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Q62 (IAS/2023) Geography › World Physical Geography › Atmospheric heat balance Official Key

With reference to the Earth's atmosphere, which one of the following statements is correct?

Result
Your answer: —  ·  Correct: C
Explanation

The correct answer is Option 3.

The Earth's atmosphere is transparent to shortwave solar radiation but opaque to longwave terrestrial radiation. Water vapour, concentrated primarily in the troposphere (lower atmosphere), acts as a potent greenhouse gas. It possesses high absorption bands for infrared radiation, trapping heat and maintaining the Earth's thermal balance. This process is crucial for the greenhouse effect.

Other options are incorrect because:

  • Option 1: Insolation at the equator is roughly 2.4 times (not 10 times) that at the poles.
  • Option 2: Infrared radiation accounts for approximately 45-50% of the total solar spectrum, while visible light accounts for about 40%; thus, it is not two-thirds.
  • Option 4: Infrared waves are not part of the visible spectrum; they have longer wavelengths and are located beyond the red end of the visible light range in the electromagnetic spectrum.
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Don’t just practise – reverse-engineer the question. This panel shows where this PYQ came from (books / web), how the examiner broke it into hidden statements, and which nearby micro-concepts you were supposed to learn from it. Treat it like an autopsy of the question: what might have triggered it, which exact lines in the book matter, and what linked ideas you should carry forward to future questions.
Q. With reference to the Earth's atmosphere, which one of the following statements is correct? [A] The total amount of insolation received …
At a glance
Origin: Books + Current Affairs Fairness: Moderate fairness Books / CA: 6/10 · 2/10

This is a classic 'NCERT Fundamental' check, specifically targeting the confusion between Incoming Solar Radiation (Insolation) and Outgoing Terrestrial Radiation. It combines concepts from Chapter 7 (Composition) and Chapter 8 (Heat Budget) of Class XI Physical Geography. The trap lies in mixing up the properties of Shortwave (Solar) and Longwave (Terrestrial) radiation.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1
In the context of Earth's atmosphere, is the total solar insolation received at the equator roughly ten times the insolation received at the poles?
Origin: Web / Current Affairs Fairness: CA heavy Web-answerable

Web source
Presence: 5/5
"The total amount of insolation received by the latitudes of the Earth does not vary a great deal, but the curvature of the Earth ahs a vital role to play in determining how much Insolation is"
Why this source?
  • Explicitly states that total insolation by latitude "does not vary a great deal", which argues against a large (≈10×) difference.
  • Notes curvature is important but implies latitudinal differences are moderate rather than an order-of-magnitude.
Web source
Presence: 4/5
"Because the Earth is a sphere, the surface gets much more intense sunlight (heat) at the equator than at the poles. ... the Sun rises no higher than 30° above the horizon at noon and heats a given parcel of ground with only a half"
Why this source?
  • States equator gets more intense sunlight but gives a concrete example at 60°N where the sun "heats a given parcel of ground with only a half", implying roughly ~2× differences in intensity at mid-latitudes rather than ~10×.
  • Supports the view that differences are substantial but not order-of-magnitude.
Web source
Presence: 3/5
"The amount of insolation decreases from the equator towards the poles."
Why this source?
  • Notes simply that "The amount of insolation decreases from the equator towards the poles," confirming a gradient but giving no support for a 10× factor.
  • Provides general confirmation of latitudinal decrease but no large-magnitude claim.

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > Spatial Distribution of Insolation at the Earth's Surface > p. 68
Strength: 5/5
“The insolation received at the surface varies from about 320 Watt/m2 in the tropics to about 70 Watt/m2 in the poles. Maximum insolation is received over the subtropical deserts, where the cloudiness is the least. Equator receives comparatively less insolation than the tropics. Generally, at the same latitude the insolation is more over the continent than over the oceans. In winter, the middle and higher latitudes receive less radiation than in summer.”
Why relevant

Gives actual measured surface insolation ranges: ~320 W/m² in the tropics and ~70 W/m² in the poles, showing a multi‑fold difference between low and high latitudes.

