NDA (II) 2021 General Ability Test · Question 63 of 149

Correct — C, Long wave terrestrial radiation. Incoming short-wave solar radiation passes largely through the atmosphere without heating it much and warms the Earth's surface instead. The warmed surface then re-radiates energy as long-wave (infrared) terrestrial radiation, which greenhouse gases such as water vapour and carbon dioxide absorb strongly. So the atmosphere is heated mainly from below by long-wave terrestrial radiation — which is exactly why air temperature normally falls with height in the troposphere.

• (a)Short wave solar radiation — The atmosphere is fairly transparent to incoming short-wave solar radiation; most of it passes through to heat the ground rather than the air itself.
• (b)Reflected solar radiation — Reflected solar radiation is short-wave energy bounced back to space; it is redirected, not absorbed as heat by the atmosphere.
• (d)Scattered solar radiation — Scattering merely changes the direction of short-wave sunlight (it is why the sky is blue); it does not account for the bulk of atmospheric heating.

The atmosphere absorbs only a small share of the incoming short-wave solar radiation, so sunlight mostly heats the Earth's surface. The heated surface emits long-wave (infrared) terrestrial radiation, and greenhouse gases absorb this efficiently. The atmosphere is therefore warmed chiefly from below — this is the greenhouse effect.

The three tempting options all involve incoming solar energy, which feels like the obvious heat source. The key insight is that the air is heated indirectly: the ground absorbs sunlight and then radiates long-wave heat that the atmosphere can trap.

• The atmosphere is largely transparent to incoming short-wave solar radiation.
• The Earth's surface, once warmed, emits long-wave (infrared) terrestrial radiation.
• Greenhouse gases such as water vapour and carbon dioxide strongly absorb long-wave radiation.
• Because heating comes from below, temperature usually falls with height in the troposphere (the normal lapse rate).

• Assuming the Sun's incoming short-wave radiation heats the air directly — it mostly heats the ground first.
• Confusing reflected and scattered solar radiation (redirected short-wave energy) with the long-wave heat the atmosphere actually absorbs.

Asked as 'the atmosphere is mainly heated by' — recall that the air is warmed from below by terrestrial long-wave radiation.

• practice — not a real PYQ

  Temperature normally decreases with height in the troposphere mainly because

  • (a)the Sun heats the upper atmosphere first
  • (b)the atmosphere is heated from below by terrestrial radiation
  • (c)air pressure increases with height
  • (d)solar radiation is reflected upward

  Answer(b) the atmosphere is heated from below by terrestrial radiation — so lower layers are warmer.

• practice — not a real PYQ

  Greenhouse gases in the atmosphere are strong absorbers of

  • (a)short-wave solar radiation
  • (b)long-wave (infrared) terrestrial radiation
  • (c)reflected sunlight
  • (d)ultraviolet radiation only

  Answer(b) long-wave (infrared) terrestrial radiation — the basis of the greenhouse effect.