Climate & atmosphere

Answer: (a) 1 only

The Bharat Forecast System (BharatFS) runs at 6 km resolution, fine enough to give forecasts at the panchayat or cluster-of-panchayats level, so Statement 1 is correct. It was developed by scientists at the Indian Institute of Tropical Meteorology (IITM), Pune, under the Ministry of Earth Sciences, not by IIT Delhi.

• ✓ 1. The Ministry of Earth Sciences says BharatFS operates at 6 km resolution (against 12 km earlier) and would help provide monsoon rainfall forecasts at the panchayat or cluster-of-panchayats level.
• ✗ 2. The model's grid structure and development were carried out by scientists at the Indian Institute of Tropical Meteorology, Pune, not IIT Delhi.

Remember · BharatFS: MoES model built at IITM Pune, 6 km resolution (earlier global model 12 km), aimed at block and panchayat-level forecasts; operational since May 2025.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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Answer: (c) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.

Statement (c) is correct: water vapour, ozone and other gases in the lower atmosphere (the troposphere) absorb much of the near-infrared radiation passing through. The other options misstate insolation values or the nature of infrared light.

• ✓ (c) NCERT says water vapour, ozone and other gases within the troposphere absorb much of the near-infrared radiation. Water vapour is concentrated in the lower atmosphere.
• ✗ (a) NCERT gives insolation at the surface as about 320 W/m2 in the tropics and about 70 W/m2 at the poles, a ratio of under five, and it says the equator receives less than the tropics. Ten times is far too high.
• ✗ (d) Infrared waves lie just beyond the visible spectrum; the human eye cannot see them. They are not part of the visible spectrum.

Remember · In the troposphere, water vapour, ozone and other gases absorb much of the near-infrared radiation. Insolation is about 320 W/m2 in the tropics and 70 W/m2 at the poles.

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

The land-ocean temperature contrast is sharper in winter than in summer, so Statement-I is wrong. Statement-II is right: water has a higher specific heat than land and therefore heats up and cools down slowly.

• ✗ Statement-I In January the isotherms bend north over the warm oceans and south over the cold continents, and the departure from the latitudinal pattern is stronger in January than in July. The contrast is therefore greater in winter.
• ✓ Statement-II Water needs more heat than land to rise by one degree, so it heats up and cools down more slowly than the landmass. This is why continents swing more in temperature than oceans.

Remember · Higher specific heat makes water heat and cool slowly, so continents swing more. The land-ocean contrast is sharpest in northern winter (January), when isotherms bend most.

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Answer: (b) Lake Faguibine

Lake Faguibine, near Timbuktu in northern Mali, was fed by overflow from the Niger. Droughts in the 1970s dried it, and sand then choked the channels from the river, so even when rains returned the water could not reach it; the lake bed has largely turned to desert.

• ✓ (b) UNEP reports that Lake Faguibine has been dry since the 1970s: droughts emptied the lake system and sand-filled channels cut it off from the Niger's floods.
• ✗ (a) Lake Victoria is in East Africa (shared by Uganda, Kenya and Tanzania), not West Africa, and it has not dried up.
• ✗ (d) Lake Volta in Ghana is a large artificial reservoir behind the Akosombo Dam and still holds water for hydropower; Lake Oguta in Nigeria is also a living lake.

Remember · Lake Faguibine (Mali, near Timbuktu): Niger-fed lake system dried since the 1970s droughts — a Sahel symbol of desertification; UNEP has worked on reviving it.

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Answer: (b) Second half of the month of June

The Northern Hemisphere's longest day is the summer solstice, around 21 June, when the North Pole is tilted most towards the Sun and the Sun is overhead at the Tropic of Cancer. 21 June falls in the second half of June.

• ✓ (b) The summer solstice falls on 20, 21 or 22 June; after it, days in the Northern Hemisphere grow shorter and nights longer.
• ✗ (a) Days are still lengthening through the first half of June; the maximum comes only at the solstice around 21 June.
• ✗ (c) July is often the hottest month because of the lag in heating, but day length is already decreasing by then.

Remember · Summer solstice ~21 June: Sun overhead at the Tropic of Cancer, longest day in the Northern Hemisphere; winter solstice ~22 December: shortest day.

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Answer: (d) Neither 1 nor 2

Both statements swap the roles of the two cloud types. It is low, thick clouds that mainly bounce sunlight back and cool the surface, while high, thin clouds let sunlight through but hold back part of the Earth's outgoing heat and so warm the surface.

• ✗ 1. Reflection of sunlight (the cooling effect) is the main job of low, thick clouds. High, thin clouds such as cirrus mostly transmit incoming solar radiation.
• ✗ 2. The warming effect comes from high, thin clouds, which trap some of the infrared radiation leaving the Earth and send it back down. Low clouds are dominated by the cooling effect.