How to extend

A student can compare these numbers (320 vs 70 W/m² → ~4.6×) with the claimed 10× ratio to judge that the claim seems larger than the values cited.

Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 18: Latitudes and Longitudes > Temperature Falls as We Move From The Equator Towards The Poles > p. 242
Strength: 5/5
“• Temperature falls at the surface of the earth as one moves away from the equator towards the poles. This is because of the spherical (geoid) shape of the earth and the position of the sun relative to earth.• The energy received per unit area decreases from the equator towards the poles as the equator receives direct sunlight and the sun's rays become slant or oblique as we move poleward.”
Why relevant

States the general geometric rule that energy per unit area decreases from equator to poles because rays are direct at the equator and increasingly oblique toward poles.

How to extend

Combine this geometric rule with the cosine (angle) effect from basic geometry or a world map to estimate relative insolation at different latitudes and check plausibility of a 10× factor.

Exploring Society:India and Beyond ,Social Science-Class VII . NCERT(Revised ed 2025) > Chapter 3: Climates of India > a) Latitude > p. 49
Strength: 4/5
“In the At the equator exposure to the sun's rays is concentrated, but near the poles it is dispersed over a wide area Fig. 3.4 polar regions, the rays are inclined or oblique, and the energy is distributed over a larger surface. Moreover, they have to pass through more of the Earth's atmosphere, as the diagram shows, which further dissipates their energy. As a result, the polar regions receive less heat than the equatorial zone. We can see this in India too: Kanniyakumari and the Nicobar Islands being close to the Equator are warm or hot almost throughout the year, whereas places in the north, such as Srinagar, are much cooler.”
Why relevant

Explains that at the equator solar rays are concentrated while near the poles they are spread over a larger area and pass through more atmosphere, both reducing polar insolation.

How to extend

Use the two effects (area spreading + longer atmospheric path) together with simple trigonometry or known latitude differences to estimate how much weaker polar insolation should be.

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > SOLAR RADIATION > p. 67
Strength: 4/5
“The earth's surface receives most of its energy in short wavelengths. The energy received by the earth is known as incoming solar radiation which in short is termed as insolation. As the earth is a geoid resembling a sphere, the sun's rays fall obliquely at the top of the atmosphere and the earth intercepts a very small portion of the sun's energy. On an average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere. The solar output received at the top of the atmosphere varies slightly in a year due to the variations in the distance between the earth and the sun.”
Why relevant

Gives the average incoming solar radiation at the top of the atmosphere (1.94 cal/cm²/min ≈ solar constant at TOA), providing a baseline before atmospheric attenuation.

How to extend

A student can compare TOA values with surface values (from other snippets) to see how atmospheric losses alter latitudinal contrasts and whether a 10× surface ratio is feasible.

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > Variation in the Net Heat Budget at the Earth's Surface > p. 70
Strength: 3/5
“As explained earlier, there are variations in the amount of radiation received at the earth's surface. Some part of the earth has surplus radiation balance while the other part has deficit. Figure 8.3 depicts the latitudinal variation in the net radiation balance of the earth the atmosphere system. The figure shows that there is a surplus of net radiation balance between 40 degrees north and south and the regions near the poles have a deficit. The surplus heat energy from the tropics is redistributed pole wards and as a result the tropics do not get progressively heated up due to the accumulation of excess heat or the high latitudes get permanently frozen due to excess deficit.”
Why relevant

Describes latitudinal patterns of net radiation (surplus in tropics, deficit near poles), confirming systematic decrease poleward though not giving exact multipliers.

How to extend

Use this qualitative pattern plus quantitative surface numbers (snippet 1) to infer that while poles receive markedly less, the difference is on the order of a few times rather than an order of magnitude.

Statement 2
In the context of Earth's atmosphere, do infrared rays constitute roughly two-thirds of the incoming solar radiation (insolation)?
Origin: Weak / unclear Fairness: Borderline / guessy
Indirect textbook clues
FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > SOLAR RADIATION > p. 67
Strength: 5/5
“The earth's surface receives most of its energy in short wavelengths. The energy received by the earth is known as incoming solar radiation which in short is termed as insolation. As the earth is a geoid resembling a sphere, the sun's rays fall obliquely at the top of the atmosphere and the earth intercepts a very small portion of the sun's energy. On an average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere. The solar output received at the top of the atmosphere varies slightly in a year due to the variations in the distance between the earth and the sun.”
Why relevant

States that the Earth's surface receives most of its energy in short wavelengths and defines insolation as incoming solar radiation.

