Climatology and Atmospheric Processes

The phenomenon has two necessary conditions acting together: the inclination of the earth's axis at about 23.5 degrees to the perpendicular to the plane of the ecliptic, and the earth's revolution around the sun. Because the axis maintains a fixed orientation in space through the orbit, a condition called parallelism, the circle of illumination cuts the parallels of latitude unequally at all positions except the equinoxes, and the proportion of each parallel lying in daylight therefore changes through the year.

• Option (d) captures both conditions and is correct.
• Option (a) is incorrect because rotation produces the alternation of day and night and fixes its twenty four hour period, but a rotating earth with an upright axis would give every place a twelve hour day throughout the year.
• Option (b) is incorrect because orbital eccentricity varies the earth's distance from the sun and therefore the intensity of insolation and the speed of orbital motion, which is why the seasons are of slightly unequal length, but it does not vary day length; a circular orbit with a tilted axis would still produce the full seasonal swing.
• Option (c) is incorrect because latitude explains why the seasonal variation in day length is negligible at the equator and extreme at the poles, that is, it explains the spatial pattern of the variation, but it cannot explain why the variation occurs at all at a single fixed place, which is what the question asks. The elimination route is to ask, of each option, whether removing that factor while keeping the others would abolish the phenomenon; only the tilt survives that test.

• Statement 1 is correct and is the entire explanation. Land and water differ fundamentally in their thermal behaviour: the specific heat capacity of water is roughly four times that of dry soil or rock, so a given input of energy raises the temperature of water far less; solar radiation penetrates several metres into water and is distributed further by turbulence and currents, while it heats only a thin surface layer of land; and evaporation from a water surface consumes a large part of the incoming energy as latent heat. Water therefore heats slowly and cools slowly, land heats rapidly and cools rapidly, and coasts under maritime influence have a small annual range while continental interiors beyond that influence have a large one. This is continentality, and Verkhoyansk in Siberia is the standard illustration.
• Statement 2 is incorrect. Continents do stand higher than ocean basins, but altitude lowers mean temperature at the lapse rate; it does not by itself widen the annual range, and the comparison intended is between the interior and the coast of the same landmass, where no systematic altitude difference is asserted.
• Statement 3 is incorrect and is nearly the opposite of the truth: strong winds promote mixing and, where they blow from the sea, carry maritime influence inland and moderate the range.
• Statement 4 is incorrect in fact; interiors are drier than coasts as a rule, and heavy rain and its attendant cloud and humidity would in any case reduce the range by damping both daytime heating and nocturnal cooling. The official answer (a) follows. The governing principle is that anything increasing moisture, cloud or maritime exchange narrows the annual range, so three of the four statements can be rejected from that single idea.

Thunder is an acoustic phenomenon with a single mechanical cause. The lightning discharge heats the air in the narrow channel it passes through to something of the order of thirty thousand kelvin in a few microseconds, roughly five times the surface temperature of the sun. Air at that temperature expands explosively, and the supersonic expansion generates a shock wave which decays into the sound wave heard as thunder. The rumble rather than a single crack is a consequence of the length of the channel, sound arriving from successively more distant portions of it at successively later times, and of reflection from terrain and cloud.

• Item 1 is incorrect: clouds are suspensions of water droplets and ice crystals with no rigid surfaces, and they do not collide and produce sound in any mechanical sense; this is a folk explanation the question is deliberately offering.
• Item 2 is incorrect and is close to nonsense as drafted, lightning being the discharge that follows the separation of charge, not an agency that separates clouds.
• Item 3 is incorrect as an explanation of thunder, though it is a correct account of something else and is the most tempting of the three: violent updraughts carrying water and ice are exactly what produces the charge separation in a cumulonimbus, by collision between graupel and ice crystals in the presence of supercooled water, and so they cause the lightning. But they do not cause the sound; the sound is caused by the discharge. Since none of the three is the cause of thunder, the official answer is (d). The lesson worth drawing is the distinction between the cause of the electrification, which is item 3, and the cause of the sound, which is the thermal expansion the option set never mentions.

