UPPSC 2021 Prelims GS Paper-I · Question 13 of 150

Correct — C, Land covered with fresh snow. The share of incoming sunlight a surface reflects is its albedo. Fresh, clean snow has the highest albedo of any common land surface — about 0.80 to 0.90 (80 to 90 percent). Sand deserts (about 0.25 to 0.40), paddy / crop land (about 0.15 to 0.25) and prairie / grassland (about 0.20) reflect far less and absorb the rest. So land under fresh snow reflects back the most sunlight.

• (a)Sand Desert — Bright to the eye, but a sand desert's albedo is only about 0.25 to 0.40 — well below fresh snow; it absorbs most of the sunlight that strikes it (hence very hot days).
• (b)Paddy crop land — A vegetated, often wet surface with low albedo (about 0.15 to 0.25); green leaves absorb most incoming light for photosynthesis and heating.
• (d)Prairie land — Grassland / prairie has an albedo around 0.20 — it reflects only a small fraction of sunlight compared with the roughly 0.85 of fresh snow.

Albedo is the fraction of incident solar radiation a surface reflects (0 = perfect absorber, 1 = perfect reflector). Light, white, dry surfaces have high albedo; dark, wet, vegetated surfaces have low albedo. Fresh snow sits at the very top of the scale; as snow ages and gets dirty its albedo falls. Albedo drives the ice-albedo feedback: when snow or ice melts it exposes darker ground or water that absorbs more heat, which accelerates further warming.

Rank the four options by how white, bright and dry each is. Fresh snow is the whitest, so it has the highest albedo and reflects the most sunlight. This is a recurring physical-geography idea — and UPSC asked this exact question, word for word, in 2010.

• Albedo = fraction of sunlight reflected; fresh snow is highest at about 0.80 to 0.90.
• Sand desert about 0.25 to 0.40; grassland / prairie about 0.20; paddy / crop land about 0.15 to 0.25.
• Dark, wet, vegetated surfaces have low albedo and absorb more sunlight.
• The ice-albedo feedback amplifies polar warming as reflective ice is lost.

• Thinking a bright sand desert reflects the most (fresh snow is roughly twice to three times higher)
• Confusing albedo (reflectivity) with emissivity (how a surface radiates heat)

UPPSC and UPSC ask 'which surface reflects most sunlight / has the highest albedo', or give a set of ecosystems to arrange in decreasing order of albedo.

• practice — not a real PYQ

  Which of the following surfaces has the LOWEST albedo (absorbs the most sunlight)?

  • (a)Fresh snow
  • (b)Dense evergreen forest / open ocean
  • (c)Dry desert sand
  • (d)Dry grassland

  Answer(b) Dense evergreen forest / open ocean — dark surfaces reflect very little (ocean albedo is only about 0.06).

• practice — not a real PYQ

  The ice-albedo feedback tends to:

  • (a)Slow polar warming
  • (b)Accelerate polar warming as reflective ice melts
  • (c)Have no effect on temperature
  • (d)Cool the tropics

  Answer(b) Accelerate polar warming — melting ice exposes darker surfaces that absorb more heat.