Solar Radiation, Heat Balance and Temperature – Class 11 Geography Chapter 8 Notes & NCERT PDF

Class 11GeographyChapter 8NCERT book: Fundamentals of Physical Geography - Geography Class 11

Notes and a simple summary of Chapter 8 of the NCERT Class 11 Geography book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 8: Solar Radiation, Heat Balance and Temperature

Introduction

We live at the bottom of a huge pile of air. The earth receives almost all of its energy from the sun, and it radiates that energy back to space. Because the energy received and the energy lost balance out, the earth as a whole neither warms up nor cools down over time. But different parts of the earth receive different amounts of heat. These differences create pressure differences, which set the air moving as winds that carry heat from one region to another. This chapter explains how the atmosphere is heated and cooled and how temperature is distributed over the earth.

Key Concepts

1. Solar Radiation (Insolation)

The earth's surface receives most of its energy in short wavelengths.

The energy received by the earth is called incoming solar radiation, or insolation for short.

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 portion of the sun's energy.

On average, the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.

Aphelion and perihelion

On 4th July the earth is farthest from the sun (152 million km) — this is aphelion.

On 3rd January it is nearest to the sun (147 million km) — this is perihelion.

So annual insolation on 3rd January is slightly more than on 4th July. But this variation is masked by the distribution of land and sea and by atmospheric circulation, so it has little effect on daily weather.

2. Variability of Insolation at the Surface

Insolation varies during a day, a season and a year. The book lists five factors:

(i) the rotation of the earth on its axis;

(ii) the angle of inclination of the sun's rays;

(iii) the length of the day;

(iv) the transparency of the atmosphere;

(v) the configuration of land in terms of its aspect.

The last two have less influence. The earth's axis makes an angle of 66½° with the plane of its orbit, which strongly affects how much insolation different latitudes receive.

The higher the latitude, the smaller the angle the rays make with the surface — the rays become slant. Slant rays spread the same energy over a larger area and pass through a greater depth of atmosphere, so more energy is lost to absorption, scattering and diffusion.

3. Passage of Solar Radiation through the Atmosphere

The atmosphere is largely transparent to short-wave solar radiation.

In the troposphere, water vapour, ozone and other gases absorb much of the near-infrared radiation.

Very small suspended particles scatter the visible spectrum. This gives the blue colour of the sky and the red colour of the rising and setting sun.

4. Spatial Distribution of Insolation

Insolation at the surface varies from about 320 W/m² in the tropics to about 70 W/m² at the poles.

Maximum insolation is received over the subtropical deserts, where cloudiness is least.

The equator receives comparatively less than the tropics.

At the same latitude, insolation is more over continents than over oceans.

In winter, middle and higher latitudes receive less radiation than in summer.

5. Heating and Cooling of the Atmosphere

Conduction — the air in contact with the heated land gets heated slowly; heat flows from the warmer to the cooler body until both are at the same temperature. Important for heating the lower layers.

Convection — heated air rises vertically in currents and carries heat upward. It is confined to the troposphere.

Advection — transfer of heat by horizontal movement of air. It is more important than vertical movement; in middle latitudes most day-and-night weather variation is caused by advection. The 'loo' of northern India in summer is an outcome of advection.

6. Terrestrial Radiation

The heated earth becomes a radiating body and gives off energy in long-wave form. This heats the atmosphere from below. Long-wave radiation is absorbed mainly by carbon dioxide and other greenhouse gases, so the atmosphere is indirectly heated by the earth. The atmosphere in turn radiates heat to space, keeping temperatures constant overall.

7. Heat Budget of the Planet Earth

Take insolation at the top of the atmosphere as 100 units:

35 units are reflected back to space before reaching the surface — 27 from the tops of clouds and 2 from snow- and ice-covered areas (the remaining 6 are scattered back by the atmosphere, as shown in the book's figure). This reflected amount is the albedo of the earth.

65 units are absorbed: 14 within the atmosphere and 51 by the earth's surface.

The earth radiates back 51 units: 17 go directly to space and 34 are absorbed by the atmosphere (6 directly, 9 through convection and turbulence, 19 through latent heat of condensation).

The atmosphere radiates 48 units (14 + 34) back to space.

Total returned: 17 + 48 = 65 units, balancing the 65 units received. This is the heat budget or heat balance.

8. Variation in the Net Heat Budget

There is a surplus of net radiation between 40° N and 40° S, and a deficit near the poles. The surplus heat from the tropics is redistributed poleward, so the tropics do not keep heating up and the high latitudes do not freeze permanently.

9. Temperature and Its Controls

Heat is the molecular movement of particles; temperature is the measurement in degrees of how hot or cold something is. Factors controlling temperature:

Latitude — insolation varies with latitude, so temperature does too.

Altitude — the atmosphere is heated from below, so temperature generally falls with height. The normal lapse rate is 6.5°C per 1,000 m.

Distance from the sea — land heats and cools quickly, the sea slowly; coastal places are moderated by land and sea breezes.

Air-masses and ocean currents — warm air-masses and warm currents raise temperature; cold ones lower it.

Local aspects.

10. Distribution of Temperature

Isotherms are lines joining places of equal temperature. They are generally parallel to latitudes.

Deviation is greater in January than July, especially in the northern hemisphere, which has more land.

In January the isotherms bend north over the ocean (the warm Gulf Stream and North Atlantic Drift warm the North Atlantic) and south over the continent (Europe, Siberian plain).

The mean January temperature along 60° E is minus 20°C at both 80° N and 50° N.