Tropical cyclogenesis requires sea surface temperatures of about 26.5 degrees Celsius through a deep mixed layer, a pre-existing low level disturbance to organise convection, weak vertical wind shear, sufficient Coriolis force and a moist middle troposphere. In the South Atlantic and the south-eastern Pacific the seed disturbance is missing, because the Inter-Tropical Convergence Zone in these longitudes remains north of the equator through almost the whole year and rarely migrates into the southern tropics, so option (b) is the answer.

• Option (c) is wrong on the stem's own terms: the stem specifies tropical latitudes, where Coriolis force is adequate; it is only within about five degrees of the equator that Coriolis vanishes.
• Option (d) is wrong because tropical cyclones form over open ocean and land is not a requirement but a destroyer of them.
• Option (a) is the strongest distractor and is not wholly false, since the Benguela and Humboldt upwelling systems do keep coastal waters cool in exactly these two regions, but low sea surface temperature is a contributory condition confined to the eastern margins rather than the reason covering the whole region, and strong vertical shear operates alongside it.

• Statement 1 is incorrect as written. The westerlies are the prevailing winds of the mid latitudes, blowing from the subtropical high towards the subpolar low and deflected eastward by the Coriolis force, and they occupy roughly thirty to sixty degrees in each hemisphere separately, that is, thirty to sixty degrees North and thirty to sixty degrees South. The belt described in the statement, running from thirty degrees North across the equator to sixty degrees South, would swallow both trade wind belts and the equatorial trough, where the winds are easterly or calm.
• Statement 2 is correct. Winter rain over Punjab, Haryana, western Uttar Pradesh and Jammu and Kashmir is brought by western disturbances, extratropical low pressure systems that form over the Mediterranean and the Caspian region and are steered across Iran, Afghanistan and Pakistan into north western India by the subtropical westerly jet stream, which shifts south of the Himalaya in winter. That rain matters out of proportion to its quantity because it falls on the standing rabi wheat crop. The official answer (b) follows. Elimination route: statement 2 is standard monsoon material, and its acceptance removes options (a) and (d).

• Statement 1 is incorrect, and the error is a single substituted word. The Indian Ocean Dipole is defined by the sea surface temperature gradient between the tropical western Indian Ocean, off the Horn of Africa, and the tropical eastern Indian Ocean, off Sumatra and Java. The statement replaces the eastern Indian Ocean with the eastern Pacific, which is the pole used to define El Nino, not the Dipole. The Dipole is measured by the Dipole Mode Index, the difference of the anomalies between those two Indian Ocean boxes; a positive phase has the western pole warmer and the eastern cooler.
• Statement 2 is correct. A positive Dipole strengthens moisture convergence over the Indian subcontinent and has repeatedly offset the suppressing effect of El Nino on the monsoon, the drought year of 1997 despite a very strong El Nino, and the deficient year of 2002 despite a weak one, being the standard contrasting pair. The two modes are not independent, and forecasting practice treats their combined state rather than either alone. Rejecting statement 1 eliminates (a) and (c), leaving (b) against (d). Governing principle for the subject teacher: the Dipole is an Indian Ocean mode and both its poles lie in the Indian Ocean. Any definition reaching into the Pacific has confused it with the Southern Oscillation.

On 21 June the Sun is vertically overhead at the Tropic of Cancer, the northern hemisphere summer solstice. The circle of illumination is then tangential to the Arctic and Antarctic Circles in such a way that the entire area north of the Arctic Circle receives continuous daylight and the entire area south of the Antarctic Circle receives continuous darkness.

