World physical geography

The Fujiwhara effect is the binary interaction of two tropical cyclones close enough to rotate about a common centre; Japan's Dr Sakuhei Fujiwhara showed it in water-vortex experiments of 1921–23. Its mark on track and strength follows one rule: the stronger steers, the weaker pays.

What the Fujiwhara effect is

• Threshold: interaction begins at about 12 degrees of latitude (~1,350 km) apart (Hong Kong Observatory).
  • US National Weather Service: 300–750 nautical miles, depending on the cyclones' sizes.
• Mechanism: each vortex is carried along by the other's wind field, so the two orbit a common centre.
• Sense of rotation: anticlockwise in the northern hemisphere, clockwise in the southern, mirroring the cyclones' own spin.

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Tundra (Köppen ET) is the treeless polar belt on permafrost, where a short growing season supports only mosses, lichens and low flowering plants (NCERT). The cold that keeps it fragile also locks in resources the world wants, so the statement needs weighing on both sides.

Why tundra is ecologically fragile

• Permafrost dependence: plants and settlements rest on frozen ground; thaw turns firm land into thermokarst lakes and slumps.
• Slow recovery: thin, nutrient-poor soils and short food chains; a single spill or vehicle track scars the land for decades.
• Climate amplifier: the Arctic has warmed nearly four times faster than the globe since 1979 (Rantanen et al., 2022); thaw releases carbon dioxide and methane.
  • IPCC SROCC (2019): permafrost temperatures have increased to record high levels since the 1980s.
• Fragile livelihoods: about 500,000 of the Arctic's 4 million people are Indigenous; herding and hunting need intact tundra.

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Aeolian processes, the erosion, transport and deposition of material by wind, dominate where plant cover is thin, soils loose and rainfall scanty. UNCCD defines desertification as land degradation in dry areas from climate and human action; wind is its chief agent in India's west, where degraded land covers 97.85 million hectares (ISRO Atlas, 2018-19).

How wind erosion and transport degrade land

• Deflation: wind lifts silt, clay and humus off bare soil, leaving coarse, infertile lag gravel and deflation hollows.
• Abrasion: saltating sand sand-blasts seedlings, crops and soil crusts, exposing fresh soil to further loss.
• Transport: saltation and creep move sand locally; suspension carries fine dust and nutrients hundreds of kilometres downwind.
  • The 1930s Dust Bowl of the US Great Plains showed how ploughed, drought-hit grassland becomes airborne soil.

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Local winds blow over a limited area, driven by regional pressure gradients and relief. The Loo, Chinook and Foehn are all hot and dry, yet they spring from opposite roots: the Loo from a heated continental plain, the Chinook and Foehn from air descending mountain lee slopes. Comparing them on region, nature and impact shows why.

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Regions of prevalence

• Loo: the northern plains from Punjab to Bihar, strongest between Delhi and Patna, through May and June before the monsoon breaks.
• Chinook: the eastern, leeward slopes of the Rocky Mountains across the Canadian Prairies and the US Great Plains, mainly in winter and early spring.
• Foehn: the leeward northern valleys of the Alps, where it was first studied; the name now covers lee-slope winds worldwide.
  • The Chinook is itself the Rockies' foehn: the same wind under a local name.
  • Kin elsewhere: the Zonda of the Andes, the Santa Ana of California, the Nor'wester of New Zealand.
• Contrast: the Loo is tied to a plain and a season; the Chinook and Foehn are tied to a mountain wall and can arrive in any month.

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NCERT traces the idea in three steps: Wegener's continental drift, Hess's sea-floor spreading (1961) and plate tectonics (1967). Together they show that continents and ocean basins are constantly made, moved and destroyed by the motion of crustal plates.

How the crustal plates move

• Rigid plates: the lithosphere is broken into plates riding the soft asthenosphere, driven by mantle convection, slab pull and ridge push.
• Measured speeds: magnetic stripes beside ridges give rates from under 2.5 cm/yr (Arctic Ridge) to over 15 cm/yr (East Pacific Rise).

