Climate & atmosphere

Answer: (c) Statement I is correct but Statement II is not correct

Dust is indeed more concentrated over subtropical and temperate belts, but the reason is that these regions have more dry winds, not fewer. Dry winds lift fine soil from arid and semi-arid land, while rain in equatorial regions and ice cover near the poles keep dust down, so Statement II gets the cause backwards.

• ✓ Statement-I The atmosphere holds more dust over subtropical and temperate regions than over equatorial and polar regions; the dust mostly stays in the lower layers.
• ✗ Statement-II These regions have plenty of dry winds, which is exactly why they carry more dust; saying they have 'less dry winds' is false.

Remember · Dust concentration is highest in subtropical and temperate regions because of dry winds; dust and salt particles act as hygroscopic nuclei for cloud formation.

📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 7 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (a) Both Statement I and Statement II are correct and Statement II explains Statement I

In the northern winter, land loses heat fast while the oceans, helped by warm currents such as the Gulf Stream and North Atlantic Drift, stay relatively warm. A given temperature is therefore found farther north over the sea and farther south over land, so the isotherms bend poleward over oceans and equatorward over continents; the warmer ocean air is the reason.

• ✓ Statement-I January isotherms swing north over the North Atlantic and south over Europe and especially the Siberian plain.
• ✓ Statement-II Oceans cool slowly and warm currents add heat, so in January the air over them is warmer than over the cold northern landmasses; this contrast is what bends the isotherms, so II explains I.

Remember · January (NH winter): isotherms bend poleward over oceans, equatorward over land; July reverses it. Deviation is sharper in the Northern Hemisphere because it has more land.

📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 8 (practise this chapter) · Class 11 Fundamentals of Physical Geography, Ch 8 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (c) I, II and III

All three describe the natural greenhouse effect. The Earth's surface re-radiates sunlight as infrared heat; gases such as carbon dioxide, methane and water vapour absorb much of it, which keeps the global average near 15°C. Remove that blanket and the surface would be about 33°C colder, below freezing.

• ✓ I Without the atmosphere's greenhouse effect the average surface temperature would fall by roughly 33°C, from about 15°C to around −18°C, too cold for life.
• ✓ II The heat that the atmosphere absorbs and traps is what holds the planet's average temperature at a life-supporting level.
• ✓ III Carbon dioxide, methane and water vapour absorb the infrared heat re-radiated by the surface and stop it escaping to space.

Remember · Natural greenhouse effect: CO2, CH4, water vapour trap outgoing infrared; keeps Earth ≈ 15°C instead of ≈ −18°C (a 33°C difference).

📘 Read it in NCERT: Class 9 Exploration, Ch 13 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (b) Both Statement II and Statement III are correct but only one of them explains Statement I

Both Statement II and Statement III are true, but only Statement III explains Statement I. Melting polar ice moves mass from the poles towards the equatorial ocean, and such shifts of mass change Earth's rotation and axis. A solar storm heats the upper atmosphere but does not shift the planet's mass, so it cannot explain a change in rotation or axis.

• ✓ I NASA-funded studies show Earth's spin axis drifts (polar motion) and its rotation slows, as mass is redistributed on and within the planet.
• ✓ II Solar flares and coronal mass ejections do hit Earth's upper atmosphere with charged particles and energy and can heat it. This is a space-weather effect and does not move Earth's mass, so it does not explain Statement I.
• ✓ III Faster melting of ice sheets has shifted mass from the poles towards the equatorial ocean. This redistribution of mass is the cause of the change in rotation and axis, so it explains Statement I.

Remember · Ice melt shifts mass from the poles to the equatorial oceans, changing Earth's spin axis and lengthening the day; solar storms only affect the upper atmosphere.

Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error