GeographyCBSE Class 11 · Solar Radiation, Heat Balance and Temperature

Insolation, Heating of the Atmosphere and the Heat Budget (Class 11 Geography, Fundamentals of Physical Geography, Chapter 8)

Updated 17 October 2026 · Fact-checked

The earth receives energy as short-wave insolation, 1.94 calories per sq cm per minute at the top of the atmosphere. Insolation varies with angle of rays, day length and atmosphere. The atmosphere is heated indirectly by conduction, convection, advection and terrestrial radiation. Of 100 units, 35 are reflected (albedo), 14 absorbed by the atmosphere and 51 by the surface, and the budget balances at 65.

Understand Insolation, Heating of the Atmosphere and the Heat Budget of the Earth

Why this topic matters. The heat budget figures (35, 14, 51, 17, 34, 48) are tested directly.

1. Insolation. The earth receives most energy in short wavelengths. Average at the top of the atmosphere is 1.94 calories per sq cm per minute. The earth is at aphelion on 4 July (152 million km) and at perihelion on 3 January (147 million km), so it receives slightly more on 3 January, but this is masked by land-sea distribution and atmospheric circulation.

2. Variability. Insolation varies with rotation, angle of the sun's rays, day length, transparency of the atmosphere and aspect. The axis makes 66.5 degrees with the orbital plane. Higher latitudes get slant rays which spread over more area and pass through more atmosphere (more absorption, scattering and diffusion). At the surface insolation ranges from about 320 W/m2 in the tropics to 70 W/m2 at the poles, highest over subtropical deserts where cloudiness is least.

3. Passage through the atmosphere. The atmosphere is largely transparent to short-wave radiation. Scattering of visible light gives the blue sky and the red rising and setting sun.

4. Heating of the atmosphere. Conduction: heat moves between bodies in contact; heats the lower layers. Convection: vertical rise of heated air, confined to the troposphere. Advection: horizontal movement of air, more important than vertical movement, e.g. the *loo* of northern India in summer. Terrestrial radiation: long-wave energy from the heated earth absorbed by carbon dioxide and other greenhouse gases. So the atmosphere is heated indirectly from below. Planck's law: the hotter a body, the shorter the wavelength it radiates.

5. Heat budget. Insolation at the top = 100 units. 35 units are reflected (the albedo: 27 from cloud tops, 2 from snow and ice and the remainder from other reflection and scattering). 65 are absorbed: 14 by the atmosphere and 51 by the surface. The surface radiates 51 units: 17 directly to space and 34 absorbed by the atmosphere (6 directly, 9 by convection and turbulence, 19 as latent heat of condensation). The atmosphere radiates 48 (14 + 34) to space. Returned: 17 + 48 = 65 = absorbed.

6. Latitudinal variation. There is a net surplus between 40 degrees N and 40 degrees S and a deficit near the poles; heat is redistributed poleward.

Insolation, Heating of the Atmosphere and the Heat Budget of the Earth: key facts

Earth-sun distance: Aphelion: 4 July, 152 million km | Perihelion: 3 January, 147 million km; More insolation on 3 January.
Insolation at the surface: Average at top: 1.94 cal per sq cm per minute
About 320 W per sq m in tropics
About 70 W per sq m at poles
Maximum over subtropical deserts; Numbers.
Four ways of heating: Conduction contact
Convection vertical, troposphere
Advection horizontal, loo
Terrestrial radiation long-wave; Processes.
Heat budget (100 units): $R e f l ec t e d 35∣ A b sor b e d b y a t m os p h er e 14∣ A b sor b e d b y s u r f a ce 51∣ S u r f a cer a d ia t es 17 t os p a ce an d 34 t o a t m os p h er e ∣ A t m os p h er er e t u r n s 48∣17 + 48 = 65$; Balance.

Conduction	contact
Convection	vertical, troposphere
Advection	horizontal, loo
Terrestrial radiation	long-wave

How to answer Insolation, Heating of the Atmosphere and the Heat Budget of the Earth questions

Heat budget questions are sums with names.

1Start with 100 units at the top.
2Subtract albedo 35 to get 65.
3Split 65 into 14 and 51.
4Show the surface giving back 17 and 34.
5Show 17 + 48 = 65.

Insolation, Heating of the Atmosphere and the Heat Budget of the Earth in real life

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

Midday versus evening sun

Suppose a torch beam held straight down makes a bright small circle, while the same beam tilted makes a larger dimmer oval.

Which aspect of insolation does this show?

Try it first, then show the solutionSolution

Vertical rays cover less area.
Slant rays spread energy over more area.

Answer: It shows why higher latitudes receive less insolation per unit area.

Memory hook: Slant spreads energy.

Bank balance

Suppose an account receives 100 rupees a month and spends 35 on one item, 14 on another and the rest, 51, on a third, and ends up returning exactly what it holds.

Which concept is like this balance?

Try it first, then show the solutionSolution

Heat budget: 35 reflected, 14 + 51 absorbed.
Total returned equals the 65 absorbed.

Answer: It resembles the heat budget of the earth with its units of insolation.

Memory hook: Income equals outflow.

Tricks to remember Insolation, Heating of the Atmosphere and the Heat Budget of the Earth

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

35-14-51
Reflected, atmosphere, surface
Use it when: Budget split.
17 + 48 = 65
Returned to space equals absorbed
Use it when: Balance.
4 July far, 3 January near
Aphelion and perihelion
Use it when: Distances.

Insolation, Heating of the Atmosphere and the Heat Budget of the Earth: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Albedo · 1 mark · MCQBoard level

The 35 units of insolation reflected to space before reaching the surface are called:
(A) latent heat
(B) albedo
(C) lapse rate
(D) advection

Try it first, then show the solutionSolution

Reflected amount is the albedo (1 mark).

