about 320 W/m² in the tropics to about 70 W/m² at the poles, and middle and high latitudes get less in winter than summer. Clouds modify this, so subtropical deserts get the most.

6. Q6 · 2 marks · Short answerHarder exam level

   In July, where on land are temperatures above 30°C, and why do isotherms run more nearly parallel to the latitudes than in January? (2 marks)

   Hint

   Subtropical Asia; land-sea contrasts are weaker.

   Answer

   Over the subtropical continental region of Asia along about 30°N. In July isotherms generally run parallel to the latitudes because the deviations caused by land and ocean currents are less pronounced than in the northern winter; equatorial oceans exceed 27°C.

7. Q7 · 3 marks · Short answerHOTS / competency

   Suppose two farms lie in the same hill district: one on a valley floor and one halfway up a slope. A frost warning is issued for a clear, calm night. Which farm is at greater risk, and why? (3 marks)

   Hint

   Air drainage.

   Answer

   The valley-floor farm. On clear, calm nights cold, dense air flows downslope under gravity and piles up in valley bottoms (air drainage), with warmer air above. The valley floor gets frost, while the slope stays warmer and is protected.

8. Q8 · 3 marks · Short answerHOTS / competency

   Why do winds and ocean currents exist at all? Answer using the unequal heating of the earth in space and time. (3 marks)

   Hint

   Surplus, deficit, pressure, transfer.

   Answer

   Insolation differs with latitude, season and land or sea, so some areas (between about 40°N and 40°S) have a heat surplus and others (near the poles) a deficit. These temperature differences create pressure differences in the atmosphere. Winds and ocean currents move heat from surplus to deficit areas, and this transfer produces changing weather and distinct climates.

9. Q9 · 1 mark · MCQExam level

   A snow-covered field reflects much of the sunlight falling on it. Which term describes this, and which term would you use for a line on a map joining places that all recorded 15°C?
   (a) Insolation; isobar
   (b) Albedo; isotherm
   (c) Terrestrial radiation; isohyet
   (d) Annual range; isotherm

   Hint

   Reflected share; equal temperature.

   Answer

   (b) Albedo; isotherm.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.How much insolation reaches the top of the atmosphere on average?Show answer

   1.94 calories per sq cm per minute.

2. 2.What angle does the earth's axis make with its orbital plane?Show answer

   66½°.

3. 3.Where is insolation maximum at the surface?Show answer

   Over subtropical deserts, where cloudiness is least.

4. 4.What is advection?Show answer

   Transfer of heat through horizontal movement of air.

5. 5.What is the normal lapse rate given in this chapter?Show answer

   6.5°C per 1,000 m.

6. 6.Where is the annual range of temperature least?Show answer

   Between 20°S and 15°N, about 3°C.

7. 7.Where is temperature inversion normal all year?Show answer

   Over polar areas.

8. 8.State Planck's law as given in the chapter.Show answer

   The hotter a body, the more energy it radiates and the shorter the wavelength of that radiation.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Insolation: short waves; 1.94 cal/sq cm/min at the top of the atmosphere.
• ☐Aphelion 4 July (152 million km); perihelion 3 January (147 million km); effect masked.
• ☐Insolation varies with rotation, angle of rays, day length, transparency, aspect; axis tilt 66½°.
• ☐Surface insolation: 320 W/m² tropics to 70 W/m² poles; maximum over subtropical deserts.
• ☐Scattering gives blue sky and red sunrise/sunset.
• ☐Heating: conduction, convection (troposphere only), advection (loo), terrestrial radiation (heated from below).
• ☐Heat budget: 35 reflected (albedo), 14 + 51 absorbed; 17 + 48 = 65 returned.
• ☐Surplus 40°N-40°S, deficit at poles; heat moves poleward.
• ☐Temperature factors: latitude, altitude (6.5°C/1,000 m), sea, air masses and currents, local aspects.
• ☐Range: over 60°C NE Eurasia; 3°C between 20°S and 15°N. Inversion: clear calm winter nights, polar areas, air drainage.

Solar Radiation, Heat Balance and Temperature: frequently asked questions