• Option (a) is therefore correct, since at the Arctic Circle the Sun grazes the horizon at midnight without setting.
• Option (b) is wrong for the opposite reason, since the Antarctic Circle is then in polar night and the corresponding statement about it holds on 22 December, not 21 June.
• Option (c) is wrong because the Sun is overhead at the Equator only at the equinoxes, on about 21 March and 23 September.
• Option (d) is wrong because the Sun is overhead at the Tropic of Capricorn on about 22 December, the winter solstice. The governing principle is the annual migration of the vertical rays between the two tropics, and every option is generated by displacing that fact to the wrong date or the wrong hemisphere.

Dew forms when a surface loses heat by outgoing longwave radiation during the night, cools below the dew point temperature of the adjacent air, and water vapour condenses directly on it. The condition for dew is therefore rapid nocturnal radiative cooling, which requires a clear sky, calm air and high relative humidity. On a cloudy night the outgoing terrestrial radiation is intercepted by the cloud layer and returned downward, the surface does not cool to the dew point, and no dew forms.

• Option (b) states this and is the official answer.
• Option (c) is not merely wrong but inverted, since cloudy nights are warmer at the surface than clear nights, which is precisely why dew fails to form.
• Option (d) is wrong because clouds do not deflect wind to ground level, and although wind does inhibit dew by mixing the air near the surface, that is not a cloud effect.
• Option (a) is the real distractor and deserves a note, since clouds do absorb terrestrial longwave radiation. It is rejected because absorption alone, without re emission back towards the surface, would not keep the ground warm; the operative mechanism is the return of energy downward.

• Statement 1 is incorrect. Jet streams are a consequence of the thermal gradient between equator and pole and of the Coriolis deflection, both of which operate in both hemispheres, so polar front and subtropical jets exist north and south of the equator.
• Statement 2 is correct. The eye is a feature of intense and well organised tropical cyclones, appearing once the system is mature and the circulation symmetric; weaker tropical storms, and the extratropical and temperate cyclones which form on fronts, have no eye at all.
• Statement 3 is incorrect and is the discriminating statement. The eye is the warmest part of the storm, not the coldest, because air subsides in the centre and warms adiabatically as it descends, which is exactly why the eye is cloud free and calm; the warm core is what distinguishes a tropical cyclone from a temperate one. Only statement 2 stands, giving (c). The physical reasoning on subsidence disposes of statement 3 without any recall of figures.

The question turns on net cloud radiative forcing, which is the balance between a cloud's shortwave albedo effect and its longwave greenhouse effect.

• Statement 1 is incorrect. High clouds are cirriform, thin and composed of ice crystals, so they are largely transparent to incoming solar radiation and reflect comparatively little of it. Their tops are very cold, so they radiate to space at a much lower temperature than the surface whose outgoing longwave they intercept, and the resulting deficit in emission traps energy in the system. The net effect of high cloud is therefore warming, not cooling, and the statement reverses it.
• Statement 2 is incorrect as a whole, though its first clause is sound. Low clouds are optically thick and do behave almost as black bodies in the infrared, so they absorb the longwave radiation leaving the surface. But their tops are warm, being close to surface temperature, so they re-emit at nearly the same rate and their greenhouse effect is correspondingly small, while their high albedo reflects a large fraction of incoming solar radiation. The albedo term dominates, and the net effect of low cloud is cooling, so the conclusion drawn in the statement does not follow from its premise. Both statements failing, (d) is correct; options (a), (b) and (c) each accept one of the two reversed net effects.

• Option (c) is correct. Water vapour is the dominant absorber of infrared radiation in the atmosphere, with strong absorption bands in the infrared, and it is concentrated in the lower troposphere because its concentration falls steeply with temperature and hence with altitude. This is the physical basis of the natural greenhouse effect and of why humid nights cool less than dry ones.
• Option (a) is wrong on magnitude. Annual insolation at the equator exceeds that at the poles by a factor of roughly two and a half, not ten, since the poles receive continuous summer daylight which partly compensates for the very low solar elevation.
• Option (b) is wrong. Infrared accounts for roughly half of incoming solar energy, with visible light around forty per cent and ultraviolet under ten per cent, so two thirds overstates it.
• Option (d) is wrong by definition, infrared lying beyond the red end of the visible spectrum at longer wavelengths and therefore outside it.