Answer: (B)

Example 2: Aphelion · 2 marks · Very short answerBoard level

On what date is the earth at aphelion, and how far is it from the sun? (2 marks)

Try it first, then show the solutionSolution

4 July (1 mark).
152 million km (1 mark).

Answer: On 4 July, at 152 million km.

Example 3: Heat budget · 3 marks · Short answerHarder board level

Show that the earth neither warms nor cools, using the heat budget. (3 marks)

Try it first, then show the solutionSolution

65 units absorbed: 14 by atmosphere, 51 by surface (1 mark).
Surface returns 17 directly; atmosphere radiates 48 (14 + 34) (1 mark).
17 + 48 = 65, equal to absorbed (1 mark).

Answer: Of 100 units, 35 are reflected, so 65 are absorbed: 14 by the atmosphere and 51 by the surface. The surface sends 17 directly to space and 34 to the atmosphere, which with its own 14 radiates 48 to space. The returned 17 + 48 = 65 equals the 65 absorbed, so the earth neither gains nor loses heat.

Example 4: Assertion and reason: advection · 1 mark · Assertion-reasonHarder board level

Assertion (A): Advection is more important than convection in middle latitudes for day-to-day weather.
Reason (R): Convection is confined to the troposphere only.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true

Try it first, then show the solutionSolution

Advection is more important than vertical movement; the reason about convection is true but is not why (1 mark).

Answer: (B)

Example 5: HOTS: surplus and deficit · 5 marks · Long answerHOTS / competency

The tropics have a surplus and the poles a deficit of radiation, yet the tropics do not keep getting hotter. Explain. (5 marks)

Try it first, then show the solutionSolution

Net radiation surplus between 40 N and 40 S (1 mark).
Deficit near the poles (1 mark).
Surplus is redistributed poleward (1 mark).
By winds and ocean currents (1 mark).
So tropics do not heat endlessly and poles do not freeze permanently (1 mark).

Answer: There is a net radiation surplus between 40 degrees N and S and a deficit near the poles. The surplus heat is redistributed poleward by atmospheric circulation (winds) and ocean currents, which keep the tropics from heating up continuously and the poles from freezing permanently. (The role of ocean currents here comes from the chapter's later treatment of temperature.)

Where students lose marks in Insolation, Heating of the Atmosphere and the Heat Budget of the Earth

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

Mistake: Saying the earth is hottest at aphelion
Distance intuition.
Wrong working
Aphelion in July is when it is closest.
Right working
Aphelion is farthest (4 July); perihelion nearest (3 January).
Do this: Fix dates.
Usually costs: 1 mark.
Mistake: Budget totals not balanced
Units confused.
Wrong working
51 + 14 + 35 = 100 so 51 returned.
Right working
Returned 17 + 48 = 65.
Do this: Show the sum.
Usually costs: 1 mark.
Mistake: Calling the atmosphere directly heated by the sun
Sun looks direct.
Wrong working
Atmosphere is heated mainly by the sun directly.
Right working
Heated indirectly from below.
Do this: Say "indirectly".
Usually costs: 1 mark.

Exam tips

Show the heat budget as a small table with numbers.
Quote 4 July and 3 January with distances.
Use correct units: W per sq m, calories per sq cm per minute.

Practice questions on Insolation, Heating of the Atmosphere and the Heat Budget of the Earth

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

Q1 · 1 mark · MCQBoard level
The loo of northern India is an example of:
(A) conduction
(B) advection
(C) terrestrial radiation
(D) latent heat
Hint
Horizontal air movement.
Answer
(B)
Q2 · 2 marks · Very short answerBoard level
Of 100 units of insolation, find the percentage reaching the surface as absorbed radiation, and the units absorbed by the atmosphere. (2 marks)
Hint
51 and 14.
Answer
51 per cent is absorbed by the surface; 14 units by the atmosphere.
Q3 · 3 marks · Short answerHarder board level
Why does the atmosphere get heated from below? (3 marks)
Hint
Short-wave through, long-wave absorbed.
Answer
The atmosphere is largely transparent to short-wave insolation, which heats the surface. The surface then radiates long-wave energy, which greenhouse gases absorb, and heat also passes to air by conduction, convection and advection.
Q4 · 3 marks · Short answerHarder board level
List four factors by which insolation varies, and explain why higher latitudes receive less. (3 marks)
Hint
Angle and path.
Answer
Insolation varies with rotation, the angle of the sun's rays, the length of day and the transparency of the atmosphere. At higher latitudes rays are slant, spreading energy over a larger area and passing through a greater depth of atmosphere with more absorption, scattering and diffusion.
Q5 · 4 marks · Short answerHOTS / competency
Why is the equator receiving less insolation than the tropics even though the sun is overhead? (4 marks)
Hint
Cloud.
Answer
Insolation at the surface is highest over subtropical deserts, where cloudiness is least. The equator has more cloud, which reflects and absorbs insolation, so it receives less at the surface than the tropics.

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.What is the average insolation at the top of the atmosphere?Show answer
1.94 calories per sq cm per minute.
2.What does Planck's law say?Show answer
The hotter a body, the more it radiates, with shorter wavelength.
3.What gives the blue sky?Show answer
Scattering of visible light by tiny particles.
4.Where is insolation highest?Show answer
Over subtropical deserts.

Insolation, Heating of the Atmosphere and the Heat Budget of the Earth: frequently asked questions

What is specific heat?

Energy needed to raise one gram of a substance by one degree Celsius.

What is terrestrial radiation?

Long-wave energy radiated by the heated earth.

Is the 35 units only cloud reflection?

27 from cloud tops and 2 from snow and ice are named; the rest is other reflection and scattering.