The Insolation Archives

1. Which one of the following is not a factor that causes variations in

Which one of the following is not a factor that causes variations in ‘insolation’?

Rotation of the Earth on its axis

Angle of inclination of the Sun's rays

Length of the day

Vegetation cover on the Earth's surface

This question was previously asked in

UPSC CBI DSP LDCE – 2023

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The correct option is D.

– Rotation of the Earth on its axis causes the daily cycle of day and night, leading to daily variations in insolation received at any point.
– Angle of inclination of the Sun’s rays varies with latitude and season (due to Earth’s tilt), affecting the intensity of insolation received per unit area. Lower angles (higher latitudes/winter) result in less intense insolation.
– Length of the day varies with latitude and season, determining the duration for which insolation is received, thus affecting the total amount of insolation received daily.
– Vegetation cover on the Earth’s surface affects how much of the received insolation is absorbed, reflected (albedo), or used in processes like transpiration. It does **not** affect the amount of insolation arriving from the sun at the top of the atmosphere or at the surface before interaction with the cover.

Factors affecting insolation received from the sun include solar constant, distance from the sun (Earth’s elliptical orbit), angle of incidence of sun’s rays (latitude, season, time of day), length of the day, and atmospheric transparency (clouds, aerosols, gases). Vegetation affects the Earth’s energy balance *after* insolation arrives.

2. With reference to the Earth’s atmosphere, which one of the following s

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

The total amount of insolation received at the equator is roughly about 10 times of that received at the poles.

Infrared rays constitute roughly two-thirds of insolation.

Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.

Infrared waves are a part of visible spectrum of electromagnetic waves of solar radiation.

This question was previously asked in

UPSC IAS – 2023

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The correct answer is C) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.

Statement C is correct. Water vapour is a potent greenhouse gas and strongly absorbs infrared radiation, particularly the outgoing longwave radiation emitted by the Earth’s surface. Water vapour is indeed concentrated in the lower atmosphere (troposphere). This absorption is a key mechanism of the greenhouse effect.

Statement A is incorrect; the total amount of insolation at the equator is roughly double (not 10 times) the average received at the poles over a year, due to the lower angle of incidence at the poles and longer atmospheric path. Statement B is incorrect; while infrared is part of insolation, it constitutes roughly half (around 49-50%) of the total solar radiation received at the top of the atmosphere, not two-thirds. Visible light constitutes about 43%. Statement D is incorrect; infrared waves have longer wavelengths than visible light and are not part of the visible spectrum. The visible spectrum ranges from approximately 400 nm (violet) to 700 nm (red), while infrared radiation has wavelengths from about 700 nm up to 1 mm.

3. Consider the following statements regarding the temperature of the Ear

Consider the following statements regarding the temperature of the Earth’s surface:

1. The temperature of a surface is determined by net radiation
2. Net radiation produces a radiant energy flow that can heat or cool a surface
3. Net radiation balance is usually positive during the daytime and negative during night time
4. Soil surface loses energy as temperature increases during the daytime

Which of the statements given above are correct?

1 and 2 only

3 and 4 only

1, 2 and 3 only

1, 2, 3 and 4

This question was previously asked in

UPSC CAPF – 2024

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The correct answer is C) 1, 2 and 3 only.

Statements 1, 2, and 3 accurately describe aspects of the Earth’s surface temperature and net radiation. The temperature of a surface is fundamentally determined by the balance of incoming and outgoing energy fluxes, which is often dominated by radiation (net radiation). A positive net radiation leads to a net gain of energy and thus heating, while a negative net radiation leads to a net loss of energy and thus cooling. During the daytime, incoming solar radiation is high, typically resulting in a positive net radiation balance. At night, with no incoming solar radiation, the surface loses energy primarily through longwave radiation, leading to a negative net radiation balance.

Statement 4 is incorrect. During the daytime, as the soil surface temperature increases due to a positive net energy balance, the rate of outgoing longwave radiation and other energy losses (like sensible and latent heat) increases. However, the surface is *gaining* energy overall (positive net radiation minus other fluxes) which *causes* the temperature to increase. The statement “loses energy as temperature increases during the daytime” is misleading and contradicts the fact that the surface is heating up due to a net energy gain.

4. The following item consists of two statements, Statement I and Stateme

The following item consists of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below :
Statement I : All the energy received by the earth is from the Sun through electro-magnetic radiation
Statement II : The earth also radiates back all the received energy through various ways to maintain the heat budget of the planet

Both the statements are individually true and Statement II is the correct explanation of Statement I

Both the statements are individually true but Statement II is NOT the correct explanation of Statement I

Statement I is true but Statement II is false

Statement I is false but Statement II is true

This question was previously asked in

UPSC CAPF – 2015

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The correct answer is C. Statement I is true, but Statement II is false based on a strict interpretation.

Statement I: All the energy received by the earth is from the Sun through electro-magnetic radiation. This statement is true. The Sun is Earth’s primary energy source, and solar energy reaches Earth as electromagnetic radiation (visible light, infrared, UV, etc.). Internal heat and tidal energy sources are negligible in comparison to the solar input in the context of the overall energy budget driving climate.
Statement II: The earth also radiates back all the received energy through various ways to maintain the heat budget of the planet. This statement is false as worded. The Earth does radiate energy back into space (primarily infrared radiation) to maintain a thermal balance. However, it does not radiate *all* the received energy. A significant portion of incoming solar radiation (about 30%) is reflected back to space by clouds, aerosols, and the Earth’s surface (this is known as albedo). The Earth absorbs the remaining energy (about 70%) and then radiates an equivalent amount of energy as infrared radiation on average to achieve radiative equilibrium. The statement “all the received energy” is inaccurate because it ignores the reflected portion. Furthermore, the current climate change indicates that the heat budget is currently not perfectly maintained; the Earth is absorbing slightly more energy than it radiates back due to the greenhouse effect.

