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Solar Radiation, Heat Balance and Temperature

NCERT Class 11 · Geography Based on NCERT Class 11 Geography textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explains how the Earth receives energy from the Sun, how that energy is distributed, and how the Earth-atmosphere system maintains a heat balance. It covers the processes of insolation, heating and cooling of the atmosphere, and the factors controlling temperature distribution. The one big idea a student must walk away with is that the Earth neither warms up nor cools down over time because the amount of heat received equals the amount lost, a concept known as the heat budget.
Key Concepts (14)
Insolation
The incoming solar radiation received by the Earth's surface in short wavelengths.
e.g. On a sunny day in Delhi, the energy from the sun that reaches the ground is insolation.
Aphelion
The position of the Earth when it is farthest from the Sun, at 152 million km, occurring on 4th July.
e.g. During aphelion, the Earth receives slightly less insolation than at perihelion.
Perihelion
The position of the Earth when it is nearest to the Sun, at 147 million km, occurring on 3rd January.
e.g. During perihelion, the Earth receives slightly more insolation than at aphelion.
Conduction
The process of heat transfer when two bodies of unequal temperature are in contact, with energy flowing from the warmer to the cooler body until both attain the same temperature or contact is broken.
e.g. When you touch a hot iron rod, heat transfers to your hand by conduction.
Convection
The process of vertical heating of the atmosphere where air in contact with the Earth rises vertically in the form of currents, transmitting heat upwards.
e.g. Hot air rising from a heated road in summer forms convection currents.
Advection
The transfer of heat through the horizontal movement of air.
e.g. In northern India during summer, the local wind called 'loo' is an outcome of advection.
Terrestrial Radiation
The process by which the Earth, after being heated by insolation, radiates energy back to the atmosphere in long wave form.
e.g. At night, the ground cools by emitting terrestrial radiation, which warms the air above.
Albedo
The percentage of incoming solar radiation that is reflected back to space by the Earth's surface and atmosphere.
e.g. Fresh snow has a high albedo, reflecting most sunlight, while dark soil has a low albedo.
Heat Budget
The balance between the amount of insolation received by the Earth and the amount of terrestrial radiation returned to space, ensuring the Earth neither warms up nor cools down.
e.g. The Earth receives 65 units of energy and returns 65 units, maintaining a heat balance.
Normal Lapse Rate
The rate at which temperature decreases with increasing height, which is 6.5°C per 1,000 m.
e.g. If you climb a mountain 1,000 m high, the temperature will drop by about 6.5°C.
Isotherm
A line on a map joining places having equal temperature.
e.g. On a January temperature map, the isotherm of 20°C runs parallel to 35°S latitude.
Inversion of Temperature
A reversal of the normal lapse rate where temperature increases with height instead of decreasing.
e.g. On a clear winter morning, cold air near the ground with warmer air above is an inversion.
Air Drainage
The flow of cold, dense air under gravity down slopes to pile up in valleys and pockets, with warm air above.
e.g. In hill stations like Shimla, cold air drains into valleys at night, protecting plants from frost.
Summer Solstice
The position of the Earth on 21st June when the sun is directly overhead at 23.5°N (Tropic of Cancer), resulting in the longest day in the Northern Hemisphere.
e.g. On 21st June, Delhi experiences the longest day of the year.
How It Works
Heating of the Atmosphere by Conduction
  1. The Earth's surface is heated by insolation.
  2. The air in contact with the land gets heated slowly by conduction.
  3. Upper layers in contact with lower layers also get heated.
  4. Heat transfer continues until both bodies attain the same temperature or contact is broken.
💡 Conduction is important only for heating the lower layers of the atmosphere, not the entire atmosphere.
Heating of the Atmosphere by Convection
  1. Air in contact with the Earth's surface gets heated.
  2. Heated air rises vertically in the form of currents.
  3. The heat is transmitted to higher layers of the atmosphere.
  4. This vertical transfer is confined only to the troposphere.
💡 Convection is a vertical process and is limited to the troposphere; it does not occur in the stratosphere.
Heating of the Atmosphere by Advection
  1. Air moves horizontally from one region to another.
  2. Heat is transferred through this horizontal movement.
  3. In middle latitudes, most daily weather variations are caused by advection.
  4. In tropical regions like northern India, local winds such as 'loo' result from advection.
💡 Advection is horizontal movement, while convection is vertical; students often confuse the two.
Terrestrial Radiation and Heating of the Atmosphere
  1. The Earth receives insolation in short wave form and heats up.
  2. The heated Earth radiates energy back to the atmosphere in long wave form.
  3. This long wave radiation is absorbed by atmospheric gases, especially carbon dioxide and other greenhouse gases.
  4. The atmosphere is thus indirectly heated from below by the Earth's radiation.
💡 The atmosphere is heated from below, not from above; this is why temperature decreases with height normally.
Heat Budget of the Earth
  1. The top of the atmosphere receives 100 units of insolation.
  2. 35 units are reflected back to space (albedo): 27 from clouds, 6 from atmosphere, 2 from snow/ice areas.
  3. 65 units are absorbed: 14 within the atmosphere and 51 by the Earth's surface.
