UPPSC 2022 Prelims GS Paper-I · Question 93 of 150

Correct — C, Long wave terrestrial radiation. The atmosphere is heated from below, not from above. Incoming sunlight arrives as short-wave radiation, and the air is largely transparent to it — the atmosphere absorbs only about twenty-three per cent of the incoming short-wave stream, and most of the rest either reflects away or passes straight through to the ground. The land and sea surface absorbs that energy, warms, and re-radiates it as long-wave infrared, and to this the atmosphere is nearly opaque: it absorbs roughly ninety per cent of the long-wave radiation emitted by the surface, because water vapour, carbon dioxide, methane, nitrous oxide and ozone all have vibrational modes that take up infrared energy. So the air gets its heat second-hand, from the ground that the Sun warmed. The everyday proof is the normal lapse rate. Temperature falls with height in the troposphere at about 6.5 degrees Celsius per kilometre, and a mountaintop is colder than the plain below it even though it is nearer the Sun. If sunlight heated the air directly, the opposite would be true.

• (a)Scattered solar radiation — Scattering redirects short-wave radiation without converting it into heat in the air. Molecules and fine particles deflect sunlight in all directions — the process that makes the daytime sky blue and gives us diffuse daylight in shade — but the photon carries on with its energy intact until something absorbs it. Redirected energy is not absorbed energy, so scattering contributes almost nothing to warming the atmosphere.
• (b)Short wave solar radiation — This is the intuitive answer and the one the question is built to catch. Short-wave solar radiation is the ultimate source of nearly all the energy in the system, but it is not what the atmosphere absorbs. Direct absorption of incoming sunlight accounts for only about a quarter of the short-wave stream, mainly by ozone in the stratosphere and by water vapour and cloud droplets lower down; the atmosphere is heated several times more by the long-wave radiation coming back up from the surface. UPSC set exactly this trap in 2024 and marked the corresponding statement incorrect.
• (d)Reflected solar radiation — Reflected sunlight is energy the Earth loses, not energy it gains. About thirty per cent of incoming solar radiation is returned to space by clouds, aerosols, ice, snow and bright land surfaces — that fraction is the planetary albedo. Because it leaves without being absorbed, it plays no part in heating the air.

Radiation is short-wave or long-wave according to the temperature of the body emitting it. The Sun, at about 6,000 kelvin, emits mostly in the visible and near-infrared — short wavelengths. The Earth's surface, at roughly 288 kelvin, emits in the thermal infrared — long wavelengths. The atmosphere's absorbing gases are selective: they let short waves through but intercept long waves. That asymmetry is the greenhouse effect. Of the sunlight reaching the top of the atmosphere, about thirty per cent is reflected straight back (the albedo), about twenty-three per cent is absorbed within the atmosphere, and roughly the remaining half reaches and is absorbed by the surface. The surface then passes that heat upward in three ways — long-wave radiation, conduction to the air in contact with it, and convection, which lifts warmed and moist air — with long-wave radiation the dominant term.

Most candidates lose this question by reasoning that the Sun heats everything, so sunlight must heat the air. The examiner wants the intermediate step. Ask instead: what does the atmosphere actually absorb? Two of the four options — scattered and reflected radiation — describe energy that is redirected rather than taken up, so they can be eliminated on principle before any figures are needed. That leaves the short-wave and long-wave streams, and the decisive evidence is that air temperature decreases upward in the troposphere. The layer nearest the ground is warmest because the ground is the heat source. Note where this reasoning stops: in the stratosphere temperature rises with height, because ozone absorbs ultraviolet directly there, which is a genuine case of the atmosphere being heated by solar radiation and is exactly why the stratosphere is stable and cloud-free.

• The atmosphere absorbs only about twenty-three per cent of incoming short-wave solar radiation, but about ninety per cent of the long-wave infrared radiation emitted by the Earth's surface.
• The Earth's planetary albedo is about 0.30 — roughly thirty per cent of incoming solar radiation is reflected back to space by clouds, aerosols, ice, snow and bright surfaces.
• Because the atmosphere is heated from below, temperature falls with height in the troposphere at a normal lapse rate of about 6.5 degrees Celsius per kilometre.
• The infrared-absorbing gases are water vapour, carbon dioxide, methane, nitrous oxide and ozone; nitrogen, oxygen and argon are transparent to infrared and are not greenhouse gases.
• In the stratosphere the pattern reverses and temperature increases with height, because ozone there absorbs ultraviolet radiation from the Sun directly.

• Answering 'short wave solar radiation' because the Sun is the ultimate energy source — the question asks what the atmosphere absorbs, not where the energy originates
• Treating scattered or reflected radiation as heating agents; both redirect energy without the air taking it up
• Generalising 'the atmosphere is heated from below' to the whole atmosphere — in the stratosphere ozone absorbs ultraviolet directly and temperature rises with height

UPPSC asks this as a one-line recall question about the heat budget — what heats the atmosphere, what albedo means, which layer holds the ozone — while UPSC builds it into two-statement or statement-and-explanation formats that pair the heating mechanism with the infrared-absorbing property of greenhouse gases.

• practice — not a real PYQ

  In the troposphere, temperature normally decreases with increasing altitude. The principal reason is that

  • (a)the air becomes thinner and therefore holds fewer greenhouse gases at the top of the atmosphere
  • (b)the atmosphere is heated mainly by long-wave radiation from the Earth's surface, so the layers nearest the ground are warmest
  • (c)ultraviolet radiation is absorbed only at high altitude
  • (d)convection currents carry heat downward from the tropopause

  Answer(b) the atmosphere is heated mainly by long-wave radiation from the Earth's surface, so the layers nearest the ground are warmest — the normal lapse rate of about 6.5 degrees Celsius per kilometre is the direct evidence for heating from below.

• practice — not a real PYQ

  The 'albedo' of the Earth refers to

  • (a)the fraction of incoming solar radiation reflected back to space without being absorbed
  • (b)the total long-wave radiation emitted by the Earth's surface in a year
  • (c)the rate at which temperature falls with height in the troposphere
  • (d)the proportion of solar radiation absorbed by ozone in the stratosphere

  Answer(a) the fraction of incoming solar radiation reflected back to space without being absorbed — about 0.30 for the Earth as a whole, higher over fresh snow and cloud and lower over dark ocean and forest.