Remember · Low, thick clouds cool the surface (they reflect sunlight); high, thin clouds warm it (they trap outgoing infrared). Deep convective clouds are roughly neutral.

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Answer: (c) Both 1 and 2

Both statements are correct: winds push warm surface water to the western side of tropical oceans, while in the temperate zone the westerlies carry warm water to the eastern side, that is, to the west coasts of continents.

• ✓ 1. In the tropics the easterly trade winds push warm water west and hold it there, so the western side of an ocean is warmer than the east. This matches warm currents along the east coasts of continents in tropical latitudes and cool waters off their west coasts.
• ✓ 2. Ocean circulation follows the planet's wind pattern, and in the middle latitudes the surface winds are the westerlies, which carry warm water eastwards, so the eastern side of the ocean is the warm side. NCERT notes that west coasts of continents in middle and higher latitudes are bordered by warm waters, as in north-western Europe.

Remember · Tropics: trade winds pile warm water in the west of the ocean. Temperate zone: westerlies carry warm water to the east, so west coasts of continents there are mild.

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Answer: (c) 2, 3 and 4

Why not the tempting option · UPSC's key is (c). The general checks on tree growth in savannas are fire, grazing and browsing animals, and the long dry season of a seasonal-rainfall climate — a university biome guide puts it as frequent fires and large grazing mammals killing seedlings, with climate the most important factor. Soil does shape the tree–grass balance locally (thin or poorly drained soils carry no trees in some East African savannas), but the question asks for the general conditions and no option pairs soil with fire and grazing; termites build the mounds on which trees grow rather than checking them. In the exam, pick the option that holds the two universal controls, fire and grazing, together.

Fire, grazing and browsing animals, and the long dry season of a seasonal-rainfall climate are the recognised checks that keep savannas open and stop them turning into forest, so 2, 3 and 4 is the answer.

• ✗ 1. Termites are decomposers that break down about 30 per cent of dead organic matter; in parts of East Africa trees grow only on termite mounds, so termites aid rather than check tree growth. Burrowing animals are not a recognised control.
• ✓ 2. Wildfire is an important part of savanna ecosystems in all regions; annual burning of dry grass and litter helps keep woody plants from taking over.
• ✓ 3. Browsing and grazing animals alter the balance between trees and grass. Rising elephant numbers in Virunga National Park, for example, destroyed woody plants and turned a heavily wooded savanna into grass savanna with very few trees.
• ✓ 4. Savanna is a vegetation type of hot, seasonally dry climates: a long dry season each year limits tree growth and keeps the grass cover open.
• ✗ 5. Soil shapes the tree–grass balance only locally — thin or poorly drained soils carry no trees in some savannas — so it is not one of the general checks, and no option pairs it with fire and grazing.

Remember · Savannas stay open through fire, grazing and browsing animals, and a long dry season; without these, woody plants tend to spread and the savanna turns to forest.

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Answer: (b) 2 only

Statement 2 is right and statement 1 is wrong. OMT (the average temperature of the ocean from the surface down to the 26°C isotherm) in the south-western Indian Ocean during January–March helps predict whether the coming monsoon rainfall will be above or below average, but the 26°C isotherm there lies at about 59 m, not 129 m.

• ✗ 1. OMT is the mean temperature of the layer from the surface down to the 26°C isotherm, which is the first half of the statement. But the January–March climatic mean depth of that isotherm in the south-western Indian Ocean is about 59 m, not 129 m.
• ✓ 2. The January–March south-western Indian Ocean OMT is a better qualitative predictor of Indian summer monsoon rainfall than sea surface temperature; it correctly signals above-average or below-average rain about 80% of the time, against 60% for SST.

Remember · OMT = mean temperature from the surface to the 26°C isotherm; south-western Indian Ocean OMT in January–March forecasts monsoon rain (isotherm depth about 59 m there).

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Answer: (c) 2 only

Only statement 2 is correct. Jet streams blow in both hemispheres, and the eye of a tropical cyclone is warmer, not colder, than its surroundings; but an eye forms only once a cyclone has organised and intensified.

• ✗ 1. Jet streams exist in both hemispheres. The polar jet lies between about 50° and 60° latitude in the northern and the southern hemispheres alike.
• ✓ 2. An eye develops only when the inner spiral bands close into an eyewall, which usually happens around the time winds reach hurricane force. Weak tropical systems and temperate (extra-tropical) cyclones generally have no eye.
• ✗ 3. The eye is the region of lowest surface pressure and, aloft, the warmest air; a tropical cyclone has a warm core, so the eye is warmer than its surroundings, not cooler.

Remember · Jet streams occur in both hemispheres. A cyclone's eye forms only in mature tropical cyclones, and it is a calm, warm-cored centre.