  4. The Earth radiates back 51 units as terrestrial radiation.
  5. Of these 51 units, 17 go directly to space, and 34 are absorbed by the atmosphere.
  6. The atmosphere absorbs a total of 48 units (14 from insolation + 34 from terrestrial radiation).
  7. These 48 units are radiated back into space.
  8. Total radiation returned to space = 17 (from Earth) + 48 (from atmosphere) = 65 units, balancing the 65 units received.
💡 The heat budget explains why the Earth neither warms up nor cools down; the numbers 65 and 100 are frequently tested.
Inversion of Temperature
  1. Normally, temperature decreases with height (normal lapse rate).
  2. During a long winter night with clear skies and still air, the Earth's surface cools rapidly by radiation.
  3. By early morning, the Earth is cooler than the air above.
  4. This creates a situation where temperature increases with height, reversing the normal lapse rate.
  5. In polar areas, inversion is normal throughout the year.
  6. In hills and mountains, cold air drains down slopes under gravity (air drainage), piling up in valleys with warm air above.
💡 Inversion is temporary and usually lasts a few hours until the sun warms the Earth; it promotes stability and fog formation.
Exam Traps — Don't Make These Mistakes
⚠ Confusing Aphelion and Perihelion Dates
✗ Aphelion occurs on 3rd January and Perihelion on 4th July.
→ The dates are reversed; aphelion (farthest) is on 4th July, and perihelion (nearest) is on 3rd January.
✓ Aphelion is on 4th July (Earth farthest from Sun at 152 million km), and Perihelion is on 3rd January (Earth nearest to Sun at 147 million km).
⚠ Mixing Up Conduction, Convection, and Advection
✗ Convection is horizontal movement of air, and advection is vertical movement.
→ This is exactly opposite; convection is vertical movement of air, while advection is horizontal movement of air.
✓ Convection is the vertical transfer of heat through rising air currents, while advection is the horizontal transfer of heat through moving air masses.
⚠ Thinking the Atmosphere is Heated Directly by the Sun
✗ The atmosphere is heated directly by incoming solar radiation (short wave).
→ The atmosphere is largely transparent to short wave solar radiation; it is heated indirectly from below by long wave terrestrial radiation from the Earth's surface.
✓ The atmosphere is heated indirectly by the Earth's terrestrial radiation (long wave), not directly by insolation.
⚠ Misinterpreting the Heat Budget Numbers
✗ The Earth absorbs 100 units of insolation and radiates back 100 units.
→ Only 65 units are absorbed by the Earth-atmosphere system; 35 units are reflected back as albedo. The balance is between 65 units received and 65 units returned.
✓ Out of 100 units of insolation at the top of the atmosphere, 35 are reflected (albedo) and 65 are absorbed. The Earth and atmosphere together return 65 units to space, maintaining heat balance.
⚠ Confusing Normal Lapse Rate with Inversion
✗ Inversion of temperature means temperature decreases with height at a rate of 6.5°C per 1,000 m.
→ That is the normal lapse rate, not inversion. Inversion is the reversal where temperature increases with height.
✓ Normal lapse rate is a decrease of 6.5°C per 1,000 m with height. Inversion of temperature is when temperature increases with height, reversing the normal pattern.
Compare & Contrast
Distance from the Sun
Aphelion
Perihelion
Aphelion is when the Earth is farthest from the Sun (152 million km on 4th July), while Perihelion is when the Earth is nearest to the Sun (147 million km on 3rd January).
Direction of heat transfer
Convection
Advection
Convection is the vertical transfer of heat through rising air currents, while advection is the horizontal transfer of heat through moving air masses.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 SA (3 marks), 1 LA (5 marks) — total 10 marks typically
Most asked: Heat budget of the Earth (numerical values and balance) · Factors controlling temperature distribution · Inversion of temperature and its effects · Difference between conduction, convection, and advection · Aphelion and Perihelion · Isotherms and their deviation in January and July
Don't skip: Explain the heat budget of the Earth with a diagram (5 marks); Discuss the factors affecting temperature distribution (5 marks); What is inversion of temperature? Explain its causes and effects (3 marks)
One-Line Revision (read aloud before the exam)
  1. Insolation is the incoming solar radiation received by the Earth in short wavelengths.
  2. Aphelion occurs on 4th July when Earth is farthest from the Sun at 152 million km.
  3. Perihelion occurs on 3rd January when Earth is nearest to the Sun at 147 million km.
  4. Conduction is heat transfer between two bodies in contact, important for heating lower atmospheric layers.
  5. Convection is vertical heat transfer through rising air currents, confined to the troposphere.
  6. Advection is horizontal heat transfer through moving air, causing 'loo' winds in northern India.
  7. Terrestrial radiation is the long wave energy emitted by the Earth after being heated by insolation.
  8. Albedo is the percentage of solar radiation reflected back to space; Earth's average albedo is 35%.
  9. The heat budget balances 65 units received with 65 units returned, so Earth neither warms nor cools.
  10. Normal lapse rate is a temperature decrease of 6.5°C per 1,000 m with height.
  11. Inversion of temperature is when temperature increases with height, common on clear winter nights.
  12. Isotherms are lines joining places of equal temperature on a map.
  13. Air drainage is the downhill flow of cold air under gravity, protecting plants from frost.
  14. The summer solstice on 21st June has the sun directly overhead at 23.5°N.
  15. The atmosphere is heated indirectly from below by terrestrial radiation, not directly by the sun.

