CDS (I) 2022 General Knowledge · Question 88 of 120

Correct — A, A snow covered mountain. Albedo is the fraction of incoming solar radiation a surface reflects rather than absorbs, and it depends on how bright and how smooth the surface is. Fresh clean snow is the brightest natural land surface on Earth and reflects roughly 80 to 90 per cent of the sunlight falling on it, which is an albedo of about 0.8 to 0.9. Nothing else on the list comes close. A sandy desert is bright but nowhere near as bright, reflecting something like a quarter to two-fifths of incoming light; a barren rocky surface is darker still; and a mangrove forest, with a dense dark canopy standing over water, is among the lowest-albedo surfaces there is, absorbing the great majority of what reaches it. For scale, the Earth as a whole has a mean albedo of about 0.30. The high albedo of snow is also why snow cover is self-reinforcing while it lasts and why its loss accelerates warming — the surface underneath absorbs far more of the same sunlight.

• (b)A barren rocky piedmont surface — A barren rocky piedmont surface reflects only a modest share of the light falling on it — bare rock and soil are typically well under half as reflective as fresh snow, and darker rock is lower still.
• (c)A sandy desert — A sandy desert is the strongest of the three challengers and the intended trap, since deserts look dazzling. Dry light sand reflects roughly 25 to 40 per cent of incoming radiation, which is high for a land surface but only about half what fresh snow reflects.
• (d)A mangrove forest — A mangrove forest is the lowest of the four. A dense dark canopy traps light by multiple reflection within the foliage, and the water beneath it absorbs strongly at most angles, so very little is returned to the sky.

Albedo governs how much of the sun's energy a surface keeps. It is measured as a fraction between 0 and 1, or as a percentage, and it varies with the colour and roughness of the surface, the wetness of the ground, and the angle at which the light arrives — a low sun over water gives a much higher reflection than a high one. In the planetary energy budget, about 30 per cent of the incoming solar radiation is returned to space by clouds, aerosols and the surface together, and the rest is absorbed and later re-radiated as long-wave infrared, which is what actually warms the atmosphere from below.

Any albedo ranking question can be answered by asking which surface looks brightest to the eye, because visible brightness under reflected light is what albedo measures. Snow first, then dry pale sand, then bare rock and soil, then grass, then dense forest, with dark coniferous forest and open water at low sun angle near the bottom. Sandy desert is the option designed to draw candidates who over-correct — deserts do have a notably high albedo for a land surface, and their strong radiative cooling at night is one consequence, but they are still only about half as reflective as fresh snow. The word fresh matters too: old snow that has compacted and picked up dust falls a long way, and that darkening is a live concern in the Himalaya, where deposited black carbon lowers the albedo of glacier surfaces and speeds their melting.

• Fresh clean snow has an albedo of about 0.8 to 0.9 — it reflects 80 to 90 per cent of incoming solar radiation.
• Dry sandy desert reflects roughly 25 to 40 per cent; bare rock and soil are lower.
• Dense forest, including mangrove, is among the lowest-albedo land surfaces.
• The Earth's mean albedo is about 0.30.
• Albedo measures visual brightness under reflected light, which is why the eye is a reliable guide to ranking surfaces.

• Choosing sandy desert because deserts appear dazzling; their albedo is high for land but roughly half that of fresh snow.
• Assuming a forest has a high albedo because it is green; dense canopies trap light and reflect very little.
• Treating all snow alike; aged, compacted or dust-laden snow has a much lower albedo than fresh snow.

As a highest-or-lowest ranking among surfaces, as a decreasing-order sequence of ecosystems, or as a statements item on what albedo measures.

• practice — not a real PYQ

  Which one of the following surfaces has the lowest albedo?

  • (a)Fresh snow
  • (b)Dry sandy desert
  • (c)Dense evergreen forest
  • (d)Bare light-coloured soil

  Answer(c) Dense evergreen forest — a dark canopy traps light within the foliage and reflects very little back to the sky.

• practice — not a real PYQ

  The average albedo of the Earth as a whole is closest to

  • (a)0.10
  • (b)0.30
  • (c)0.50
  • (d)0.80

  Answer(b) 0.30 — about thirty per cent of incoming solar radiation is returned to space by clouds, aerosols and the surface together.