The pattern of planetary winds largely depends upon which of the following factors? 1. Latitudinal variation of atmospheric heating 2. The distribution of continents and oceans 3. The rotation of earth Which of the above is/are correct?

Q: What is the answer to CDS (II) 2025 GK Q95?

D. Correct — D, 1, 2 and 3. All three are on NCERT's own list of what the pattern of planetary winds depends on, and each does a distinct job. Latitudinal variation of atmospheric heating is the engine: the sun's rays strike the low latitudes nearly vertically and the high latitudes at a slant, so the tropics gain more heat than they lose and the poles lose more than they gain, and the pressure difference that follows is what sets air moving at all. The rotation of the earth is the steering: the Coriolis deflection, to the right in the northern hemisphere and the left in the southern, bends what would otherwise be a straight run from high pressure to low into the trade winds, the westerlies and the polar easterlies. The distribution of continents and oceans is the reason the real pattern is untidy. Land heats and cools far faster than water, so the pressure belts break into separate cells over the northern hemisphere — the Azores and Pacific highs in summer, the great Siberian high in winter — instead of running as unbroken bands, and in southern Asia that seasonal contrast reverses the winds outright to give the monsoon. The southern hemisphere, with far less land, keeps its belts almost intact, which is exactly why its westerlies blow so much more steadily. (Provisional — pending the official UPSC key.)

(a)1 only
(b)1 and 2 only
(c)2 and 3 only
(d)1, 2 and 3

Answer

Why

Correct — D, 1, 2 and 3. All three are on NCERT's own list of what the pattern of planetary winds depends on, and each does a distinct job. Latitudinal variation of atmospheric heating is the engine: the sun's rays strike the low latitudes nearly vertically and the high latitudes at a slant, so the tropics gain more heat than they lose and the poles lose more than they gain, and the pressure difference that follows is what sets air moving at all. The rotation of the earth is the steering: the Coriolis deflection, to the right in the northern hemisphere and the left in the southern, bends what would otherwise be a straight run from high pressure to low into the trade winds, the westerlies and the polar easterlies. The distribution of continents and oceans is the reason the real pattern is untidy. Land heats and cools far faster than water, so the pressure belts break into separate cells over the northern hemisphere — the Azores and Pacific highs in summer, the great Siberian high in winter — instead of running as unbroken bands, and in southern Asia that seasonal contrast reverses the winds outright to give the monsoon. The southern hemisphere, with far less land, keeps its belts almost intact, which is exactly why its westerlies blow so much more steadily.

Why the others are wrong

(a)1 only — Heating alone would give a simple flow from equator to pole and back, with no trades and no westerlies. Rotation and the land-sea pattern are both needed to produce the belts we actually observe.
(b)1 and 2 only — Drops rotation, which is the factor that turns a north-south pressure-driven flow into the east-west wind belts. Without the Coriolis deflection there would be no trade winds at all.
(c)2 and 3 only — Drops the differential heating that starts everything. Rotation can only deflect air that is already moving, and the land-sea contrast can only modify a circulation that already exists.

Concept

Planetary or prevailing winds are the permanent wind belts of the globe — trade winds between the subtropical highs and the equatorial low, westerlies between the subtropical highs and the subpolar lows, and polar easterlies beyond. They are the surface expression of a three-cell circulation in each hemisphere: the Hadley cell in the tropics, the Ferrel cell in the mid-latitudes and the polar cell. Uneven solar heating creates the pressure belts, the earth's rotation deflects the resulting flow, and the arrangement of land and sea distorts the belts into cells.

The statement most students hesitate over is the second one, because the word 'planetary' suggests something that ought to be the same all the way round a latitude circle. Comparing the two hemispheres settles it. In the southern mid-latitudes there is almost nothing but ocean, and the westerlies there run so strongly and so continuously that sailors named them the Roaring Forties. At the same latitudes in the north, the land masses of North America and Eurasia break the belt into travelling highs and lows, and winter builds an enormous cold anticyclone over Siberia that no southern latitude produces. Same latitude, same heating, same rotation, different winds — and the only variable left is the distribution of continents and oceans. NCERT lists two further factors this question does not print: the emergence of the pressure belts themselves, and their seasonal migration with the sun's apparent path, which is what gives the Mediterranean lands rain in winter and drought in summer.