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NCERT calls a tropical cyclone a heat engine run on latent heat from condensing sea moisture; a sea surface above 27°C is its first condition. Warmer seas therefore feed it directly.

Meaning of SST rise

• Meaning: sea surface temperature (SST) is the temperature of the ocean's top layer; its rise is sustained warming of that layer.
• Drivers: greenhouse warming, marine heatwaves and warm ENSO and Indian Ocean Dipole phases.

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Aurora borealis (northern lights) and aurora australis (southern lights) are natural light displays in the thermosphere, seen mostly near the poles. On 10–11 May 2024, cameras of the Indian Institute of Astrophysics recorded one even over Hanle in Ladakh, a reminder that their trigger lies on the Sun.

What auroras are

• Definition: aurora is Latin for dawn; NCERT names the display Aurora Borealis in the Northern Hemisphere and Aurora Australis in the Southern.
• Location: two ovals roughly centred on the magnetic poles: over Scandinavia, Alaska and Canada in the north; over Antarctica, seen from Tasmania and New Zealand, in the south.
• Height: the aurora typically forms 80 to 500 km above the surface (NOAA), mostly within the thermosphere.
• Colours: arcs, curtains and rays; oxygen glows green and red, nitrogen blue and purple.

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A twister, or tornado, is a violently rotating column of air that descends from a severe thunderstorm like an elephant's trunk, with very low pressure at its centre. NOAA counts more than 1,200 tornadoes a year in the United States, concentrated on the plains north of the Gulf of Mexico.

What a twister is

• Local but violent: a storm of short duration over a small area, which releases heat and potential energy as kinetic energy; over the sea it is called a waterspout.
• Parent storm: the most destructive tornadoes come from supercells, rotating thunderstorms with a radar-visible circulation called a mesocyclone (NOAA).

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Ancient India grew inside a clear physical frame: mountains and seas enclosed it, while passes, rivers and coasts kept its regions connected. Each feature shaped settlement, production and power.

Mountains: shield and gateway

• Himalayan wall: blocked easy entry from the north and traps the monsoon winds, making them shed their moisture within the subcontinent.
• North-western passes: the Khyber and the Bolan admitted migrants, traders and armies, and with them new crafts and ideas.

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NCERT describes fjords as very deep glacial troughs filled with sea water along high-latitude shorelines, with famous examples in Norway, Chile and New Zealand. Ice made them, and ice also made them beautiful.

How fjords are formed

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• Glacial erosion: moving ice plucks and abrades rock, turning V-shaped river valleys into steep U-shaped troughs.
• Over-deepening: thick ice cuts deepest inland; near the mouth it thins, leaving a shallow rock sill behind which the water stays deep.
• Drowning: post-glacial sea-level rise floods the trough, giving an indented coast where hillsides drop sharply into the water.
• Hanging valleys: smaller tributary glaciers cut shallower troughs that now hang above the fjord.

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In the middle Ganga plain the rain-bearing monsoon blows from the east, so Bhojpuri speech calls it Purvaiya, the easterly. The name is geography; its songs are culture.

Why the south-west monsoon is 'Purvaiya'

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• Deflection: the Arakan Hills turn the Bay of Bengal branch into West Bengal from the south and south-east.
• Channelling: the Himalaya and the north-west thermal low send one arm westward along the Ganga plain.
• Local contrast: the local westerly is the dry summer loo, so only the easterly means rain.

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India has temperate vegetation on Himalayan heights, tropical rain forests in the Western Ghats and the Andaman and Nicobar Islands, thorn forests in the dry north-west and mangroves in deltas. This diversity flows from climate, relief, soil and history, and its rain-forest core needs protection.

Factors behind diversity of natural vegetation

• Rainfall: the decisive control; over 200 cm with mean temperature above 22°C gives tropical evergreen forest, while under 50 cm gives thorn forest.
• Seasonality: the monsoon's long dry spell makes most Indian forests deciduous, shedding leaves to save water.
• Altitude: the Himalaya stacks tropical, temperate and alpine belts; deodar, a valued endemic, grows mainly in the western Himalaya.
• Relief and aspect: windward slopes are wetter than leeward ones, and north- and south-facing slopes receive different sunlight.
• Soil and drainage: laterite, black, alluvial and desert soils carry different plant communities; waterlogged deltas grow mangroves.
• Isolation and people: island arcs develop distinct vegetation, while clearing, grazing and fire leave secondary growth.