• Statement II is correct and is elementary. The specific heat capacity of water is roughly four to five times that of dry soil and rock, and water is further moderated by vertical mixing, transparency to solar radiation and evaporative cooling, so oceans heat and cool far more slowly than land.
• Statement I is incorrect, and this is the harder judgement. The land and sea thermal contrast in the middle and higher latitudes is greatest in winter, not summer, because continental interiors cool dramatically under long nights, snow cover and radiative loss, reaching values tens of degrees below the adjacent ocean, whereas in summer land warms above the ocean by a much smaller margin. The strength of the winter Siberian high against the relatively mild North Atlantic and North Pacific illustrates the point.
• Statement I false and Statement II true gives (d).
• Options (a) and (b) require the summer contrast to dominate, and (c) denies a settled physical property.

• Statement-I is incorrect. The atmosphere is largely transparent to incoming short wave solar radiation, which passes through and heats the earth's surface. The atmosphere is then heated chiefly from below by outgoing long wave terrestrial radiation, through absorption, conduction and convection.
• Statement-II is correct. Carbon dioxide, water vapour, methane and other greenhouse gases absorb strongly in the infrared and are therefore good absorbers of long wave radiation, which is precisely the mechanism by which terrestrial radiation heats the atmosphere. Since Statement-I is false and Statement-II is true, (d) is correct.
• Options (a) and (b) both require Statement-I to be true and fail immediately.
• Option (c) inverts the truth values and fails on both counts. Note that Statement-II, correctly read, is the reason Statement-I is wrong.

• Statement-I is correct. The troposphere extends to roughly 18 km at the equator against roughly 8 km at the poles.
• Statement-II is correct. Intense insolation at the equator drives strong convection which carries heat, moisture and turbulence to great heights, pushing the tropopause upward.
• Statement-II is the causal explanation for Statement-I, so (a) is correct.
• Option (b) fails because the explanatory link is genuine and is the standard textbook reason.
• Options (c) and (d) fail because both statements are true.

• Statement 1 is correct. In January, the northern hemisphere winter, oceans retain heat while continents cool rapidly. Isotherms therefore bend poleward, that is northward, over the warmer oceans and equatorward, that is southward, over the colder land masses.
• Statement 2 is incorrect and internally contradictory. The Gulf Stream and North Atlantic Drift are warm currents, not cold, and they make the North Atlantic warmer rather than colder. Describing them as cold currents that produce cooling is a factual error, even though the concluding clause about isotherms bending north is itself true. A statement containing a false premise cannot be accepted. Hence (a).
• Options (b) and (c) require statement 2 to be true, and (d) wrongly rejects statement 1.

• Statement 1 is correct. Water vapour is concentrated in the lower troposphere and its quantity falls sharply with height, with about half of all atmospheric water vapour lying below roughly 2 km and almost none above 12 km.
• Statement 2 is incorrect. Water vapour content is maximum over the warm humid equatorial and tropical oceans, where it can reach about four per cent of air by volume, and minimum over the cold polar regions, where it falls to a fraction of one per cent. The saturation capacity of air rises with temperature, so cold polar air simply cannot hold much vapour. Hence (a).
• Options (b) and (c) require the false polar claim, and (d) wrongly rejects the correct altitude relationship.

• Statement I is correct and reproduces the standard textbook position: dust concentration is highest over subtropical and temperate latitudes and lowest over equatorial and polar regions.
• Statement II is incorrect, and it inverts the very reason Statement I holds. Subtropical and temperate areas have more dry winds, not less. The subtropical belt is dominated by descending limbs of the Hadley circulation, producing arid and semi-arid surfaces with sparse vegetation cover from which fine particles are readily lifted. By contrast the equatorial belt has heavy convectional rainfall, dense vegetation and high humidity, so particles are both less available and continuously scavenged from the air by precipitation, while polar regions have frozen or snow-covered surfaces offering little loose material. Since I holds and II fails, the answer is (c).
• Options (a) and (b) both require Statement II to be correct.
• Option (d) rejects the dust distribution itself, which is well established. The trap rewards a candidate who notices that the given explanation contradicts rather than supports the observation.