MCQs — 10 Questions with Answers

Q1. What is the average insolation received at the top of Earth's atmosphere?

  • A. 1.94 calories per sq.cm per minute ✓
  • B. 2.50 calories per sq.cm per minute
  • C. 1.50 calories per sq.cm per minute
  • D. 3.00 calories per sq.cm per minute

Answer: A — The NCERT textbook explicitly states Earth receives on average 1.94 calories per sq.cm per minute at the top of the atmosphere.

Q2. Which position of Earth receives slightly more insolation annually — aphelion or perihelion?

  • A. Aphelion (152 million km from Sun)
  • B. Perihelion (147 million km from Sun) ✓
  • C. Both receive equal insolation
  • D. It depends on the atmospheric conditions

Answer: B — Perihelion (January 3rd, 147 million km) is when Earth is nearest the Sun, receiving more insolation than aphelion, though this effect is masked by land-sea distribution.

Q3. Why does maximum insolation occur over subtropical deserts rather than the equator?

  • A. Subtropical deserts receive vertical sun rays
  • B. Equatorial regions have persistent cloud cover and high atmospheric moisture reducing surface insolation ✓
  • C. Deserts have higher temperature
  • D. Deserts are closer to the Sun

Answer: B — Although the equator receives vertical rays, cloud cover and moisture reduce actual surface insolation, while subtropical deserts (320 W/m²) have clear skies allowing maximum radiation penetration.

Q4. A student compares insolation at 0°, 30°, and 60° latitude. If the same solar intensity hits all three locations, which location receives least energy per unit area and why?

  • A. 0° latitude — highest altitude reduces insolation
  • B. 60° latitude — slant rays spread energy over larger area and pass through greater atmospheric depth ✓
  • C. 30° latitude — tropical clouds block radiation
  • D. All receive equal energy per unit area

Answer: B — At 60° latitude, slant rays spread the same energy over a larger ground area and travel through greater atmospheric depth causing more absorption and scattering, resulting in minimum energy per unit area.

Q5. Which of the following is NOT correct about the passage of solar radiation through the atmosphere?

  • A. Water vapour and ozone absorb near-infrared radiation
  • B. The atmosphere is completely opaque to short-wave solar radiation ✓
  • C. Suspended particles scatter visible light in all directions
  • D. Scattering of light by atmospheric particles creates the blue colour of the sky

Answer: B — The atmosphere is largely TRANSPARENT to short-wave solar radiation, not opaque; only 14 units out of 100 are absorbed in the atmosphere while the rest reaches the surface.

Q6. If insolation = 100 units, atmospheric absorption = 14 units, and Earth's surface absorption = 51 units, then the amount reflected back to space (albedo) is:

  • A. 35 units ✓
  • B. 65 units
  • C. 49 units
  • D. 51 units

Answer: A — Total insolation (100) minus absorbed units (14 + 51 = 65) equals reflected radiation: 100 - 65 = 35 units, which is the Earth's albedo.

Q7. In the Earth's heat budget, terrestrial radiation is primarily absorbed by which atmospheric component?

