With reference to the Earth's atmosphere, which one of the following statements is correct?
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- AThe total amount of insolation received at the equator is roughly about 10 times of that received at the poles.
- BInfrared rays constitute roughly two-thirds of insolation.
- CInfrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.
- DInfrared waves are a part of visible spectrum of electromagnetic waves of solar radiation.
Show answer
Answer: (C) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.
Option (a) is wrong:
The equator receives about 5 times more insolation than the poles, not 10 times.
Option (b) is wrong:
Infrared radiation is longwave (not shortwave).
Option (c) is correct:
Water vapour in the atmosphere is a major absorber of infrared (longwave) radiation emitted by Earth's surface.
Option (d) is wrong:
Infrared is NOT part of the visible spectrum.
Answer is (c).
Water vapour in the lower atmosphere absorbs most of Earth's outgoing infrared radiation, creating the natural greenhouse effect that keeps Earth warmer than it would otherwise be.
UPSC is testing whether students can distinguish between incoming solar radiation (mostly visible light) and outgoing terrestrial radiation (infrared), plus which atmospheric gases absorb each type.
Solar Insolation Distribution
Geography insolation equator poles
Solar Insolation Distribution: Equator vs Poles
Equator receives 5 times more insolation than poles (not 10 times)
Poles receive zero insolation during their respective winters
Maximum insolation occurs at 23.5°N/S during respective solstices
Why Distribution Varies
Insolation (incoming solar radiation) varies dramatically with latitude due to Earth's spherical shape and axial tilt. The equatorial regions consistently receive more concentrated solar energy.
Equator vs Poles Comparison
Factor | Equator (0°) | Poles (90°N/S) |
|---|---|---|
Insolation Ratio | 5 times higher | Baseline |
Sun's Angle | Nearly 90° (perpendicular) | 0° to 23.5° (oblique) |
Atmospheric Path | Shortest thickness | Longest thickness |
Seasonal Variation | Minimal (±23.5°) | Extreme (0° to 47°) |
Day Length | 12 hours constant | 0 to 24 hours |
Global Insolation Pattern
Source: INFORSE — Solar · www.inforse.org
Trap: Option A claims 10 times difference — actual ratio is 5 times
Maximum insolation shifts between Tropic of Cancer and Tropic of Capricorn, not always at equator
Poles can receive more insolation than equator during summer solstice (24-hour daylight vs 12-hour)
Electromagnetic Spectrum & Solar Radiation
Geography Infrared rays insolation visible spectrum
Electromagnetic Spectrum: Solar vs Terrestrial Radiation
Visible light (45%) dominates incoming solar radiation, not infrared
Infrared is NOT part of visible spectrum — it's beyond red light
Earth emits longwave infrared after absorbing shortwave solar radiation
Solar Radiation Composition
Component | Wavelength | % of Insolation | Visibility |
|---|---|---|---|
Ultraviolet | < 0.4 μm | 9% | Invisible |
Visible Light | 0.4 - 0.7 μm | 45% | Violet to Red |
Near Infrared | 0.7 - 4.0 μm | 46% | Invisible |
Far Infrared | > 4.0 μm | Negligible | Invisible |
Radiation Classification
# Electromagnetic Radiation
## Shortwave (Solar)
- UV (9%)
- Visible (45%)
- Near-IR (46%)
## Longwave (Terrestrial)
- Far Infrared
- Peak at 10 μm
- Earth's emission
## Visible Spectrum
- Violet (0.4 μm)
- Blue
- Green
- Yellow
- Orange
- Red (0.7 μm)Trap: Option B claims infrared dominates insolation — actually visible light (45%) dominates
Trap: Option D puts infrared in visible spectrum — infrared starts beyond red (> 0.7 μm)
Don't confuse incoming shortwave (solar) with outgoing longwave (terrestrial) radiation
Atmospheric Absorption of Infrared
Geography Infrared waves water vapour lower atmosphere
Infrared Absorption by Atmospheric Gases
Water vapour is the major absorber of infrared radiation in atmosphere
Most water vapour concentrated in troposphere (lower atmosphere)
CO₂, CH₄, O₃ also absorb infrared but less than water vapour
Why Water Vapour Dominates
Water vapour is Earth's most abundant greenhouse gas, making up 1-3% of atmosphere. It strongly absorbs longwave infrared radiation emitted by Earth's surface, creating the natural greenhouse effect.
Infrared Absorption by Gases
Gas | Concentration | IR Absorption | Atmospheric Layer |
|---|---|---|---|
Water Vapour (H₂O) | 0.1-3% | Strongest | Troposphere (0-12 km) |
Carbon Dioxide (CO₂) | 0.04% | Moderate | Well-mixed throughout |
Methane (CH₄) | 0.0002% | Strong per molecule | Well-mixed throughout |
Ozone (O₃) | Variable | Moderate | Stratosphere peak |
Nitrogen (N₂) | 78% | None | Throughout atmosphere |
Greenhouse Process
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Solar Radiation In**
**Shortwave** radiation penetrates atmosphere`"]
s2["`**Surface Heating**
Earth's surface absorbs solar energy and warms up`"]
s3["`**Infrared Emission**
Warm surface emits **longwave infrared** radiation upward`"]
s4["`**Atmospheric Absorption**
**Water vapour** and other greenhouse gases absorb infrared`"]
s5["`**Re-radiation**
Atmosphere radiates energy both **up** and **down**`"]
s6["`**Surface Warming**
Downward radiation **warms** Earth's surface further`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5
s5 --> s6Question Connection
This question tested understanding that water vapour concentrated in lower atmosphere (troposphere) is the primary absorber of infrared radiation — making Option C correct.
Water vapour vs carbon dioxide: H₂O is the dominant greenhouse gas, not CO₂
Lower atmosphere = troposphere (0-12 km) where most water vapour exists
Don't confuse absorption (trapping energy) with scattering (redirecting light)
Shortwave vs Longwave Radiation
Geography
Shortwave vs Longwave Radiation: Key Differences
Sun emits shortwave (0.15-4 μm), Earth emits longwave (4-100 μm)
Temperature determines wavelength: hotter objects emit shorter wavelengths
Atmosphere is transparent to shortwave, opaque to longwave
Shortwave vs Longwave Comparison
Property | Shortwave | Longwave |
|---|---|---|
Source | Sun (5,800K) | Earth (288K) |
Wavelength Range | 0.15 - 4.0 μm | 4.0 - 100 μm |
Peak Wavelength | 0.5 μm (visible) | 10 μm (thermal infrared) |
Atmospheric Behavior | Mostly transmitted | Largely absorbed |
Key Absorbers | O₃, water droplets | H₂O, CO₂, CH₄ |
Common Name | Solar radiation | Terrestrial radiation |
Radiation Spectra
Source: SunWind Solar — Solar Energy • SunWind Solar · sunwindsolar.com
Wien's Law Application
Wien's displacement law explains the difference: peak wavelength = 2,900/T(K). Sun at 5,800K peaks at 0.5 μm (visible), Earth at 288K peaks at 10 μm (infrared).
Shortwave = solar radiation, longwave = terrestrial radiation (not about distance traveled)
4 μm is the boundary: shorter = shortwave, longer = longwave
Visible light is shortwave, thermal infrared is longwave — don't mix them up