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Igneous rocks form when magma or lava cools and solidifies; they are called primary rocks because sedimentary and metamorphic rocks derive from them. Depth and rate of cooling shape their character.

Characteristics

• Crystalline and massive: interlocking mineral crystals, with no strata, bedding planes or fossils.
• Texture follows cooling: slow cooling at depth gives large grains; quick cooling at the surface gives fine or glassy texture.
• Hard and resistant: compact and mostly impermeable, though joints admit water; basalt flows build plateaus such as the Deccan.
• Useful: they host metallic ores and supply building stone.

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The Deccan Traps are a flood basalt province, built by lava flows fed through dykes, covering most of the Maharashtra plateau. This weathered, jointed basalt stores soil, stone, water and energy.

Natural resource potentials

• Black soil (regur): basalt weathers into clay-rich, moisture-retentive black cotton soil across the Maharashtra, Saurashtra and Malwa plateaus, ideal for cotton.

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• Building stone: durable basalt gives road metal and railway ballast; Ellora's 34 caves were dug into a high basalt cliff.
• Minerals: laterite caps on plateaus hold bauxite, the aluminium ore; gas cavities in the basalt hold agate and zeolites.
• Water: jointed basalt stores groundwater for dug wells; the Krishna rises near Mahabaleshwar.
• Hydel and wind: the steep western scarp powers the Koyna project, and (PIB, 2026) Maharashtra alone has 173.9 GW of wind potential at 150 m.

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The sun heats the oceans unevenly, winds drag their surface and Earth's rotation bends every moving mass. Together these set water flowing in currents, and where currents meet, rise or sink, they decide where fish gather.

Forces influencing ocean currents

• Solar heating: equatorial water expands and stands slightly higher, so it flows down the gentle slope towards the poles.
• Wind: planetary winds, the trades and westerlies, drag surface water into equatorial currents and drifts.
• Gravity: pulls water down pressure slopes and makes cold, dense water sink.
• Coriolis force: deflects moving water to the right in the northern hemisphere and left in the southern, closing the great gyres.
• Secondary forces: temperature, salinity and density differences drive slow thermohaline circulation; coasts and sea-floor relief deflect flow; the monsoon's seasonal reversal reverses currents in the northern Indian Ocean.

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Nearly 95% of India's trade by volume moves by sea (PIB, 2025), and most world shipping must pass a few narrow gateways. Straits (natural sea channels) and isthmuses (land necks, once cut by canals) decide the distance, cost and safety of that trade.

Straits: natural gateways of trade

• Shortcuts between seas: the Strait of Malacca, between Malaysia and Sumatra, joins the Indian Ocean to the South China Sea and East Asia's factories.
• Energy lifelines: about 21 million barrels of oil a day crossed the Strait of Hormuz in 2018, roughly a fifth of world petroleum use (US EIA).
• Openings of enclosed seas: Gibraltar opens the Mediterranean to the Atlantic; the Turkish Straits are the Black Sea's outlet for grain and oil.
• Entrepot ports: Singapore on Malacca and Rotterdam near the English Channel earn from transshipment, bunkering and ship repair.

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The troposphere reaches about 8 km over the poles and 18 km over the equator and holds nearly all the air's dust and water vapour (NCERT). Heat, moisture and motion meet only here, so every change of weather happens in this layer.

Heated from below: the weather engine

• Normal lapse rate: the sun warms the ground, the ground warms the air, so temperature falls by 6.5°C per 1,000 m. Warm, light air beneath cold air tends to rise.
• Moisture and dust: water vapour condenses on dust and salt particles, giving clouds, rain, snow, hail, fog and dew, all absent higher up.
• Latent heat: condensation releases stored heat; of the 34 heat units the earth passes to the air, 19 come this way (NCERT heat budget), feeding storms.