  • A. Nitrogen and oxygen gases
  • B. Greenhouse gases, particularly carbon dioxide ✓
  • C. Dust particles in the troposphere
  • D. Water droplets in clouds only

Answer: B — Long-wave terrestrial radiation is selectively absorbed by greenhouse gases like CO₂ and methane, not by the major atmospheric gases (N₂, O₂) which are transparent to long-wave radiation.

Q8. Assertion: Conduction heats only the lower atmospheric layers slowly. Reason: Conduction requires direct contact between warm and cool bodies with energy flowing from warmer to cooler body.

  • A. Both assertion and reason are correct, and reason explains the assertion ✓
  • B. Both are correct, but reason does not explain the assertion
  • C. Assertion is correct but reason is incorrect
  • D. Assertion is incorrect but reason is correct

Answer: A — Conduction limited to lower layers occurs precisely because it requires direct contact (reason), so only air touching warm Earth gets heated (assertion); both statements are correct and logically linked.

Q9. Which heating mechanism is primarily responsible for day-to-night temperature variations in middle latitudes and why?

  • A. Conduction — direct heating from Earth's surface
  • B. Convection — vertical air circulation brings warm air down at night
  • C. Advection — horizontal air movement rapidly changes temperature over short time periods ✓
  • D. Radiation — direct absorption of terrestrial radiation by air

Answer: C — In middle latitudes, advection (horizontal wind movement) is the dominant mechanism causing most diurnal weather changes, not vertical processes like conduction or convection.

Q10. Using the heat budget data provided (insolation 100 units → 65 units absorbed, Earth radiates 51 units → 17 units escape directly + 34 units absorbed by atmosphere), explain why the Earth's temperature remains in steady state and calculate total radiation escaping to space.

  • A. Earth warms because 65 units are absorbed; total escaping = 51 units
  • B. Heat balance achieved because total escaping (65 units) equals total absorbed (65 units after albedo); calculations: 17 (direct) + 48 (from atmosphere) = 65 units ✓
  • C. Earth cools because 34 units are absorbed by atmosphere; total escaping = 34 units
  • D. Heat balance is impossible due to continuous energy transfer

Answer: B — The heat budget balances when total radiation escaping to space (17 units from Earth + 48 units from atmosphere that received 14 from insolation + 34 from terrestrial radiation = 65 units) equals net insolation (100 - 35 albedo = 65 units), maintaining Earth's thermal equilibrium.

Flashcards

What is insolation?

Insolation is the incoming solar radiation received by Earth, averaging 1.94 calories per square centimetre per minute at the top of the atmosphere.

Why does the equator receive less insolation than the tropics despite being closer to the sun?

The equator has persistent cloud cover and high atmospheric moisture that reduces actual surface insolation, while subtropical deserts have clear skies allowing maximum radiation to reach the ground.

Define conduction in the context of atmospheric heating.

Conduction is the transfer of heat from the warm Earth surface to cooler air in direct contact, heating only the lower atmospheric layers slowly.

How does convection differ from conduction in heating the atmosphere?

Convection involves warm air rising vertically as currents to heat upper layers, while conduction transfers heat horizontally through direct contact between bodies.

What is terrestrial radiation?

Terrestrial radiation is the long-wave energy emitted by Earth's heated surface back toward the atmosphere, which is absorbed by greenhouse gases.

Define albedo.

Albedo is the total amount of solar radiation reflected back to space before reaching Earth's surface, approximately 35 units out of 100 incoming units.

Why is the atmosphere mostly transparent to incoming solar radiation but opaque to outgoing terrestrial radiation?

The atmosphere is transparent to short-wave solar radiation but absorbs long-wave terrestrial radiation through greenhouse gases like CO₂, creating the greenhouse effect.

What does heat budget or heat balance of the Earth explain?

Heat balance explains that total radiation escaping to space (65 units) equals total insolation received (100 units minus 35 units reflected), preventing Earth from warming or cooling.

Why do slant sun rays at high latitudes produce less heating than vertical rays at the equator?

Slant rays spread energy over a larger area and must pass through greater atmospheric depth, causing more absorption and scattering, resulting in less energy per unit area.

What is advection and give one Indian example.

Advection is heat transfer through horizontal air movement; the 'loo' winds of northern India during summer are a classic example of local heating through advection.

Important Board Questions

Define insolation and state how the angle of sun's rays affects the amount of insolation received at different latitudes. [2 marks]

Define insolation as incoming solar radiation. Explain that vertical rays concentrate energy over smaller area, while slant rays at high latitudes spread energy over larger area and pass through greater atmospheric depth, reducing insolation per unit area.

