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.
- (a)1 only
- (b)2 only
- (c)Both 1 and 2
- (d)Neither 1 nor 2
Correct — A, 1 only. Statement 2 is the easy kill and it inverts a real cause. Because the Earth's surface is curved, the Sun's rays fall almost vertically at the equator and at a steep slant near the poles, so the same beam of energy is smeared over a much larger area at high latitudes. Curvature is exactly why insolation is unevenly distributed, not why it is even. Statement 1 is the one the key accepts: insolation is incoming solar radiation, and it is short-wave energy — short, that is, by comparison with the long-wave infrared the Earth radiates back to space. With statement 2 rejected, only statement 1 stands, giving (a).
- (b)2 only — Statement 2 is the false one. The Earth's curvature makes insolation uneven; it is the reason the tropics receive far more energy per unit area than the poles.
- (c)Both 1 and 2 — Cannot be right because statement 2 states the opposite of the truth — the curved surface causes unevenness, not evenness.
- (d)Neither 1 nor 2 — Rejects statement 1 as well, but insolation genuinely is short-wave radiation as against the Earth's long-wave outgoing radiation, and the key accepts it.
Insolation is the solar energy received at the Earth. It arrives as short-wave radiation, peaking near half a micrometre, while the warmed Earth re-radiates in the long-wave infrared. The amount received varies with latitude, season, day length, the angle at which the rays strike, and the transparency of the atmosphere — the equator gets far more per unit area than the poles.
One honest caveat about statement 1, worth knowing even though it does not change the answer. Insolation genuinely is short-wave when set against terrestrial radiation, so the thrust of the statement is sound. But 0.39 to 0.76 micrometre is the visible band alone; measured at the surface, sunlight is roughly 42 per cent visible, 49 per cent infrared and 8 per cent ultraviolet, so short-wave solar energy runs well past the range the paper prints. The official key nevertheless marks statement 1 correct, because the point being tested is short-wave against long-wave and not the exact end-points. Author your reasoning to that distinction, answer (a), and do not let the narrow range tempt you into rejecting statement 1.
- Measured at the Earth's surface, sunlight is about 49 per cent infrared, 42 per cent visible and 8 per cent ultraviolet; the visible band runs roughly 380 to 700 nanometres.
- Solar radiation is called short-wave only relative to terrestrial radiation — the Earth re-radiates in the long-wave infrared.
- The atmosphere absorbs only about 23 per cent of incoming short-wave solar radiation but about 90 per cent of the long-wave radiation emitted by the surface, which is why the air is heated from below.
- Because the Earth is nearly spherical, the angle of incidence of sunlight falls away from the equator towards the poles, spreading the same energy over a wider area.
The curvature of the Earth is the reason insolation is unevenly distributed — which is why statement 2 is false.
- Reading statement 2 too fast and accepting the curvature clause without noticing it has been attached to the wrong conclusion.
- Assuming insolation means only visible light — visible is under half of it, with infrared the largest share.
- Confusing short-wave incoming solar radiation with long-wave outgoing terrestrial radiation when reasoning about what heats the air.
Asked as a two-statement item in which one statement quotes a real cause (the curved surface) but pairs it with the opposite effect.
With reference to the Earth's atmosphere, which one of the following statements is correct?
- (a) The total amount of insolation received at the equator is roughly about 10 times of that received at the poles
- (b) Infrared rays constitute roughly two-thirds of insolation
- (c) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere
- (d) Infrared waves are a part of visible spectrum of electromagnetic waves of solar radiation
Answer(c) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere
Same concept, and it settles the caveat behind this CDS statement. That UPSC item rejects option (b) because infrared is roughly half of insolation rather than two-thirds, and rejects option (d) because infrared lies outside the visible band — which is precisely why 0.39 to 0.76 micrometre names the visible part of insolation rather than the whole of it.
- practice — not a real PYQ
Which of the following mainly explains why the equator receives more insolation per unit area than the poles?
- (a)The equator is closer to the Sun throughout the year
- (b)Sunlight strikes the equator more nearly vertically, so a given beam covers a smaller area
- (c)The equator has a thinner atmosphere
- (d)The Earth rotates faster near the equator, drawing in more radiation
Answer(b) Sunlight strikes the equator more nearly vertically — the same beam is spread over a smaller area than at high latitudes.
- practice — not a real PYQ
The Earth's outgoing radiation differs from incoming solar radiation chiefly in that it is
- (a)long-wave infrared
- (b)ultraviolet
- (c)in the visible band
- (d)radio waves
Answer(a) long-wave infrared — the cooler Earth radiates at much longer wavelengths than the Sun, which is why solar radiation is called short-wave.