How to extend

A student could combine this with a standard solar spectrum (or TOA spectral fractions) to judge whether infrared could be as large as two-thirds if most energy is shortwave.

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > The Passage of Solar Radiation through the Atmosphere > p. 68
Strength: 4/5
“The atmosphere is largely transparent to short wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth's surface. Within the troposphere water vapour, ozone and other gases absorb much of the near infrared radiation. Very small-suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth surface. This process adds colour to the sky. The red colour of the rising and the setting sun and the blue colour of the sky are the result of scattering of light within the atmosphere.”
Why relevant

Says the atmosphere is largely transparent to short-wave solar radiation and that tropospheric gases absorb much of the near-infrared.

How to extend

Use the note that shortwave predominates at TOA but near-IR is partially absorbed in the atmosphere to estimate the split between visible/UV and IR in incoming solar energy.

Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 14: Climate > lnsolation > p. 131
Strength: 4/5
“The only source of energy for the earth's atmosphere comes from the sun, which has a surface temperature of more than 5982 °C (10,800 'F). This energy travels through space for a distance of 150 million km (93 million miles) and reaches us as solar energy or radiant energy in the process called insolation. This radiation from the sun is made up of three parts: the visible 'white' light that we see when the sun shines, and the less visible ultra-violet and infra-red rays. What matters most is the effect of the atmosphere upon the incoming solar radiation. It is estimated that of the total radiation coming to us, 35 per cent reaches the atmosphere and is directly reflected back to space by dust, clouds and air molecules.”
Why relevant

Explicitly lists the Sun's radiation components as visible, ultraviolet and infrared and notes a substantial fraction (~35%) is reflected back from atmosphere/particles.

How to extend

Combine the qualitative component list and the reflected fraction with external spectral data (e.g., solar spectral irradiance) to infer the fraction of incoming energy in infrared.

Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 21: Horizontal Distribution of Temperature > Insolation > p. 282
Strength: 4/5
“• Insolation is the amount of sun's energy received in the form of radiation by the earth. It is measured as the amount of solar energy received per square centimetre per minute.• Earth receives the sun's radiation in the form of short waves (visible light or wavelengths below visible light — most of it is ultraviolet radiation) which are electromagnetic.• The earth absorbs short wave radiation during daytime and reflects the heat received into space as long-wave radiation (mostly infrared radiation which is nothing but heat) during the night.”
Why relevant

States Earth receives solar radiation in the form of short waves (visible and ultraviolet), implying incoming solar energy is dominated by shortwave rather than longwave/IR.

How to extend

A student could take this rule and compare it to known percent distributions of solar spectrum to test whether IR can plausibly be two-thirds.

Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Water Vapour > p. 272
Strength: 3/5
“• Water Vapour is one of the most variable gaseous substances present in the atmosphere constituting between 0.02% and 4% of the total volume (in cold dry, and humid tropical climates, respectively).• 90% of moisture content in the atmosphere exists within 6 km of the surface of the earth. Like carbon dioxide, water vapour plays a significant role in the insulating action, of the atmosphere. It absorbs not only the long-wave terrestrial radiation (infrared or heat emitted by the earth during the nights) but also a part of the incoming short-wave solar radiation (visible and UV radiation).”
Why relevant

Notes water vapour absorbs not only outgoing long-wave terrestrial radiation but also part of incoming short-wave (visible and UV), implying some incoming energy in near-IR/shortwave is absorbed in atmosphere.

How to extend

Use this to refine an estimate of how much incoming IR is removed before reaching surface versus how much of total insolation is IR at TOA.