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Answer: (a) does not set below the horizon at the Arctic Circle

21 June is the June (northern summer) solstice: the North Pole is tilted towards the Sun, the noon Sun is overhead at the Tropic of Cancer, and every place on or north of the Arctic Circle (about 66½° N) has the Sun above the horizon for the full 24 hours.

• ✓ (a) The Arctic Circle is defined as the southern limit of the zone where the Sun does not set on at least one day a year — the June solstice.
• ✗ (b) On the same day the Antarctic Circle has the opposite: the Sun does not rise, giving 24 hours of night.
• ✗ (c) The noon Sun is overhead at the Equator only at the equinoxes (around 21 March and 23 September).
• ✗ (d) The Sun is overhead at the Tropic of Capricorn at the December solstice (around 22 December); on 21 June it is over the Tropic of Cancer.

Remember · 21 June: Sun overhead at Tropic of Cancer; midnight sun at and north of the Arctic Circle; polar night at the Antarctic Circle. Reversed around 22 December.

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Answer: (d) Reducing the global warming

Both are proposed geoengineering methods of 'solar radiation modification', meant to cool the planet. Stratospheric sulphate aerosols would reflect sunlight back to space, copying the cooling seen after big volcanic eruptions; thinning heat-trapping cirrus clouds would let more of the Earth's heat escape.

• ✓ (d) The IPCC lists stratospheric aerosol injection and cirrus cloud thinning among the most-studied SRM methods, whose aim is to reduce warming.
• ✗ (a) Artificial rain uses cloud seeding with silver iodide or salt in rain-bearing clouds, not stratospheric sulphate.
• ✗ (b) These methods target the planet's overall heat budget; they are not cyclone-suppression tools.

Remember · Solar radiation modification (geoengineering to cut warming): stratospheric aerosol injection, marine cloud brightening, cirrus cloud thinning, surface albedo change. Carbon dioxide removal is the other branch.

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Answer: (b) Clouds reflect back the Earth's radiation.

Dew forms only when the ground and grass cool below the dew point, and that needs a clear night in which the Earth's heat escapes freely to space. On a cloudy night the clouds send the Earth's heat back down, so the surface stays warmer and no dew forms.

• ✓ (b) Clouds hold back the Earth's outgoing radiation and return part of it towards the surface, keeping the ground from cooling enough for water vapour to condense on it. UPSC treats this as the cause.
• ✗ (a) This is the tempting option. Clouds do take in some of the Earth's radiation, but the answer UPSC accepts is that they send it back, which is what keeps the surface warm.
• ✗ (c) It is the reverse of the truth: with cloud cover the surface stays warmer, and it is a clear, calm, cold night that gives low surface temperature and dew.
• ✗ (d) Dew needs calm air, but clouds do not act by blowing wind down to the ground; the deciding factor is heat loss.

Remember · Dew needs a clear, calm, cold night; cloud cover returns the Earth's heat and stops the surface cooling to the dew point.

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Answer: (c) 1 and 3 only

Statements 1 and 3 are correct. The Earth's magnetic field has flipped repeatedly in the geological past, and early life released oxygen that changed the atmosphere. Statement 2 is false because the early atmosphere had carbon dioxide and very little free oxygen.

• ✓ 1. Rocks record past polarity flips of the magnetic field. NASA describes a history of flipping every few hundred thousand years, though the flips are irregular and the last was about 780,000 years ago.
• ✗ 2. The early atmosphere was mostly water vapour, nitrogen, carbon dioxide, methane and ammonia, with very little free oxygen. It was not 54% oxygen and it did contain carbon dioxide.
• ✓ 3. Once life evolved in the oceans, photosynthesis added oxygen to the water and later the air. About 2,000 million years ago oxygen began to flood the atmosphere.

Remember · Early atmosphere: water vapour, nitrogen, CO2, methane, ammonia, almost no free oxygen. Oxygen came from photosynthesis by early life.

📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 2 (practise this chapter) · Class 11 Fundamentals of Physical Geography, Ch 2 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2018, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (b) 2 only

The Indian Ocean Dipole is a sea-surface temperature difference between the western and the south-eastern tropical Indian Ocean, not the Eastern Pacific, so statement 1 is wrong. A positive or negative IOD can strengthen or weaken the effect of an El Nino on the Indian monsoon, so statement 2 is correct.

• ✗ 1. The IOD is measured as the temperature difference between the tropical western Indian Ocean and the tropical south-eastern Indian Ocean. The Eastern Pacific is the region watched for El Nino and La Nina, which is a different phenomenon.
• ✓ 2. The El Nino-induced circulation over India can be countered or supported by the circulation the IOD sets up, depending on the phase and strength of each. This is why a positive IOD can soften an El Nino's damage to the monsoon.

Remember · IOD = west-minus-east sea-surface temperature contrast inside the Indian Ocean. It can counter or reinforce El Nino's effect on the Indian monsoon.

Question and answer: UPSC's official GS Paper I (2017, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error