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A subcontinent is a large, distinct part of a continent. Bounded by the Himalaya and the Indian Ocean, India and its neighbours form a great geographic entity within Asia, with India at its core.

Physical separation

• Mountain wall: the Himalaya and other ranges formed a formidable barrier, crossed only through passes such as Khyber, Bolan, Shipkila, Nathula and Bomdila.
• Seas on three sides: the Arabian Sea, Bay of Bengal and Indian Ocean frame the peninsula.

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Temperature falls with height at a normal lapse rate of 6.5°C per 1,000 m, and moist air forced up a slope cools until its vapour condenses. A range's alignment to the prevailing winds therefore decides where rain falls and how local weather behaves.

Alignment of major ranges

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The Circum-Pacific Zone, or Ring of Fire, is a horseshoe of more than 450 volcanoes stretching nearly 40,250 km around the Pacific. One process explains its character: plates meeting along the ocean's rim.

Plate-tectonic setting

• Convergent margins: the Pacific plate and minor plates such as the Cocos (off Central America) and Nazca (off South America) sink beneath neighbouring plates at subduction zones.
• Transform margin: along the San Andreas Fault, plates slide past each other, causing earthquakes without volcanoes.

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The UNCCD defines desertification as land degradation caused by climatic variations and human activities. ISRO's atlas puts India's degraded land at 97.85 million hectares in 2018-19, which shows why no climatic line contains it.

Why climate does not bound it

• Human drivers: overgrazing, deforestation, mining and poor irrigation work in every climate, wet or dry, hot or cold.
• Shifting margins: climate variability moves dryland edges; the Sahel drought south of the Sahara (1967–1977) degraded vast rangelands.

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Young, steep and seismically active, the Himalaya are being reshaped by roads, hydropower, construction and mass tourism. The 2013 Kedarnath disaster and, since then, land subsidence in Joshimath in January 2023 showed how such pressure turns natural hazards into disasters.

Damage from development and tourism

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• Slopes: cutting, tunnelling and muck dumping destabilise hillsides, and buildings spread onto landslide-prone land.
• Water base: NITI Aayog's working group (2018) reported that half of the Himalayan region's three million-plus perennial springs had dried or turned seasonal.
• Tourism pressure: visitors beyond carrying capacity bring plastic, sewage, traffic and hotel sprawl.
• Cumulative risk: dams and roads crowded into one valley multiply hazards; in 2013, a glacial burst and landslides destroyed the area around Kedarnath temple.

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Ocean currents are moving streams of water driven by winds, density differences and the earth's rotation; water masses are large bodies of water with their own temperature, salinity and density. Currents act by transport, water masses by layering and mixing, and their impacts differ accordingly.

How currents and water masses differ

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A mantle plume is a narrow column of unusually hot, buoyant rock rising from deep in the mantle, possibly near the core–mantle boundary. Where it meets the plate above, it creates a hotspot, linking deep-Earth heat to plate behaviour.

What a mantle plume is

• Mechanism: the plume head spreads beneath the lithosphere and partly melts, feeding volcanoes above.
• Fixed source: plumes stay roughly stationary while plates drift over them, the hotspot idea J. Tuzo Wilson proposed in 1963.

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NCERT traces Earth's origin to the nebular hypothesis, revised by Laplace in 1796: planets formed from a cloud around a young Sun. NASA's Juno, launched on 5 August 2011 and orbiting Jupiter since 4 July 2016, tests this on the oldest planet.

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Why Jupiter holds Earth's story

• Oldest planet: NASA dates Jupiter to leftovers of the Sun's formation 4.6 billion years ago.
• Time capsule: unlike Earth, its huge mass kept its original composition, preserving the early nebula's chemistry.

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The cryosphere is the frozen part of the Earth system: ice sheets, glaciers, sea ice, snow cover and permafrost. Since the exam, the IPCC's special report on the ocean and cryosphere records a widespread shrinking of the cryosphere with warming, and that shrinking feeds back on climate in several ways.

Energy balance: albedo

• Reflection: snow- and ice-covered areas add to the Earth's albedo, the share of sunlight reflected back to space.