Statement 3
In Earth's atmosphere, are infrared waves largely absorbed by atmospheric water vapour?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > The Passage of Solar Radiation through the Atmosphere > p. 68
Presence: 5/5
“The atmosphere is largely transparent to short wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth's surface. Within the troposphere water vapour, ozone and other gases absorb much of the near infrared radiation. Very small-suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth surface. This process adds colour to the sky. The red colour of the rising and the setting sun and the blue colour of the sky are the result of scattering of light within the atmosphere.”
Why this source?
  • Explicitly says water vapour in the troposphere absorbs much of the near-infrared radiation.
  • Places the absorption process within the atmospheric layer where most water vapour resides (troposphere).
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Water Vapour > p. 272
Presence: 5/5
“• Water Vapour is one of the most variable gaseous substances present in the atmosphere constituting between 0.02% and 4% of the total volume (in cold dry, and humid tropical climates, respectively).• 90% of moisture content in the atmosphere exists within 6 km of the surface of the earth. Like carbon dioxide, water vapour plays a significant role in the insulating action, of the atmosphere. It absorbs not only the long-wave terrestrial radiation (infrared or heat emitted by the earth during the nights) but also a part of the incoming short-wave solar radiation (visible and UV radiation).”
Why this source?
  • States water vapour absorbs long-wave terrestrial radiation (infrared/heat emitted by the Earth).
  • Links water vapour with the insulating/greenhouse action that retains emitted infrared energy.
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Significance of Atmospheric Moisture > p. 326
Presence: 4/5
“• Water vapour absorbs both incoming and outgoing radiation and plays a crucial role in the earth's heat budget.• The amount of water vapour present decides the quantity of latent energy stored in the atmosphere for the development of storms and cyclones.• The atmospheric moisture affects the human body's cooling rate by influencing the sensible temperature.”
Why this source?
  • Affirms that water vapour absorbs both incoming and outgoing radiation, implicating infrared in Earth's heat budget.
  • Identifies water vapour as crucial to atmospheric radiative processes that govern heat retention.
Statement 4
In Earth's atmosphere, is atmospheric water vapour concentrated in the lower atmosphere (the troposphere)?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Water Vapour > p. 272
Presence: 5/5
“• Water Vapour is one of the most variable gaseous substances present in the atmosphere constituting between 0.02% and 4% of the total volume (in cold dry, and humid tropical climates, respectively).• 90% of moisture content in the atmosphere exists within 6 km of the surface of the earth. Like carbon dioxide, water vapour plays a significant role in the insulating action, of the atmosphere. It absorbs not only the long-wave terrestrial radiation (infrared or heat emitted by the earth during the nights) but also a part of the incoming short-wave solar radiation (visible and UV radiation).”
Why this source?
  • Explicitly quantifies that the vast majority of atmospheric moisture is close to the surface (90% within ~6 km).
  • Directly supports the idea that water vapour is concentrated in the lower atmosphere.
Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Structure of the Atmosphere > p. 7
Presence: 5/5
“For the purpose of systematic study, on the basis of temperature and lapse rate, the atmosphere may be classifed under the following layers: • (i) Troposphere, (ii) Stratosphere, (iii) Mesosphere, (iv) Exosphere• 1. Troposphere: Te lowest layer of the atmosphere is known as troposphere. It is the home of biospheric layer that supports life on Earth. Approximately 90% of the total mass of the atmosphere and the bulk of all water vapour, clouds, weather, and air pollution are contained within the troposphere. Te tropopause is the upper limit which is defned by an average temperature of –57°C, but its exact elevation varies with the season, latitude, surface temperatures and pressure.”
Why this source?
  • States that the troposphere contains the bulk of all water vapour, clouds and weather.
  • Identifies the troposphere as the primary layer for atmospheric moisture, linking moisture concentration to this lower layer.
FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 7: Composition and Structure of Atmosphere > Water Vapour > p. 64
Presence: 4/5
