NDA (I) 2018 General Ability Test · Question 78 of 150

Correct — A, Atacama. The Atacama, a narrow strip along the Pacific coast of northern Chile, is the driest non-polar desert on earth. Some of its weather stations have gone years at a stretch without measurable rain, and parts of its core are so sterile that space agencies use them as a stand-in for Martian soil. Three separate causes stack up there. It sits under the descending limb of the subtropical high, which suppresses cloud. The cold Humboldt current chills the air over the sea so that moisture condenses into low fog over the water rather than rain over the land. And the Andes wall off any moist air coming from the Amazon side. No other desert in the list has all three working at once.

• (b)Gobi — The Gobi in Mongolia and northern China is a cold, mid-latitude rain-shadow desert. It is dry because it lies far inland behind the Himalaya and the Tibetan plateau, but it receives appreciably more precipitation than the Atacama, some of it as winter snow.
• (c)Sahara — The Sahara is the largest hot desert in the world, which is the fact most candidates remember — but largest is not driest. Its interior is arid, yet its long-term rainfall averages exceed the Atacama's, and its highland massifs such as Tibesti and Ahaggar collect more.
• (d)Kalahari — The Kalahari in southern Africa is a semi-arid sandy basin rather than a true desert over much of its extent; it supports grass and thorn scrub and receives a summer rainy season, so it is by some distance the wettest of the four.

Most of the world's hot deserts lie between about 15° and 30° of latitude on the western side of continents. Two forces put them there. The subtropical high-pressure belt sits over those latitudes, and its descending air warms as it sinks, which dries it out and kills cloud formation. On the western margins, a cold ocean current runs up the coast towards the Equator, cooling the lowest layer of air so that it becomes stable and yields fog rather than rain. The Atacama beside the Humboldt current and the Namib beside the Benguela current are the two classic examples of this pairing.

Questions in this family reward keeping four different superlatives apart, because papers rotate between them and the options usually contain the answer to a neighbouring one. The Sahara is the largest hot desert; Antarctica is the largest desert of any kind and also, on strict rainfall figures, the driest place on earth, which is why the qualifier 'non-polar' matters when a source calls the Atacama the driest; the Gobi is the largest desert of Asia; and the Thar is the most densely populated desert in the world. This item offers only the four names given, and among them the Atacama is unambiguously the driest, so the key is safe. The Atacama's aridity is also very old — the region has been arid for millions of years, which is part of why its nitrate and copper deposits, laid down and never washed away, made it economically valuable enough to fight a war over in the nineteenth century.

• The Atacama lies along the Pacific coast of northern Chile and is the driest non-polar desert on earth.
• Three causes combine there — subtropical subsidence, the cold Humboldt current offshore, and the rain shadow of the Andes.
• The Sahara is the world's largest hot desert, but it is not the driest.
• The Gobi is a cold, continental rain-shadow desert of Mongolia and northern China.
• Cold eastern-boundary currents create coastal deserts — Humboldt with the Atacama, Benguela with the Namib.

• Answering 'Sahara' because the remembered fact is largest, not driest.
• Forgetting that Antarctica outranks every hot desert on dryness, which is why sources say 'driest non-polar'.
• Treating the Kalahari as a full desert when much of it is semi-arid with a summer rainy season.

NDA asks for a single desert superlative, or pairs a desert with the ocean current or mountain range responsible for it.

• practice — not a real PYQ

  The cold ocean current chiefly responsible for the aridity of the Atacama Desert is the

  • (a)Benguela current
  • (b)Canary current
  • (c)Humboldt current
  • (d)California current

  Answer(c) Humboldt current — the cold current running north along the coast of Chile and Peru, which stabilises the air and prevents rain.

• practice — not a real PYQ

  Which one of the following is the largest hot desert in the world?

  • (a)Atacama
  • (b)Sahara
  • (c)Gobi
  • (d)Thar

  Answer(b) Sahara — largest hot desert; the Atacama is the driest and the Gobi is a cold desert.