January means: over 27°C in equatorial oceans, over 24°C in the tropics, 2°C–0°C in middle latitudes and –18°C to –48°C in the Eurasian interior.

In the southern hemisphere the isotherms of 20°C, 10°C and 0°C run parallel to 35° S, 45° S and 60° S.

In July, equatorial oceans exceed 27°C; over 30°C is noticed in subtropical Asia along 30° N.

The highest annual range (over 60°C) is over north-eastern Eurasia — due to continentality. The least range, 3°C, lies between 20° S and 15° N.

11. Inversion of Temperature

Normally temperature falls with height (normal lapse rate). When this is reversed, it is inversion of temperature.

Ideal conditions: a long winter night with clear skies and still air.

Over polar areas, inversion is normal throughout the year.

Surface inversion promotes stability; smoke and dust collect beneath the inversion layer, and dense morning fogs are common in winter.

In hills and mountains, cold dense air flows downslope at night and piles up in valleys — air drainage. It protects plants from frost damage.

Planck's law: the hotter a body, the more energy it radiates and the shorter the wavelength of that radiation. Specific heat is the energy needed to raise the temperature of one gram of a substance by one Celsius.

Worked Example (from the Project Work)

New Delhi (Safdarjung): January mean daily max 21.1°C, min 7.3°C → daily mean = (21.1 + 7.3) ÷ 2 = 14.2°C. May: (39.6 + 25.9) ÷ 2 = 32.75°C. Annual range as calculated in the book = 32.75 − 14.2 = 18.55°C.

Diagrams to Know

Summer solstice — the sun's rays vertical over the Tropic of Cancer (23.5° N) on 21 June, slanting towards the poles.

Heat budget — boxes/arrows showing 100 units in, 35 reflected, 14 + 51 absorbed, 17 + 48 radiated out.

Latitudinal net radiation — curve above zero between 40° N and 40° S, below zero towards the poles.

Key-Facts Sheet

Insolation at top of atmosphere: 1.94 cal/cm²/min.

Aphelion 4 July (152 million km); perihelion 3 January (147 million km).

Surface insolation: ~320 W/m² tropics, ~70 W/m² poles.

Albedo = 35 units; heat balance 65 in = 65 out.

Normal lapse rate = 6.5°C per 1,000 m.

Highest annual range > 60°C (NE Eurasia); least 3°C (20° S–15° N).

Exam Focus

MCQ from the exercise: the sun is overhead at noon on 21 June at 23.5° N; the atmosphere is mainly heated by long-wave terrestrial radiation; subtropics are hotter than the equator mainly because they have less cloud cover.

30-word answer — Why is India hottest in May, not after the solstice? Outline: by May the sun is nearly overhead and skies are clear; after June the monsoon clouds and rain cut insolation and cool the land.

150-word answer — How does the earth maintain heat balance? Outline: 100 units in → 35 reflected (albedo) → 65 absorbed (14 atmosphere, 51 surface) → 51 re-radiated (17 to space, 34 to atmosphere) → atmosphere radiates 48 → 17 + 48 = 65 out.

Siberian annual range — continentality: far from the moderating sea.

Key Terms

Insolation (सूर्यातप)

Aphelion (अपसौर)

Perihelion (उपसौर)

Albedo (एल्बिडो / परावर्तकता)

Conduction (चालन)

Convection (संवहन)

Advection (अभिवहन)

Terrestrial radiation (पार्थिव विकिरण)

Heat budget (ऊष्मा बजट)

Isotherm (समताप रेखा)

Normal lapse rate (सामान्य ह्रास दर)

Inversion of temperature (तापमान का व्युत्क्रमण)

Air drainage (वायु अपवाह)

Continentality (महाद्वीपीयता)

Common Mistakes

Thinking the atmosphere is heated directly by the sun — it is heated mainly from below by long-wave terrestrial radiation.

Assuming the equator gets the most insolation — the subtropical deserts do, because of clear skies.

Mixing up aphelion (farthest, July) and perihelion (nearest, January).

Confusing convection (vertical) with advection (horizontal).

Forgetting that inversion needs clear, calm, long nights.

💡 Key Learning Points

✓Insolation varies with latitude, angle of rays, day length, transparency of air and aspect.
✓The atmosphere is heated mainly from below by long-wave terrestrial radiation through conduction, convection and advection.
✓The heat budget balances: of 100 units, 35 are reflected and 65 absorbed; 17 + 48 = 65 units return to space.
✓Temperature is controlled by latitude, altitude, distance from the sea, air-masses, ocean currents and local aspects.
✓Inversion of temperature occurs on long, clear, calm winter nights and through air drainage in hilly areas.

👨‍🏫 Teaching Tips

→Use a torch shone vertically and at a slant on graph paper to show how slant rays spread energy over a larger area.
→Draw the heat budget as a 100-unit flow chart on the board and let students check that inputs equal outputs.
→Compare morning and afternoon temperatures at school to discuss conduction and convection.
→Use a world map of January isotherms to trace how they bend over the North Atlantic and Siberia.
→Do the book's project: calculate daily mean monthly temperature and annual range for the nearest observatory.
→Link 'loo' in north India to advection and ask students from other regions for local wind examples.
→Show photos of winter morning fog and smog to explain temperature inversion and air drainage.

📋 Assessment Questions

Define insolation and state two reasons why it varies at the earth's surface.
Explain the heat budget of the earth using the units given in the book.
Why is the annual range of temperature highest over north-eastern Eurasia?
Distinguish between conduction, convection and advection with one example each.
What conditions favour temperature inversion, and what are its effects?

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