Key facts

NCERT: 'The pattern of planetary winds largely depends on : (i) latitudinal variation of atmospheric heating; (ii) emergence of pressure belts; (iii) the migration of belts following apparent path of the sun; (iv) the distribution of continents and oceans; (v) the rotation of earth.'
Seven pressure belts run from the equatorial low through the subtropical highs and subpolar lows to the polar highs, and they shift north and south with the sun.
The Coriolis force is proportional to wind speed and to the sine of the latitude: zero at the equator, greatest at the poles.
Trade winds blow from the north-east in the northern hemisphere and from the south-east in the southern, both converging on the equatorial trough.
The southern hemisphere's westerlies are stronger and steadier than the northern, because there is far less land to break the belt up.

What each factor contributes to the wind belts

Latitudinal variation of heating→The engine. Surplus heat in the tropics and deficit at the poles creates the pressure belts, and pressure difference is what makes air move.

Rotation of the earth→The steering. Coriolis deflection bends the flow right in the northern hemisphere and left in the southern, turning a simple north-south exchange into trades, westerlies and polar easterlies.

Distribution of continents and oceans→The distortion. Land heats and cools faster than sea, so the belts break into cells — the Siberian high in winter, the oceanic subtropical highs in summer, and the monsoon reversal over southern Asia.

Emergence of pressure belts→On NCERT's list but not in this question. The belts themselves are the intermediate step between uneven heating and the winds.

Migration of the belts with the sun→Also on NCERT's list and not in this question. It is why the Mediterranean lands get the westerlies and their rain in winter, and the dry subtropical high in summer.

The question prints three of NCERT's five factors, and all three are genuine.

Study next

The seven pressure belts and how far they migrate with the sun's apparent pathThe three-cell model — Hadley, Ferrel and polar cells — and what happens at their boundariesThe Coriolis force: why it grows with wind speed and with latitude, and why it vanishes at the equatorJet streams, and the difference between the subtropical and polar-front jetsSeasonal and local winds — monsoon, loo, chinook, mistral, foehn — and how they sit inside the planetary pattern

Common traps

Assuming a 'planetary' wind must be unaffected by land and sea. The contrast between the two hemispheres' westerlies shows otherwise.
Confusing the Coriolis force with a real force. It is an apparent deflection arising from measuring motion on a rotating earth, and it does no work.
Forgetting that the pressure belts move. The Mediterranean climate exists only because they do.

As a multi-statement item on the controls of the general circulation, or as a single-factor question on the Coriolis force — where it is greatest, what it depends on, which way it deflects.

Related PYQs

UPSC_2024_GS1_Q142024Open & attempt →

With reference to “Coriolis force”, which of the following statements is/are correct ? 1. It increases with increase in wind velocity. 2. It is maximum at the poles and is absent at the equator. Select the answer using the code given below :

(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Answer(c) Both 1 and 2

The third factor of this question, taken on its own. The deflection is proportional to wind speed and to the sine of the latitude, which is why it disappears at the equator and is greatest at the poles — and why the earth's rotation belongs on the list of controls on the planetary wind pattern.

CDS_GK_2021_I_Q432021Open & attempt →

Which one of the following statements about the Coriolis force is not correct?

(a) It is maximum at the Poles.
(b) It is absent at the Equator.
(c) It deflects the wind to the right direction in the southern hemisphere.
(d) It deflects the wind to the right direction in the northern hemisphere.

Answer(c) It deflects the wind to the right direction in the southern hemisphere.

CDS has tested the steering factor directly before. The deflection is to the right in the northern hemisphere and to the left in the southern, which is why the northern trades arrive from the north-east and the southern trades from the south-east.

Practice

practice — not a real PYQ
The trade winds blow from the sub-tropical high pressure belt towards the equatorial low. In the northern hemisphere their direction is
- (a)north-westerly
- (b)north-easterly
- (c)south-westerly
- (d)south-easterly
Answer(b) north-easterly — air moving south from the sub-tropical high is deflected to its right by the earth's rotation, arriving as a wind from the north-east.
practice — not a real PYQ
The westerlies of the southern hemisphere are stronger and steadier than those of the northern hemisphere mainly because
- (a)the southern hemisphere receives more solar energy
- (b)the Coriolis force is greater south of the equator
- (c)there is far less land in the southern mid-latitudes to interrupt the belt
- (d)the southern hemisphere has a smaller pressure gradient
Answer(c) there is far less land in the southern mid-latitudes to interrupt the belt — an almost unbroken ocean lets the wind belt run continuously, which is why they are called the Roaring Forties.