The concept described in Statement II is related to Earth’s energy balance and radiative equilibrium. For a stable temperature, the energy absorbed by Earth must equal the energy radiated back into space. However, Statement II incorrectly states that “all the received energy” is radiated back, neglecting the reflected portion of solar radiation which is also part of balancing the energy budget. The primary way Earth radiates energy is through longwave (infrared) electromagnetic radiation. While Statement II captures the general idea of radiative balance, its specific wording makes it false.

5. Consider the following statements: 1. The Earth receives the Sun’s e

Consider the following statements:

1. The Earth receives the Sun’s energy at the infrared end of the spectrum.
2. The Earth re-radiates the Sun’s heat as ultraviolet energy.

Which of the above statements is/are correct ?

1 only

2 only

Both 1 and 2

Neither 1 nor 2

This question was previously asked in

UPSC CAPF – 2009

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Neither statement 1 nor statement 2 is correct.

The Earth receives energy from the Sun primarily in the form of visible light, with significant contributions from infrared and ultraviolet radiation. The peak emission of the Sun’s radiation is in the visible spectrum. The Earth, being much cooler than the Sun, re-radiates absorbed energy primarily as longwave infrared radiation.

Statement 1 is incorrect because the Earth receives energy across the solar spectrum, not just the infrared end. Statement 2 is incorrect because the Earth re-radiates energy as infrared (heat) radiation, not ultraviolet radiation. The greenhouse effect is caused by atmospheric gases absorbing this outgoing infrared radiation.

6. Which one of the following statements about North-facing slopes and So

Which one of the following statements about North-facing slopes and South-facing slopes in the Northern Hemisphere is not correct?

North-facing slopes are snow covered while South-facing slopes are bare.

North-facing slopes get less intense radiation as compared to South-facing slopes.

Snow on North-facing slopes is found at a lower altitude than South-facing slopes.

Tree line characteristics on North-facing slopes and South-facing slopes are similar.

This question was previously asked in

UPSC Geoscientist – 2022

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In the Northern Hemisphere, South-facing slopes receive more direct solar radiation than North-facing slopes. This difference in insolation leads to significant differences in temperature, soil moisture, snow cover duration, and ultimately, vegetation patterns and species distribution. The tree line, which is the upper limit of tree growth limited by cold temperatures and snow duration, is typically found at a lower altitude on colder, wetter North-facing slopes compared to warmer, drier South-facing slopes. Therefore, the tree line characteristics are generally *not* similar on North-facing and South-facing slopes.

– Solar radiation intensity (B) is higher on South-facing slopes in the Northern Hemisphere.
– Lower insolation on North-facing slopes leads to lower temperatures and longer-lasting snow cover (A), often resulting in snow being present at lower altitudes (C).
– These environmental differences (temperature, moisture, snow) drive differences in plant communities and ecological boundaries like the tree line.

This phenomenon of ecological differences based on aspect (the direction a slope faces) is well-studied and results in distinct microclimates and vegetation zones on opposite-facing slopes within the same mountain range or area. The effect is reversed in the Southern Hemisphere, where North-facing slopes receive more direct solar radiation.

7. Insolation percentage received on Pole as compared to Equator is

Insolation percentage received on Pole as compared to Equator is approximately:

42 percent.

68 percent.

33 percent.

47 percent.

This question was previously asked in

UPSC Geoscientist – 2020

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Due to the angle of the Earth’s tilt and the curvature of the Earth, the sun’s rays strike the poles at a much more oblique angle compared to the equator. This oblique angle means the solar radiation is spread over a larger area and passes through a thicker layer of the atmosphere, where it is absorbed, scattered, and reflected more significantly. On an annual average basis, the insolation received at the poles is significantly less than at the equator, typically around 30-40% of the equatorial value. 33% is a commonly cited approximation for the annual average insolation received at the poles compared to the equator.

The amount of solar radiation (insolation) received on the Earth’s surface varies geographically and seasonally due to factors like the angle of incidence of sunlight, atmospheric thickness, duration of daylight, and albedo.

While poles receive very low annual average insolation, they experience extreme seasonal variations, including periods of 24-hour daylight or darkness. The equator, in contrast, receives relatively consistent high levels of insolation throughout the year.

8. Which of the following statements regarding insolation is/are correct?

Which of the following statements regarding insolation is/are correct?

1. Insolation is predominantly short-wave 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.

1 only

2 only

Both 1 and 2

Neither 1 nor 2

This question was previously asked in

UPSC CDS-1 – 2024

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Statement 1 is correct regarding insolation. Statement 2 is incorrect.

Insolation (incoming solar radiation) is predominantly in the form of short-wave radiation, primarily visible light. However, its distribution across the Earth’s surface is highly uneven due to factors like the Earth’s curvature, axial tilt, rotation, and atmospheric conditions.

1. Solar radiation covers a spectrum from ultraviolet to infrared. The visible light portion (approximately 0.39 to 0.76 micrometre) constitutes the peak intensity and a large proportion of the total solar energy, hence it is considered predominantly short-wave radiation.
2. The Earth’s curved surface means that the angle at which solar rays strike the surface varies with latitude. Rays hit more directly (perpendicularly) near the equator and more obliquely near the poles, spreading the energy over a larger area and thus reducing the intensity per unit area at higher latitudes. This, along with atmospheric effects and albedo differences, leads to significant variations in insolation received across the globe, making the distribution uneven.