“Water vapour is also a variable gas in the atmosphere, which decreases with altitude. In the warm and wet tropics, it may account for four per cent of the air by volume, while in the dry and cold areas of desert and polar regions, it may be less than one per cent of the air. Water vapour also decreases from the equator towards the poles. It also absorbs parts of the insolation from the sun and preserves the earth's radiated heat. It thus, acts like a blanket allowing the earth neither to become too cold nor too hot. Water vapour also contributes to the stability and instability in the air.”
Why this source?
  • Asserts that water vapour decreases with altitude, implying higher concentrations near the surface.
  • Provides regional percentage context while confirming the vertical decline of moisture.
Statement 5
In the context of Earth's atmosphere and solar radiation, are infrared waves part of the visible spectrum of electromagnetic radiation?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 14: Climate > lnsolation > p. 131
Presence: 5/5
“The only source of energy for the earth's atmosphere comes from the sun, which has a surface temperature of more than 5982 °C (10,800 'F). This energy travels through space for a distance of 150 million km (93 million miles) and reaches us as solar energy or radiant energy in the process called insolation. This radiation from the sun is made up of three parts: the visible 'white' light that we see when the sun shines, and the less visible ultra-violet and infra-red rays. What matters most is the effect of the atmosphere upon the incoming solar radiation. It is estimated that of the total radiation coming to us, 35 per cent reaches the atmosphere and is directly reflected back to space by dust, clouds and air molecules.”
Why this source?
  • Separates visible 'white' light from less visible ultraviolet and infra-red, implying infra-red is not part of the visible band.
  • Presents infra-red as a distinct, less visible component of solar radiation.
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 21: Horizontal Distribution of Temperature > 21.5. Heat Budget > p. 293
Presence: 5/5
“• The earth receives a certain amount of insolation (short waves – UV and visible part of the electromagnetic spectrum) and gives back heat into space by terrestrial radiation (longwave or infrared radiation). Through this give and take, or the heat budget, the earth maintains a constant temperature.”
Why this source?
  • Defines insolation as short waves (UV and visible) and contrasts these with terrestrial longwave (infrared).
  • Explicitly treats infrared as longwave radiation distinct from the visible shortwave band.
Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 17: Climate Change > Emission > p. 255
Presence: 4/5
“. The Earth re-emits energy back into the atmosphere. Because the Earth is cooler than the Sun, the energy is emitted in the form of infrared radiation, at wavelengths longer than the incoming solar energy.”
Why this source?
  • States the Earth emits energy as infrared at wavelengths longer than the incoming solar energy.
  • Identifies infrared with longer wavelengths, distinguishing it from shorter-wavelength visible solar radiation.
Pattern takeaway: UPSC frequently creates 'Definition Swaps'. Option B describes Terrestrial Radiation (mostly IR) but applies it to Insolation. Option D mixes the definition of Visible Spectrum with IR. The key to cracking Physical Geography is precise terminology: know the difference between 'Insolation' and 'Terrestrial Radiation'.
How you should have studied
  1. Bullet 1. [THE VERDICT]: Sitter. Directly solvable from NCERT Class XI Fundamentals of Physical Geography (Chapter 8: Solar Radiation & Chapter 7: Composition).
  2. Bullet 2. [THE CONCEPTUAL TRIGGER]: The 'Heat Budget' and 'Atmospheric Circulation'. Specifically, the mechanism of the Greenhouse Effect (how atmosphere is transparent to insolation but opaque to terrestrial radiation).
  3. Bullet 3. [THE HORIZONTAL EXPANSION]: Memorize the 'Radiation Rules': 1) Solar = Shortwave (Visible/UV), Terrestrial = Longwave (IR). 2) Insolation Ratio: Equator (~320 W/m²) is roughly 4x Poles (~70 W/m²), not 10x. 3) Water Vapour Profile: 90% is within 6km of surface (Troposphere). 4) Albedo: Earth reflects ~30-35% (Clouds are the biggest reflector).
  4. Bullet 4. [THE STRATEGIC METACOGNITION]: When reading about 'Absorption', always ask: 'Absorbs WHAT?' Ozone absorbs UV (incoming); Water Vapour/CO2 absorb IR (outgoing/terrestrial). The exam traps you by swapping these targets.
Concept hooks from this question
📌 Adjacent topic to master
S1
👉 Latitudinal variation of insolation (oblique rays & area dilution)
💡 The insight

