CDS (II) 2024 General Knowledge · Question 78 of 120

Correct — A, Both the statements are correct and statement 2 is the correct explanation of statement 1. Take them in order. The Hadley cell is the tropical limb of the general circulation, and the surface winds of half the planet are its lower branch: air converging on the equatorial trough is deflected by the Earth's rotation into the north-east and south-east trades, and the air that sinks near 30 degrees builds the subtropical high-pressure belts from which the trades blow out and the westerlies begin. So statement 1 holds. Statement 2 gives the reason the cell turns at all. Insolation is concentrated where the Sun is most nearly overhead, so the equatorial belt is heated more strongly than anywhere else; the heated air expands, rises, spreads poleward aloft, cools and sinks in the subtropics, and flows back equatorward at the surface. That is a thermal circulation, and NCERT calls such a circulation in the tropics a Hadley cell. The second statement is therefore the cause and the first is the consequence, which is what the option asks you to affirm.

• (b)Both the statements are correct, but statement 2 is not the correct explanation of statement 1. — Accepts both facts but severs the link between them, when the differential heating in statement 2 is exactly what creates the circulation whose surface limb the trade winds are.
• (c)Statement 1 is correct, but statement 2 is false. — Statement 2 is the standard account of why the cell exists — stronger heating at the equator, rising air, a thermally driven overturning.
• (d)Statement 1 is false, but statement 2 is correct. — Denies the usefulness of the cell, when the trades, the doldrums, the subtropical highs and the belt of desert along 30 degrees are all read off it.

The atmosphere transfers heat from the tropics to the poles through three overturning cells in each hemisphere. Air rises along the intertropical convergence zone, moves poleward at height, sinks over the subtropical highs at around 30 degrees and returns as the trades — that is the Hadley cell. Between about 30 and 60 degrees the Ferrel cell has cold air sinking from the poles and warm air rising from the subtropical high, with the westerlies at the surface. At high latitudes cold dense air subsides near the poles and blows out as the polar easterlies, forming the polar cell. NCERT sets out all three and notes that together they set the pattern for the general circulation of the atmosphere.

Assertion-and-reason items on circulation are usually decided by asking whether the second sentence supplies the mechanism for the first. Here it does, though the connection is stated a little loosely — statement 2 explains why the cell exists, and the step from there to its being key to understanding the winds is left for you to make. That step is short, since the cell's surface branch is the trade-wind belt itself. The cell is named for George Hadley, who proposed the tropical overturning in the eighteenth century to explain the trade winds. One refinement worth carrying: the cells are not fixed. The whole system shifts with the Sun through the year, which is why the intertropical convergence zone moves north over India in summer and why the monsoon can be described as a seasonal migration of that zone.

• NCERT describes circulations from the surface upwards and back as cells, and calls the tropical one the Hadley cell.
• The Ferrel cell of the middle latitudes gives the westerlies at the surface; the polar cell gives the polar easterlies.
• The three cells together set the pattern for the general circulation and move heat from lower to higher latitudes.
• Air rises along the intertropical convergence zone and sinks over the subtropical high-pressure belts near 30 degrees, from where the trade winds blow back equatorward.
• The subsiding limb of the Hadley cell over the subtropics is the reason most of the world's hot deserts lie near 30 degrees north and south.

• Placing the descending limb of the Hadley cell at the poles instead of near 30 degrees latitude.
• Treating the cells as fixed in place; they migrate with the overhead Sun through the year.
• Answering an assertion-and-reason item on whether both sentences are true without checking whether the second explains the first.

As an assertion-and-reason pair on the cause of the tropical circulation, or as a direct question on where the Hadley cell's air rises and where it descends.

• practice — not a real PYQ

  In the Hadley cell, the air that rises near the equator descends principally in which zone?

  • (a)The polar front
  • (b)The subtropical high-pressure belts near 30 degrees
  • (c)The equatorial trough
  • (d)The mid-latitude westerlies near 60 degrees

  Answer(b) The subtropical high-pressure belts near 30 degrees — from which the trade winds blow back towards the equator.

• practice — not a real PYQ

  The surface winds produced by the mid-latitude cell of the general circulation are known as which of the following?

  • (a)Trade winds
  • (b)Polar easterlies
  • (c)Westerlies
  • (d)Monsoon winds

  Answer(c) Westerlies — the surface branch of the Ferrel cell, between about 30 and 60 degrees latitude.