NDA (I) 2018 General Ability Test · Question 77 of 150

Correct — C, Both 1 and 2. Statement 1 describes the equatorial trough exactly. Intense heating along the Equator makes air rise, which leaves a belt of low pressure at the surface; because the air is going up rather than sideways, surface winds there are feeble and fickle, and sailing ships could lie becalmed in them for days. That is why the belt is called the doldrums. Statement 2 describes the Chinook correctly too. It is a warm, dry wind that pours down the eastern slopes of the Rocky Mountains in North America during winter, and it can lift the air temperature by tens of degrees within a few hours, stripping snow off the ground fast enough to have earned it the local name snow-eater. Both statements stand, so the answer is both.

• (a)1 only — This rejects the Chinook description, but every part of it is right — warm, dry, a winter wind of the Rockies' eastern slope, and famous precisely for how quickly it raises the temperature.
• (b)2 only — This rejects the doldrums description, which is also right. The equatorial belt is a low-pressure zone of rising air with weak and variable surface winds.
• (d)Neither 1 nor 2 — A blanket rejection when both statements are textbook-accurate. Candidates sometimes reach for this because 'calm' feels wrong next to the heavy thunderstorms of the equatorial belt — but calm here describes the horizontal wind, not the weather.

The doldrums is the surface expression of the Inter-Tropical Convergence Zone, the belt where the north-east and south-east trade winds meet. Convergence forces air upward, so pressure at the surface is low, horizontal winds are weak, and the rising moist air builds towering cloud and heavy rain. The Chinook belongs to a different family altogether — the foehn-type winds, which form when air is forced over a mountain range, loses its moisture as rain or snow on the windward side, and then descends the lee side warming by compression. Because it has already been wrung dry, the descending air heats faster than it cooled on the way up, and arrives hot and parched.

Two things make this item easy to get wrong. The first is that 'calm' sounds like the opposite of the doldrums' weather, which is stormy and wet — but the calm refers to the horizontal wind at the surface, and the storms are the vertical motion. The second is that 'hot and dry wind ... in winter' sounds self-contradictory until the foehn mechanism is understood; the wind is hot relative to the winter air it replaces, not hot in absolute terms. Learn the foehn family as one group and the questions stop being separate facts: Chinook on the lee of the Rockies in North America, Foehn on the northern flank of the Alps, Zonda on the Argentine side of the Andes, Santa Ana in southern California, and closer to home the Loo of the north Indian plains in summer, which is hot and dry for a different reason. Set against them, the Mistral of the Rhone valley and the Bora of the Adriatic are the cold local winds.

• The doldrums is the equatorial low-pressure belt where the trade winds converge and air rises, leaving weak surface winds.
• It is the surface trace of the Inter-Tropical Convergence Zone, and it migrates north and south with the overhead sun.
• The Chinook is a warm, dry lee wind of the eastern Rocky Mountains, most noticed in winter.
• It is nicknamed the snow-eater because it can clear snow cover in hours by melting and evaporating it.
• Chinook, Foehn, Zonda and Santa Ana are all foehn-type winds; Mistral and Bora are cold local winds.

• Treating 'calm' in the doldrums as a claim about the weather rather than about the horizontal wind.
• Confusing the Chinook with the Mistral or Bora, which are cold winds, not warm ones.
• Assuming a winter wind cannot be described as hot; a foehn wind is warm relative to the air it displaces.

Local winds are set either as a straight name-to-region match or, as here, as a two-statement item where one clause about temperature or moisture is quietly reversed.

• practice — not a real PYQ

  Which one of the following is a warm, dry wind that descends the leeward slope of a mountain range?

  • (a)Mistral
  • (b)Bora
  • (c)Foehn
  • (d)Blizzard

  Answer(c) Foehn — the Alpine member of the same family as the Chinook, warmed by compression as it sinks.

• practice — not a real PYQ

  The doldrums belt is best described as

  • (a)a high-pressure belt with descending air and clear skies
  • (b)a low-pressure belt of rising air with weak surface winds
  • (c)the zone of strongest trade winds
  • (d)the boundary between the westerlies and the polar easterlies

  Answer(b) a low-pressure belt of rising air with weak surface winds — the surface expression of the Inter-Tropical Convergence Zone.