CDS (II) 2022 General Knowledge · Question 46 of 120

Correct — A, 1 and 2 only. The first two statements are the textbook description almost word for word, and the third fails on its last four words. NCERT's chapter on solar radiation and temperature says: 'Normally, temperature decreases with increase in elevation. It is called normal lapse rate. At times, the situations is reversed and the normal lapse rate is inverted. It is called Inversion of temperature. Inversion is usually of short duration but quite common nonetheless. A long winter night with clear skies and still air is ideal situation for inversion.' That gives statement 1 — temperature rising with altitude is the definition of an inversion — and statement 2, whose three conditions each do real work: a long night gives the ground time to lose heat, clear skies let that heat radiate away instead of being trapped by cloud, and still air stops mixing from evening out the layers. Statement 3 begins correctly and then makes a claim NCERT flatly contradicts in the very next sentence: 'Over polar areas, temperature inversion is normal throughout the year.' The poles are not the exception where inversion is absent; they are the one place where it is permanent.

• (b)2 and 3 only — Rejects the definition itself. Statement 1 is what an inversion is — the normal lapse rate turned upside down — so no reading of the phenomenon can leave it out.
• (c)1 only — Keeps the definition but drops the conditions, and statement 2 is the standard description of an ideal night for a surface inversion: long, clear and still.
• (d)1, 2 and 3 — The all-of-them pick, and it fails on statement 3's tail. 'Short-term' and 'common all over the globe' are both fair; 'except at the poles' is exactly wrong, because over polar areas inversion is normal all year round.

Air temperature normally falls with height at about 6.5 degrees Celsius per kilometre in the troposphere, and that is the normal lapse rate. When a layer of warm air comes to sit over a layer of cooler air the profile is inverted. The commonest kind is the surface or radiation inversion of a clear winter night: the ground radiates its heat to space, the air touching it cools by conduction, and by dawn the coldest air is at the bottom. Inversions also occur in valleys through air drainage, where cold dense air behaves almost like water and flows down the slopes to pool in the valley bottom with warmer air above — which is why orchards are often planted on hillsides rather than in the hollow. Because the cold layer at the bottom is heavy, an inversion is intensely stable: nothing rises through it, and whatever is emitted underneath stays trapped.

The item is built the way these three-statement questions usually are — two easy statements to establish confidence and a third that starts true and turns false at the end. Read the tail of every statement before marking it. The tail here, 'except at the poles', is not a small qualification but a reversal of the actual position, and the sentence it contradicts sits directly beneath the sentence the earlier statements were taken from. The practical importance of an inversion is worth carrying alongside the definition: the trapped layer is exactly why smog forms over Delhi and other north Indian cities in winter, with pollutants held under an inversion lid until the sun is strong enough to break it, and why cold air pooling in valleys causes frost damage to crops on the valley floor. One printed slip in the source text worth noting for accuracy: NCERT's own sentence reads 'the situations is reversed', which is the textbook's error rather than the paper's.

• The normal lapse rate is a fall of about 6.5 degrees Celsius per kilometre in the troposphere; an inversion is that profile reversed.
• A long winter night with clear skies and still air is the ideal situation for a surface inversion.
• Inversion is usually of short duration but quite common; over polar areas it is normal throughout the year.
• Air drainage produces valley inversions — cold dense air flows down the slopes and pools at the bottom, protecting hillside plants from frost.
• An inversion layer is very stable and prevents vertical mixing, which is why it traps smoke and pollutants and produces winter smog.

• Marking statement 3 correct because its first half sounds right; the qualification at the end is where it fails.
• Believing inversions do not occur at the poles, when polar inversions are the most persistent of all.
• Confusing an inversion with the ordinary lapse rate, or forgetting that the stratosphere is itself an inversion layer.

As a two- or three-statement item on the definition and the ideal conditions, or through its consequence — smog, frost, or the stability of the trapped layer.

• practice — not a real PYQ

  The pooling of cold, dense air in valley bottoms with warmer air above, which produces frost in the hollows while hillsides stay warmer, is called

  • (a)air drainage
  • (b)adiabatic cooling
  • (c)advection
  • (d)convection

  Answer(a) air drainage — the cold air behaves almost like water, flowing down the slope and settling at the bottom.

• practice — not a real PYQ

  Winter smog over north Indian cities is aggravated chiefly because a temperature inversion

  • (a)increases wind speed near the ground
  • (b)prevents vertical mixing and traps pollutants near the surface
  • (c)raises the temperature of the ground
  • (d)causes heavy rainfall that washes dust into the air

  Answer(b) prevents vertical mixing and traps pollutants near the surface — the inversion acts as a lid over the city.