January is winter in the Northern Hemisphere.

• Statement II is correct because of the differential heating and cooling of land and water: land has low specific heat and heats and cools rapidly, while water has high specific heat, mixes vertically and is partly transparent to insolation, so it retains heat far longer. In winter the continents therefore cool sharply while the adjacent oceans remain relatively warm.
• Statement I is correct and follows directly. An isotherm joins points of equal temperature, so where it crosses a cold landmass it must swing towards the equator to find that temperature, and where it crosses a warmer ocean it swings towards the pole. The bending is thus a map expression of the land and sea contrast, which means Statement II genuinely explains Statement I, giving (a).
• Option (b) concedes both statements but denies the causal link, which cannot stand since the temperature contrast is the sole cause of the deflection.
• Options (c) and (d) each reject one correct statement. Note that in July the pattern reverses, with isotherms bending poleward over the heated landmasses.

• Statement I is correct. Balancing absorbed solar radiation against emitted longwave radiation gives Earth an effective radiating temperature of about minus eighteen degrees Celsius, well below freezing. The observed global mean of roughly fifteen degrees Celsius is the difference made by the atmosphere, so without it the surface would indeed be frozen almost everywhere.
• Statement II is correct and states the mechanism: the atmosphere is largely transparent to incoming shortwave solar radiation but absorbs a substantial part of the outgoing longwave terrestrial radiation, re-emitting it in all directions including downward, which raises the equilibrium surface temperature.
• Statement III is correct. Molecular structure decides radiative behaviour. Nitrogen and oxygen, being homonuclear diatomic molecules with no dipole moment, are effectively transparent to longwave radiation, whereas carbon dioxide, water vapour, methane and nitrous oxide possess vibrational modes that produce a changing dipole moment and therefore absorb strongly in the infrared. All three hold, giving (c).
• Options (a), (b) and (d) each omit one correct statement. Note that the question describes the natural greenhouse effect, which makes the planet habitable, and not the enhanced anthropogenic effect.

• Statement 1 is correct, since the islands lie between about six and fourteen degrees north in the Bay of Bengal, with a small land area surrounded by warm sea, giving high humidity, a very small annual range of temperature and heavy rainfall, which is the definition of a humid tropical coastal or equatorial maritime climate.
• Statement 2 is correct, and it follows from position rather than memory. The islands lie in the path of the Bay of Bengal branch of the southwest monsoon from May onward and again in the path of the retreating northeast monsoon from October to December, when those winds pick up moisture crossing the Bay.
• Statement 3 is incorrect and is the eliminator, because it inverts the wet season. The islands are wet from roughly May to December and comparatively dry from January to April, so a December to May maximum is wrong at both ends.

Hence 1 and 2 and (b) is correct, while (c) and (d) accept the inverted rainfall regime and (a) discards a statement that follows directly from the islands' latitude and position.

• Statement 1 is correct and statement 2 is the eliminator, so (a) is the answer. The Bharat Forecast System is a high resolution numerical weather prediction model working on a grid of about six kilometres against the roughly twelve kilometre grid of the system it replaces, and that refinement is precisely what allows forecasts to be issued for panchayat clusters rather than for districts, which is the operational point for agriculture and for local disaster response.
• Statement 2 misattributes the development. The model was built by the Indian Institute of Tropical Meteorology at Pune, an institution of the Ministry of Earth Sciences, and it runs on the Arka high performance computing system, with the India Meteorological Department operationalising the forecasts. The Indian Institutes of Technology contribute to atmospheric research but are not the developers of this model. (b) and (c) accept the misattribution and (d) discards the accurate statement of purpose.