Insolation per unit area decreases from equator to poles because solar rays become more oblique, are spread over a larger surface, and traverse more atmosphere at high latitudes.

High-yield for explaining temperature gradients, climate zones and poleward heat redistribution; connects to questions on latitude controls of climate, isotherms and seasonal contrasts. Mastering this helps answer comparative reasoning and cause-effect questions on temperature and radiation.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 18: Latitudes and Longitudes > Temperature Falls as We Move From The Equator Towards The Poles > p. 242
  • Exploring Society:India and Beyond ,Social Science-Class VII . NCERT(Revised ed 2025) > Chapter 3: Climates of India > a) Latitude > p. 49
🔗 Anchor: "In the context of Earth's atmosphere, is the total solar insolation received at ..."
📌 Adjacent topic to master
S1
👉 Typical surface insolation magnitudes (tropics vs poles)
💡 The insight

Surface insolation magnitudes show that tropical values (~320 W/m2) are only a few times larger than polar values (~70 W/m2), not an order-of-magnitude higher.

Enables quantitative evaluation of claims about relative radiation (e.g., refuting exaggerated ratios); links to heat budget, energy balance and regional climate comparisons — useful for data-based reasoning and numerical options in UPSC geography questions.

📚 Reading List :
  • FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > Spatial Distribution of Insolation at the Earth's Surface > p. 68
🔗 Anchor: "In the context of Earth's atmosphere, is the total solar insolation received at ..."
📌 Adjacent topic to master
S1
👉 Solar constant and atmospheric attenuation (TOA vs surface)
💡 The insight

The incoming solar flux at the top of the atmosphere (~1.94 cal/cm2/min or ~1350 W/m2) is reduced before reaching the surface by reflection and absorption, so TOA values differ from surface insolation.

Critical for separating top-of-atmosphere figures from surface measurements in questions on energy budgets, solar energy potential and climate forcing; helps avoid mixing global constants with surface-level comparisons.

📚 Reading List :
  • FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > SOLAR RADIATION > p. 67
  • Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 14: Climate > lnsolation > p. 131
  • Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 6: Environmental Degradation and Management > i) Solar Energy or Photovoltaic (Pv) Energy > p. 52
🔗 Anchor: "In the context of Earth's atmosphere, is the total solar insolation received at ..."
📌 Adjacent topic to master
S2
👉 Short-wave (solar) vs Long-wave (terrestrial) radiation
💡 The insight

Solar radiation arrives mainly as short-wave (visible and UV) while Earth emits long-wave infrared; this distinction is central to any claim about what fraction of insolation is infrared.

High-yield: understanding the short-wave/long-wave split is essential for questions on Earth's energy balance, heat budget, and diurnal/seasonal temperature behaviour. It links radiation physics to topics like albedo, outgoing terrestrial radiation, and climate processes; mastering it helps answer questions about energy flow and greenhouse mechanisms.

📚 Reading List :
  • FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > SOLAR RADIATION > p. 67
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 21: Horizontal Distribution of Temperature > Insolation > p. 282
  • Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 17: Climate Change > Emission > p. 255
🔗 Anchor: "In the context of Earth's atmosphere, do infrared rays constitute roughly two-th..."
📌 Adjacent topic to master
S2
👉 Atmospheric absorption and scattering by specific constituents
💡 The insight

Atmospheric gases (water vapour, ozone) and particles selectively absorb or scatter certain solar wavelengths, so the composition of incoming solar bands at the surface differs from the top-of-atmosphere spectrum.

High-yield: explains why incoming solar spectrum at surface is altered (important for radiation budgets, climate, and observational interpretation). It connects chemistry (ozone, water vapour) to physical geography and climatology and supports reasoning about what portions of solar energy reach the surface.

📚 Reading List :
  • FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > The Passage of Solar Radiation through the Atmosphere > p. 68
  • Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Structure of the Atmosphere > p. 8
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Water Vapour > p. 272
🔗 Anchor: "In the context of Earth's atmosphere, do infrared rays constitute roughly two-th..."
📌 Adjacent topic to master
S2
👉 Greenhouse effect and atmospheric re-emission of infrared
💡 The insight

Greenhouse gases absorb outgoing long-wave infrared from the surface and re-radiate it, which is distinct from the incoming short-wave solar radiation composition.

