Chapter 8 — Solar Radiation, Heat Balance and Temperature
Open PDFReads in your browser→Summary
NCERT Class 11 Geography Chapter 8 Solar Radiation, Heat Balance and Temperature explains how the earth receives energy from the sun as insolation, how the atmosphere is heated through conduction, convection, advection and terrestrial radiation, and how the earth maintains a heat budget by returning exactly as much energy to space as it receives.
The chapter covers insolation — the incoming short-wave solar radiation the earth receives, averaging 1.94 calories per sq. cm per minute at the top of the atmosphere. It explains how factors such as latitude, angle of sun's rays, day length and atmospheric transparency vary the insolation reaching the surface (320 Watt/m² in tropics to 70 Watt/m² at poles). The atmosphere is heated by conduction, convection, advection and long-wave terrestrial radiation. The heat budget shows that of every 100 units received, 35 are reflected (albedo) and 65 are absorbed, with the earth ultimately radiating all 65 units back to space, maintaining a stable temperature. The chapter also discusses factors controlling temperature distribution — latitude, altitude, distance from sea, ocean currents — and the phenomenon of temperature inversion.
Key points & formulas
- 01Insolation is the incoming solar radiation received by the earth in short wavelengths; the earth receives 1.94 calories per sq. cm per minute at the top of the atmosphere.
- 02Earth is at aphelion (farthest, 152 million km) on 4th July and at perihelion (nearest, 147 million km) on 3rd January.
- 03Factors affecting insolation received: rotation of the earth, angle of inclination of sun's rays, length of day, transparency of the atmosphere, and configuration of land.
- 04The atmosphere is heated by conduction (contact with heated land), convection (vertical air currents in troposphere), advection (horizontal air movement), and terrestrial radiation (long-wave emission from the earth's surface).
- 05Heat budget: of 100 units of insolation, 35 are reflected back to space (albedo), 51 units are absorbed by the earth's surface, and the remaining 14 by the atmosphere; the earth radiates 51 units back, of which 17 go directly to space and 34 are absorbed by the atmosphere; the atmosphere in turn radiates 48 units to space — total returned = 65 units, balancing the 65 units absorbed.
- 06Surplus radiation balance exists between 40° N and 40° S; regions near the poles have a deficit; surplus heat is redistributed polewards by atmospheric and oceanic circulation.
- 07Normal lapse rate is 6.5°C per 1,000 m; temperature generally decreases with increasing altitude.
- 08Temperature inversion (the reversal of the normal lapse rate) is common on long winter nights with clear skies and still air; air drainage in hills causes cold air to collect in valley bottoms, protecting plants from frost.
Frequently asked questions
01What is insolation in NCERT Class 11 Geography Chapter 8?
Insolation is the incoming solar radiation received by the earth in short wavelengths. On average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.
02What is the difference between aphelion and perihelion?
Aphelion is when the earth is farthest from the sun (152 million km) on 4th July, and perihelion is when the earth is nearest to the sun (147 million km) on 3rd January. Insolation is slightly more on 3rd January as a result.
03What are the factors that control the amount of insolation received at the earth's surface?
The five factors are: (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, and (v) configuration of land in terms of its aspect.
04How is the atmosphere heated according to Chapter 8?
The atmosphere is heated by four processes: conduction (heat transfer by contact between the land and the air above), convection (vertical movement of heated air currents within the troposphere), advection (horizontal movement of air transferring heat), and terrestrial radiation (long-wave radiation emitted by the earth's surface that is absorbed by atmospheric gases like CO₂).
05What is the heat budget of the earth?
Of 100 units of insolation, 35 are reflected back to space (albedo) and 65 are absorbed. The earth absorbs 51 units and the atmosphere 14 units. The earth radiates 51 units back — 17 directly to space and 34 absorbed by the atmosphere. The atmosphere then radiates 48 units to space. Total returned = 17 + 48 = 65 units, balancing the 65 absorbed. This is why the earth neither warms up nor cools down.
06What is albedo?
Albedo is the reflected amount of radiation. Of 100 units of insolation, 35 units are reflected back to space before reaching the earth's surface — 27 units from the tops of clouds and 2 units from snow and ice-covered areas.
07What is the normal lapse rate?
The normal lapse rate is the rate at which temperature decreases with increasing height. It is 6.5°C per 1,000 m.
08What is temperature inversion?
Temperature inversion is when the normal lapse rate is reversed — temperature increases with altitude instead of decreasing. It is usually of short duration and commonly occurs on long winter nights with clear skies and still air. Over polar areas, temperature inversion is normal throughout the year.
09What are the factors that control temperature distribution on the earth's surface?
The five main factors are: (i) latitude, (ii) altitude, (iii) distance from the sea, (iv) air-mass circulation and the presence of warm and cold ocean currents, and (v) local aspects.
10What are isotherms?
Isotherms are lines on a map joining places that have equal temperature. They are used to show the global distribution of surface air temperature.
11Why do subtropical deserts receive the maximum insolation?
Subtropical deserts receive maximum insolation because cloudiness is least there. The equator receives comparatively less insolation than the tropics because of greater cloud cover.
12What is air drainage and why is it significant?
Air drainage is the flow of cold, dense air downslope under gravity in hills and mountains during the night, collecting in valley bottoms. It creates temperature inversion in valleys and protects plants from frost damage.
13Is the NCERT Class 11 Geography Chapter 8 PDF free to download?
Yes, it is free to download with no sign-up.
More chapters in Fundamentals of Physical Geography
This is the complete Fundamentals of Physical Geography Chapter 8 as published by NCERT — every diagram, solved example, and exercise included, free. Browse all NCERT Class 11 textbooks.
Read offline with notes, solutions & mock tests
CBSE Prepmaster — free on iOS & Android