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Solar Radiation, Heat Balance and Temperature – Class 11 Geography notes, important questions and MCQs

NCERT Class 11 Geography, chapter 8. Notes, key terms, important questions with answers and MCQs.

Chapter overview

The Earth receives energy from the Sun as short-wave radiation called insolation. The amount of insolation varies with latitude, season and time of day. The atmosphere is heated mainly by terrestrial (long-wave) radiation through conduction, convection and advection. Over a year the Earth maintains a heat balance between incoming and outgoing energy. Temperature distribution depends on latitude, altitude, distance from the sea, ocean currents and air masses.

Insolation and heating of the atmosphere

Insolation is maximum near the tropics and decreases towards the poles because of the angle of the Sun's rays; it also depends on the length of the day and the transparency of the atmosphere.
The Earth's surface absorbs insolation and radiates long-wave terrestrial radiation, which heats the atmosphere from below; this is why temperature falls with height in the troposphere.
Heat is transferred by conduction, convection (vertical transfer) and advection (horizontal transfer, such as the loo winds in north India).

Heat budget

Out of 100 units of insolation, about 35 units are reflected back to space (the albedo), and 65 units are absorbed by the atmosphere (14) and the Earth's surface (51).
The Earth radiates the absorbed energy back as terrestrial radiation, so the total heat of the Earth remains constant over time.
Surplus heat in tropical regions is transferred to higher latitudes by winds and ocean currents.

Temperature distribution

Factors include latitude, altitude, distance from the sea, air masses, ocean currents and local aspects.
Isotherms are lines joining places of equal temperature; they bend sharply where land and sea meet.
Temperature inversion occurs when temperature increases with height, usually on long, clear, calm winter nights; it causes fog and frost and traps pollutants.

Key terms

Insolation: incoming solar radiation received by the Earth
Albedo: the proportion of radiation reflected by a surface
Terrestrial radiation: long-wave radiation emitted by the Earth
Advection: horizontal transfer of heat by moving air
Temperature inversion: a condition in which temperature increases with height

Important questions with answers

What is temperature inversion?2 marks

Temperature inversion is a condition in which temperature increases with height instead of decreasing. It occurs on long, clear, calm winter nights when the ground loses heat quickly and cools the air close to it, while the air above remains warmer. It causes fog and frost.

Why does insolation vary over the Earth's surface?3 marks

Insolation varies because of the angle of the Sun's rays, which are more direct near the tropics and slanting near the poles; the length of the day, which changes with latitude and season; the transparency of the atmosphere, affected by clouds and dust; and the configuration of land.

Explain the heat budget of the Earth.5 marks

The Earth neither warms up nor cools down over a long period, because it maintains a balance between the energy it receives from the Sun and the energy it loses. If the insolation reaching the top of the atmosphere is taken as 100 units, about 35 units are reflected back to space before reaching the surface: 27 units from clouds, 6 units scattered by the atmosphere and 2 units reflected from snow and ice surfaces. This reflected part is the Earth's albedo. The remaining 65 units are absorbed: 14 units by the atmosphere and 51 units by the Earth's surface. The Earth then radiates this energy back as long-wave terrestrial radiation. Of the 51 units, 17 units go directly to space and 34 units are absorbed by the atmosphere through radiation, convection, turbulence and the release of latent heat. The atmosphere, having absorbed 48 units in total, radiates them back to space. Thus 65 units received are balanced by 65 units lost (17 + 48), keeping the Earth's average temperature constant. However, there is surplus heat between about 40°N and 40°S and a deficit near the poles; this imbalance is corrected by the transfer of heat poleward by winds and ocean currents.

Quick revision

Insolation varies with latitude, season and day length.
Atmosphere heated from below by terrestrial radiation.
Conduction, convection, advection.
Heat budget; isotherms; inversion.

Exam tips

Remember the heat budget: 35 reflected, 14 absorbed by the atmosphere, 51 by the surface.
List six factors controlling temperature distribution.

Practice MCQs

Incoming solar radiation is called
1. albedo
2. advection
3. terrestrial radiation
4. insolation
Show answer
Answer: (d) insolation. It is short-wave radiation.
Lines joining places of equal temperature are
1. isotherms
2. isobars
3. isohyets
4. contours
Show answer
Answer: (a) isotherms. They show temperature distribution.
The reflectivity of a surface is called its
1. albedo
2. insolation
3. conduction
4. lapse rate
Show answer
Answer: (a) albedo. Snow has high albedo.
Horizontal transfer of heat by moving air is
1. advection
2. convection
3. conduction
4. radiation
Show answer
Answer: (a) advection. The loo of north India is an example.
The atmosphere is mainly heated by
1. incoming short-wave radiation directly
2. terrestrial long-wave radiation from the Earth
3. the Moon
4. ocean tides
Show answer
Answer: (b) terrestrial long-wave radiation from the Earth. This is why it is heated from below.
About how many units out of 100 of insolation are reflected back to space
1. 14
2. 35
3. 51
4. 65
Show answer
Answer: (b) 35. This is the Earth's albedo.
Temperature inversion is most likely on
1. long clear calm winter nights
2. cloudy windy summer days
3. rainy afternoons
4. stormy seas
Show answer
Answer: (a) long clear calm winter nights. The ground loses heat quickly.
Why do isotherms bend sharply along coasts
1. the Moon pulls them
2. coasts have no temperature
3. isotherms follow rivers
4. land and sea heat and cool at different rates
Show answer
Answer: (d) land and sea heat and cool at different rates. Land heats and cools faster than water.

All chapters of Class 11 Geography

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.