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Salinity is the amount of salt dissolved in sea water; the open ocean normally ranges between 33‰ and 37‰, and the Indian Ocean averages 35‰. These variations reflect how water is gained or lost at the surface, and their effects run from ocean physics to the economy.

Accounting for variations in salinity

• Evaporation and precipitation: the main control on surface salinity; salinity rises under clear, dry subtropical skies and falls where heavy rain dilutes the surface.
• River inflow: the Bay of Bengal is less saline than the Arabian Sea, which loses more water to evaporation; the Baltic and Black Seas stay low for the same reason.
• Freezing and thawing: forming sea ice leaves brine behind in polar seas, while melting ice dilutes them.
• Winds and currents: they move water between regions; the North Atlantic Drift makes the North Sea saltier despite its high latitude.
• Enclosure: the land-locked Red Sea reaches 41‰, and the Mediterranean is salty from high evaporation.

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NCERT describes India's climate as tropical monsoon, with heavy rain mostly in summer. NCERT notes that intensive subsistence agriculture is found largely in the densely populated regions of monsoon Asia; a few traits of this climate explain why.

Characteristics of the monsoon climate

• Seasonal reversal of winds: differential heating of land and sea creates shifting pressure zones; moist onshore winds blow in summer, dry offshore winds in winter.
• Rain in the hot season: water and peak warmth arrive together, so the main crop season gets both moisture and heat, ideal for rice.
• Warmth almost all year: a long growing season lets farmers raise a second or even third crop wherever irrigation exists.
• A dry, mild winter: clear skies ease harvesting, drying and storing grain, and support an irrigated winter crop such as wheat.
• Rivers fed by rain and snow: the Ganga, Brahmaputra, Mekong and Yangtze carry monsoon water and silt, building fertile plains and deltas.
• A variable rhythm: onset, breaks in the monsoon and retreat vary from year to year, bringing floods in some seasons and droughts in others.

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An air mass is a vast body of air whose temperature and moisture vary little horizontally, because it has rested over one uniform surface, its source region. Carried far from home, these properties shape weather and, over time, climate.

The concept of air mass

• Source regions: tropical and subtropical oceans, subtropical hot deserts, cold high-latitude oceans, and snow- or ice-covered continents (NCERT lists five).

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• Modification: a moving air mass gains or loses heat and moisture below it; air colder than the ground turns unstable and showery, warmer air stays stable and foggy.

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NCERT notes that debris avalanches and landslides occur very frequently in the Himalayas: the range is tectonically active, built of sedimentary rock and loose deposits, and very steep. Monsoon rain and careless construction turn this natural fragility into disaster.

Natural causes

• Tectonics: the range is still rising and lies in Seismic Zones IV and V; tremors loosen rock and debris.
• Weak material: sedimentary rocks and unconsolidated or semi-consolidated deposits give way easily when wet.
• Steep slopes and river cutting: rivers undercut the toe of steep slopes, removing their support.
• Intense rain: monsoon downpours and cloudbursts saturate slopes, as in the 2013 Uttarakhand disaster.

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The South China Sea joins the Indian and Pacific Oceans through the Malacca Strait, and six claimants contest it. The July 2016 arbitral award made it a test of law against power.

Why its significance has grown

• Trade artery: a large share of world seaborne trade, including India's trade with East Asia, sails these sea lanes.
• Resources: rich fisheries and believed oil and gas reserves make exclusive economic zone rights worth contesting.
• Overlapping claims: China's 'nine-dash line' cuts across the claims of Vietnam, the Philippines, Malaysia, Brunei and Taiwan.
• Militarised reefs: China built artificial islands at seven Spratly features, which the tribunal found caused severe harm to coral reefs.
• Law versus power: in Philippines v. China (12 July 2016) the UNCLOS tribunal found no legal basis for China's historic rights within the nine-dash line; China rejected the award.
• Great-power rivalry: US freedom-of-navigation operations confront Chinese assertiveness, while a divided ASEAN has yet to agree a binding Code of Conduct.

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