CDS (I) 2020 General Knowledge · Question 68 of 120

Correct — C, subtropical high-pressure cells. The Hadley cell is the thermally driven overturning of the tropical atmosphere. Intense heating at the equator makes air expand, rise and diverge aloft; as it drifts polewards it cools, loses moisture and becomes denser, and by about 25 to 30 degrees of latitude it sinks. That descending limb is what creates the subtropical high-pressure belt — the horse latitudes — and the sinking air warms by compression, its relative humidity falls, cloud cannot form and the great desert belts of the world sit under it, the Sahara, the Arabian, the Thar, the Kalahari and the Australian interior. From the base of those highs the air returns equatorward as the trade winds, closing the loop. The other three options all name the rising end of the same cell rather than the sinking end, which is the distinction the question is testing.

• (a)inter-tropical convergence zone — That is where the Hadley cell rises, not where it descends. The ITCZ is the belt where the trade winds of the two hemispheres meet and force air upward, producing the heavy convectional rain of the equatorial zone.
• (b)doldrums — Another name for the same rising belt. The doldrums are the calm, low-pressure equatorial waters under the ITCZ, notorious in the age of sail because the surface winds are light and variable there.
• (d)equatorial troughs — The equatorial trough is the low-pressure furrow along the equator — again the ascending limb. Air converges into a trough and rises; descent belongs to a high, not a low.

The general circulation of the atmosphere is usually described as three cells in each hemisphere. The Hadley cell runs between the equator and about 30 degrees, rising at the ITCZ and sinking at the subtropical highs. The Ferrel cell occupies the mid-latitudes and the polar cell the high latitudes. The Hadley cell is the only one of the three that is directly thermally driven, which is why it is the strongest and the most regular.

The item is a single-fact question dressed as a four-option problem, because three of the four options are the same place under different names. The ITCZ, the doldrums and the equatorial trough are all descriptions of the rising, low-pressure equatorial belt; only the subtropical high is a descending, high-pressure belt. Spotting that three options are synonyms is often faster than recalling the circulation. It is worth carrying the consequence too: because the descent sits at 25 to 30 degrees, the world's hot deserts form a broken ring at those latitudes on both sides of the equator, and the trade winds blow out of the same highs towards the equator.

• The Hadley cell rises at the Inter-Tropical Convergence Zone and descends at the subtropical high-pressure belt near 25 to 30 degrees latitude.
• Descending air warms by compression, so its relative humidity falls and cloud formation is suppressed — the reason the subtropical deserts lie under this belt.
• The subtropical highs are also called the horse latitudes; the trade winds blow out of them towards the equator.
• The ITCZ, the doldrums and the equatorial trough all describe the same low-pressure, ascending equatorial belt.
• The ITCZ migrates north and south with the overhead sun, which is what drives the seasonal rhythm of the tropical wet and dry climates.

• Treating the ITCZ, doldrums and equatorial trough as different places; they describe one belt.
• Associating high pressure with cold air only — the subtropical highs are warm and dry.
• Forgetting that the deserts under the descending limb are a consequence of the circulation, not of distance from the sea.

As a one-line question on where the cell rises or descends, or indirectly through desert formation and the trade winds.

• practice — not a real PYQ

  The subtropical high-pressure belts are chiefly formed by

  • (a)convergence of the trade winds at the equator
  • (b)descent of air in the poleward limb of the Hadley cell
  • (c)cooling of air over the polar ice caps
  • (d)orographic uplift over mid-latitude mountains

  Answer(b) descent of air in the poleward limb of the Hadley cell — the sinking air warms by compression and builds high pressure near 25 to 30 degrees.

• practice — not a real PYQ

  Heavy convectional rainfall in the equatorial belt is most directly associated with

  • (a)the subtropical high-pressure cells
  • (b)the polar front
  • (c)the Inter-Tropical Convergence Zone
  • (d)the westerlies

  Answer(c) the Inter-Tropical Convergence Zone — the trade winds meet there and the forced ascent produces towering cloud and daily downpours.