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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2018 · Paper I

Q4(b)(ii). Cold air descending into a valley

  • 5 marks
  • Answered
Question

What happens to the air temperature in a valley when cold air blowing from the mountain tops descends into the valley?

Answer in brief

The air warms. As it descends it is compressed adiabatically by the rising pressure; the work done on it raises its internal energy, by about 9.8 K for every km of descent. Air that is cold on the summit arrives in the valley warm and dry (föhn or chinook wind).

Key points

  • Pressure rises towards the valley floor, so the sinking air is compressed. Large masses of air exchange little heat with their surroundings on the way down: the process is nearly adiabatic.
  • q=0, so Δ U=w, and w>0 because work is done on the air. U and so T rise: T₂=T₁(p₂/p₁)^((γ-1)/γ).
  • The warming is the dry adiabatic lapse rate run backwards, g/cₚ≈ 9.8 K per km. Air at 0°C on a ridge 2 km above the valley arrives at about +20°C, much warmer than it was on the summit.
  • The air carries no more water, but warmer air can hold more, so its relative humidity falls: the valley wind is warm and dry. Such winds (föhn in the Alps, chinook in the Rockies) melt snow quickly and dry out vegetation.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. Pressure rises towards the valley floor, so the sinking air is compressed. Large masses of air exchange little heat with their surroundings on the way down: the process is nearly adiabatic.
  2. First law: q=0, so Δ U=w, and w>0 because work is done on the air. U and so T rise: T₂=T₁(p₂/p₁)^((γ-1)/γ).
  3. The warming is the dry adiabatic lapse rate run backwards, g/cₚ≈ 9.8 K per km. Air at 0°C on a ridge 2 km above the valley arrives at about +20°C, much warmer than it was on the summit.
  4. The air carries no more water, but warmer air can hold more, so its relative humidity falls: the valley wind is warm and dry. Such winds (föhn in the Alps, chinook in the Rockies) melt snow quickly and dry out vegetation.
  5. The air is cold on the mountain top only because it is high up; compression, not heat from the ground, warms it in the valley.

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Model answer to Q4(b)(ii), 2018 Paper I: solution page 25
Solution page 25 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 4