Explain with examples how conduction, convection, and advection differ in heating the atmosphere. Which mechanism is most important in middle latitudes and why? [5 marks]

Conduction: slow direct contact heating of lower layers only. Convection: vertical air currents, important in tropics (rising warm air). Advection: horizontal wind movement, dominates in middle latitudes because it rapidly transports warm/cold air masses causing most day-to-night weather changes; example — cold wind intrusion changes temperature in hours.

Describe the Earth's heat budget with numerical values. Explain why the terrestrial radiation emitted by Earth does not escape directly to space, and how this process maintains Earth's thermal equilibrium despite continuous energy exchange. [6 marks]

Heat budget: 100 units insolation → 35 reflected, 65 absorbed (14 atmosphere, 51 surface). Earth radiates 51 units; only 17 escape directly to space while 34 are absorbed by greenhouse gases in atmosphere. Atmosphere then radiates 48 units to space. Total escaping: 17 + 48 = 65 units, balancing net insolation (65 units). Greenhouse gas absorption of terrestrial radiation prevents direct escape, re-radiating energy downward (greenhouse effect) and upward, creating the balanced budget that maintains steady Earth temperature.

Assertion-Reason Questions for Solar Radiation, Heat Balance and Temperature

Q1. Assertion (A):

The atmosphere is heated primarily by direct absorption of incoming solar radiation.

Reason (R):

Water vapour, ozone, and other gases in the troposphere absorb much of the near infrared radiation.

Answer: D — A is false but R is true — The assertion is false, but the reason is true. The atmosphere is heated indirectly by terrestrial radiation from the earth's surface, not primarily by direct solar absorption. The stated reason (water vapour absorbing near-infrared) is a real process but doesn't explain the main heating mechanism.

Q2. Assertion (A):

Albedo refers to the total amount of radiation absorbed by the earth's surface.

Reason (R):

Albedo is the reflected amount of solar radiation that never reaches the earth's surface or escapes after reaching it.

Answer: D — A is false but R is true — Both are false. Albedo specifically refers to reflected radiation (35 units out of 100), not absorbed radiation. The assertion has the definition inverted. Students often confuse albedo with absorption.

Q3. Assertion (A):

Convection is more important than conduction in heating the troposphere.

Reason (R):

Convection involves vertical movement of heated air currents and is the primary mechanism for transferring heat through the entire troposphere.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason correctly explains why convection is more important. Conduction only heats the immediate lower layers in contact with the surface, while convection distributes heat vertically throughout the troposphere.

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Case-Based Questions (CBQ) for Solar Radiation, Heat Balance and Temperature

Case Study:

A meteorological station in the Thar Desert (Rajasthan, India) recorded insolation of 310 W/m² at noon on 21st June, while a coastal station at Mumbai recorded only 240 W/m² on the same date and time. Both stations are at similar latitudes (21°N). Weather radar showed clear skies in Rajasthan but cumulus clouds over Mumbai. The Thar Desert region has very low vegetation cover and sandy soil with high albedo of 0.35, while Mumbai coastal area has water bodies with albedo of 0.06.

Q1. Why does the Thar Desert receive significantly more insolation than Mumbai despite being at the same latitude?

  • A. The Thar Desert is closer to the sun during summer solstice
  • B. Mumbai receives more slant rays due to its coastal location
  • C. Clear skies over the desert allow more solar radiation to reach the surface, while clouds over Mumbai scatter and reflect radiation back to space ✓
  • D. The desert soil absorbs more radiation than water

Answer: C

Q2. Given that the Thar Desert has an albedo of 0.35 and received 310 W/m², how much insolation (in W/m²) was actually absorbed by the desert surface? (Net absorption = Total insolation × (1 - albedo))

  • A. 108.5 W/m²
  • B. 201.5 W/m² ✓
  • C. 225.5 W/m²
  • D. 241.0 W/m²

Answer: B

Q3. The sandy desert with high albedo reflects more radiation back to space. How does this affect the heat balance of the desert region?

  • A. High albedo cools the desert because less radiation is absorbed and more is reflected to space ✓
  • B. High albedo warms the desert because reflected radiation is re-radiated as heat
  • C. Albedo has no effect on desert temperature; only atmospheric processes matter
  • D. High albedo increases insolation by concentrating sun rays

Answer: A

Q4. If monsoon clouds increase cloud cover over the Thar Desert during August, what would be the expected change in surface insolation?

  • A. Insolation would increase because clouds trap solar radiation
  • B. Insolation would decrease because clouds scatter and reflect solar radiation back to space ✓
  • C. Insolation would remain unchanged; clouds only affect temperature at night
  • D. Insolation would increase because water vapour in clouds absorbs infrared radiation

Answer: B

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