High-yield: central to questions on climate change, surface temperature regulation, and the planetary heat budget. It links radiative transfer to policy-relevant topics (warming, feedbacks) and enables tackling questions about causes and effects of atmospheric warming.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Explanation: > p. 337
  • Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 6: Environmental Degradation and Management > Greenhouse Effect and Global Warming > p. 7
  • FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > Heat Budget of the Planet Earth > p. 69
🔗 Anchor: "In the context of Earth's atmosphere, do infrared rays constitute roughly two-th..."
📌 Adjacent topic to master
S3
👉 Water vapour as a primary infrared absorber (greenhouse agent)
💡 The insight

Water vapour absorbs long-wave and near-infrared radiation and contributes strongly to the atmosphere's insulating (greenhouse) effect.

High-yield for questions on radiative forcing and greenhouse gases; links physical geography to climate change topics and helps answer why humid regions retain more heat. Mastery allows answering cause–effect questions on surface warming and diurnal temperature ranges.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Water Vapour > p. 272
  • FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > The Passage of Solar Radiation through the Atmosphere > p. 68
🔗 Anchor: "In Earth's atmosphere, are infrared waves largely absorbed by atmospheric water ..."
🌑 The Hidden Trap

The 'Solar Constant' Variation. Since the question touched on insolation amounts, the next logical question is the variation due to distance. Earth receives ~7% more insolation at Perihelion (Jan 3) than at Aphelion (July 4). This is a sibling fact in the same NCERT chapter.

⚡ Elimination Cheat Code

Use Etymology and Common Sense. Option D: 'Infra' means 'Below'. Infrared is 'Below Red'. It cannot be *part* of the visible spectrum by definition. Option A: If the Equator received 10x the energy of the Poles, the Poles would be near absolute zero or the Equator would be uninhabitable; the actual gradient is significant but not that extreme (approx 4x).

🔗 Mains Connection

Link this to Environment (Climate Change): Water Vapour is actually the most potent 'Natural' Greenhouse Gas, but it is a 'Feedback' agent, not a 'Forcing' agent. Warmer air holds more moisture (Clausius-Clapeyron relation) → more IR absorption → more warming. This is the 'Water Vapour Feedback Loop' crucial for Mains GS-3.

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SIMILAR QUESTIONS

CDS-I · 2024 · Q9 Relevance score: 2.95

Which of the following statements regarding insolation is/are correct? 1. Insolation is predominantly shortwave radiation, with wavelengths in the range of 0.39 micrometre to 0.76 micrometre. 2. Insolation is evenly distributed across the Earth because of the Earth's curved surface. Select the correct answer using the code given below.

NDA-II · 2025 · Q119 Relevance score: 1.78

Consider the following statements with reference to structure of the Earth's atmosphere : 1. Thickness of the troposphere is greatest at the equator 2. The air temperature of the tropopause is highest above the poles 3. The temperature in troposphere decreases at the rate of 1ºF for every 165 m of height Which of the statements given above is/are correct ?

IAS · 2025 · Q33 Relevance score: 0.55

Consider the following statements : Statement I : Scientific studies suggest that a shift is taking place in the Earth's rotation and axis. Statement II : Solar flares and associated coronal mass ejections bombarded the Earth's outermost atmosphere with tremendous amount of energy. Statement III : As the Earth's polar ice melts, the water tends to move towards the equator. Which one of the following is correct in respect of the above statements?

IAS · 2024 · Q12 Relevance score: -0.53

With reference to "water vapour", which of the following statements is/are correct ? 1. It is a gas, the amount of which decreases with altitude. 2. Its percentage is maximum at the poles. Select the answer using the code given below :

NDA-I · 2013 · Q66 Relevance score: -0.65

Statement I: Insolation is greatest when the sun is directly overhead and the sun`s rays are vertical. Statement II: When the sun is lower in the sky, the same amount of solar energy spreads over a greater area of ground surface, so insolation is lower.