Climate & atmosphere

Answer: (d) Statement-I is incorrect, but Statement-II is correct

The atmosphere lets most short-wave sunlight pass through; the Earth's surface absorbs it and then radiates long-wave (terrestrial) energy, which carbon dioxide and other greenhouse gases absorb. So the air is heated mainly from below by the Earth, not directly by the Sun: Statement-I is wrong, Statement-II is right.

• ✗ Statement-I The atmosphere is largely transparent to incoming short-wave solar radiation. It is heated indirectly, from below, by the long-wave radiation the warmed Earth gives out.
• ✓ Statement-II Terrestrial radiation is long-wave, and carbon dioxide and the other greenhouse gases absorb it well. This is the basis of the greenhouse effect.

Remember · Sun heats the ground (short wave); the ground heats the air (long wave, terrestrial radiation). Greenhouse gases trap that long-wave energy.

📘 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 (2024, 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

The troposphere reaches about 18 km over the equator but only about 8 km near the poles. The reason is the intense heating at the equator: strong convection currents carry heat high up, so Statement-II is the cause of Statement-I.

• ✓ Statement-I Average height of the troposphere is 13 km; it is roughly 18 km at the equator and 8 km near the poles.
• ✓ Statement-II Strong convectional currents over the hot equator lift heat to great heights, which pushes the top of the troposphere upward. This explains Statement-I.

Remember · Troposphere: ~18 km at equator, ~8 km at poles, average 13 km — thickest at the equator because of strong convection.

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

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (a) 1 only

In the northern winter the oceans stay warmer than the land, so January isotherms bulge north over the oceans and dip south over the continents. Statement 2 fails because the Gulf Stream and North Atlantic Drift are warm currents that make the North Atlantic warmer, not colder.

• ✓ 1. In January the isotherms swing northward over the oceans and southward over the land masses, most clearly over the North Atlantic and Europe.
• ✗ 2. The Gulf Stream and North Atlantic Drift are warm currents. They make the North Atlantic warmer, which is why the isotherms bend north there.

Remember · January isotherms: bend poleward over oceans, equatorward over continents. Gulf Stream and North Atlantic Drift are warm currents.

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

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (a) 1 only

Water vapour is a variable gas concentrated near the surface, so it thins out with height. It is most plentiful in the warm, wet tropics (up to about 4% of air by volume) and scarcest in cold, dry polar and desert air (under 1%).

• ✓ 1. Water vapour is a variable gas whose amount decreases with altitude, because evaporation happens at the surface and cold upper air holds little moisture.
• ✗ 2. Its share is lowest, not highest, at the poles — below 1% of the air — and highest in the warm, wet tropics at about 4%.

Remember · Water vapour: up to ~4% of air in humid tropics, under 1% in polar and desert air; decreases with altitude and from equator to poles.

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

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (d) Marine West coast climate

All three features describe the Marine West Coast (Cfb) climate of places like north-western Europe: the sea keeps both annual and daily temperature ranges small, rain falls in every month, and totals vary widely between about 50 and 250 cm.

• ✓ (d) Koeppen's Cfb climate has small annual and daily temperature ranges, precipitation throughout the year, and 50–250 cm of precipitation — exactly the three clues.
• ✗ (a) Equatorial climate also has rain all year and a small annual range, but its rainfall is much heavier and it is hot throughout the year; the 50–250 cm spread points to the mid-latitude west coasts.
• ✗ (c) Humid subtropical (China type) climate lies on eastern margins, with hot summers and about 75–150 cm of rain, often with summer thunderstorms.

Remember · Marine West Coast (Cfb): NW Europe, British Columbia, southern Chile, New Zealand — mild, small temperature ranges, rain all year, 50–250 cm.

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

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (c) Both 1 and 2

The Coriolis force comes from the Earth's rotation and deflects moving air — to the right in the northern hemisphere, to the left in the southern. Its strength rises with wind speed and with latitude, so it is greatest at the poles and zero at the equator.

• ✓ 1. The deflection grows with the speed of the wind: the faster the air moves, the stronger the Coriolis effect on it.
• ✓ 2. The force is proportional to the sine of the latitude, so it is maximum at the poles and absent at the equator. That is why tropical cyclones do not form right on the equator.

Remember · Coriolis force rises with wind speed and latitude: zero at the equator, maximum at the poles; deflects winds right (north) and left (south).

📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 9 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (d) 2 and 4

21 June is the summer solstice of the northern hemisphere, when the North Pole is tilted towards the Sun. Every place north of the equator then gets more than 12 hours of daylight, the equator gets exactly 12, and the southern hemisphere gets less. So the Tropic of Cancer and the Arctic Circle qualify.

• ✗ 1. The equator has about 12 hours of daylight and 12 hours of darkness throughout the year, never more.
• ✓ 2. The Tropic of Cancer is in the northern hemisphere, where days are longer than 12 hours in June; on 21 June the Sun is overhead there at noon.
• ✗ 3. The Tropic of Capricorn is in the southern hemisphere, where June is winter and daylight is shorter than 12 hours.
• ✓ 4. On 21 June the Arctic Circle has its longest day — the Sun stays above the horizon for about 24 hours.

Remember · 21 June: longest day north of the equator (24-hour day at and beyond the Arctic Circle); equator always ~12 hours; southern hemisphere has short winter days.

📘 Read it in NCERT: Class 7 Curiosity, Ch 12 (practise this chapter) · Class 7 Curiosity, Ch 12 (practise this chapter) · Class 7 Curiosity, Ch 12 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (b) Only two

Radar sends out radio pulses and times their echoes to find the distance and speed of whatever reflects them. That makes it excellent for watching rain, hail and snow and for following flocks of migrating birds or swarms of insects in the sky, but an echo cannot reveal what chemical a passenger is carrying, so radar is not a narcotics detector.

• ✗ 1. Drugs on passengers are found with X-ray and body scanners, trace-detection equipment and sniffer dogs; radar only tells where an object is and how it moves, not what substance it is.
• ✓ 2. Doppler weather radars measure reflected pulses from raindrops and ice; dual-polarisation radars can tell rain, hail, snow and ice pellets apart.
• ✓ 3. Weather-radar networks also pick up birds, bats and insects aloft; ornithologists use these echoes to map and forecast nightly bird migration.

Remember · Radar (RAdio Detection And Ranging) gives range and velocity of targets: rain, storms, aircraft, ships, bird and insect movements. It cannot identify substances such as drugs.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error