The earth atmosphere is mainly heated by which one of the following ?

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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 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.

Concept: 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.

State PCS 2022 Q93 — Geography PYQ with Answer

State PCS 2022 Q93 — Geography PYQ with Answer