Physical Geography

Earth has three concentric layers: Crust, Mantle, Core. Crust is thin (5-30 km; oceanic crust ~5-10 km made of SIMA—silica+magnesia, basaltic; continental crust ~30-70 km of SIAL—silica+alumina, granitic). Mantle extends to 2900 km. Core extends to 6371 km centre; outer core is liquid, inner core solid (iron-nickel, NIFE). Memory aid for discontinuities (top to bottom): 'Con-Moho-Rep-Gut-Lehmann.' Conrad (within crust), Mohorovicic/Moho (crust-mantle), Repetti (upper-lower mantle), Gutenberg (mantle-core), Lehmann (outer-inner core). The asthenosphere (upper mantle, 100-400 km) is the source of magma. S-waves cannot pass through the liquid outer core, proving its liquid state—a shadow zone forms between 105 and 142 degrees.

Earthquake (body) waves: Primary (P) and Secondary (S). P-waves are longitudinal/compressional, fastest, travel through solids, liquids and gases. S-waves are transverse, travel ONLY through solids—this is the key UPSC fact. Surface waves (L-waves) are most destructive. Shadow zones: P-wave shadow zone lies between 105 and 142 degrees from the epicentre; S-wave shadow zone is everywhere beyond 105 degrees (a much wider zone). The S-wave shadow zone existence proves the outer core is liquid; P-waves bend (refract) on entering the liquid core, creating their narrower shadow zone. Mnemonic: 'S Stops in Solids only.'

Temperature increases with depth (geothermal gradient ~1 degree C per 32 m near surface, but the rate slows sharply deeper). Sources of internal heat: radioactive decay (uranium, thorium, potassium), primordial heat from Earth's formation, and gravitational/tidal friction. Density rises with depth—from ~2.7 g/cc (crust) to ~13 g/cc (inner core). Pressure also rises with depth. Despite high temperature, the inner core stays solid due to immense pressure. Example: deepest mine (Mponeng, South Africa, ~4 km) and deepest borehole (Kola Superdeep, Russia, ~12.3 km) only scratched the crust, confirming our interior knowledge relies mainly on indirect (seismic) evidence.

The Himalayas are still rising. The Atlantic Ocean is widening by 2.5 cm every year. California periodically shudders along the San Andreas Fault. All three phenomena trace back to a single unifying theory — plate tectonics — and to the three types of boundaries where Earth's lithospheric plates meet. Mastering the boundary classification is the single most rewarding hour in your UPSC Geography prep.

Definition: A tectonic plate is a large, rigid segment of Earth's outer shell (the lithosphere) that moves slowly relative to its neighbours, riding on the more plastic asthenosphere below.
Definition: A plate boundary is the zone where two plates meet, and where almost all the world's earthquakes, volcanoes, mountain-building, and ocean-trench formation occur.

Why boundaries matter

Earth's interior is hot, and convection currents in the mantle drag the overlying plates in different directions. At plate interiors, very little dramatic happens — plates simply ride along quietly. At plate boundaries, however, plates meet, and the meeting can be one of three kinds depending on the relative motion of the two plates. The classification — divergent, convergent, transform — is the master key to global geomorphology.

Type 1 — Divergent (Constructive) Boundaries

At a divergent boundary, two plates move apart from each other. As they separate, magma from the mantle rises through the gap and solidifies, creating brand-new oceanic crust. Because new lithosphere is being built, these are also called constructive boundaries.

Examples to remember for UPSC:

• Mid-Atlantic Ridge — a 16,000 km underwater mountain chain where the North/South American plates pull away from the Eurasian/African plates. The ridge surfaces above sea level only at Iceland, which is therefore growing wider by ~2 cm/year.
• East African Rift Valley — a continental divergent boundary tearing the African Plate into the Nubian and Somali sub-plates. In a few million years, the Horn of Africa will become a separate landmass.
• Red Sea — formed when the Arabian Plate broke away from the African Plate; a young ocean in formation.

Features at divergent boundaries: mid-ocean ridges, rift valleys, shallow-focus earthquakes, basaltic (low-silica, fluid) volcanism. The eruptions are usually non-explosive because the lava flows easily.

Type 2 — Convergent (Destructive) Boundaries

At a convergent boundary, two plates move toward each other. One plate is forced under the other in a process called subduction, and lithosphere is consumed back into the mantle. Because crust is being destroyed, these are called destructive boundaries. There are three sub-cases depending on which kinds of crust collide:

2a. Ocean-continent collision

Oceanic crust is denser than continental crust, so the ocean plate slides under the continental plate. This produces:

• A deep ocean trench at the point of subduction.
• A chain of volcanic mountains on the overriding continental plate, fuelled by magma from the melting subducted slab.

Best-known example: the Andes Mountains of South America, formed by the Nazca Plate subducting under the South American Plate. Mount Fuji and the Cascades are similar.

2b. Ocean-ocean collision

When two oceanic plates collide, the older (and denser) one subducts under the younger. The result is an island arc — a curved chain of volcanic islands.

Best-known examples: Japan, the Philippines, the Aleutian Islands of Alaska, Indonesia, the Mariana Islands. The Mariana Trench — Earth's deepest point — sits at the eastern edge of the Mariana island-arc system.

2c. Continent-continent collision

When two continental plates collide, neither is dense enough to subduct cleanly. Instead, the crust crumples upward into massive fold mountain ranges. Crucially, there is no significant volcanism in this case because no slab descends deep enough to melt.

The textbook example for UPSC: the Himalayas — formed when the Indo-Australian Plate rammed northward into the Eurasian Plate around 50 million years ago. The collision continues today, lifting the Himalayas by ~5 mm per year. Other examples: the Alps (African vs Eurasian), the Urals (older collision).

Features at convergent boundaries: deep trenches, fold mountains, intermediate-to-deep earthquakes, andesitic (intermediate-silica) and explosive volcanism (except in continent-continent collisions).

Type 3 — Transform (Conservative) Boundaries

At a transform boundary, two plates slide past each other horizontally. Lithosphere is neither created nor destroyed, which is why these are called conservative boundaries. The fault is essentially a giant horizontal sliding zone.

Best-known example: the San Andreas Fault in California, where the Pacific Plate moves north-westward past the North American Plate at about 5 cm per year. Smaller transform faults exist along mid-ocean ridges, breaking them into segments offset from each other.

Features at transform boundaries: shallow but powerful earthquakes (because the plates lock and then suddenly slip), almost no volcanism, no mountain-building.

The historical road to plate tectonics

The modern theory took half a century to assemble:

1. Alfred Wegener (1912) — Continental Drift theory. Wegener, a German meteorologist, proposed that all continents were once joined in a supercontinent called Pangaea, surrounded by a single ocean Panthalassa. They later broke apart and drifted to their present positions. Wegener pointed to matching coastlines (South America and Africa), matching fossils, and matching rock formations across oceans. His weakness — he could not explain what force moved the continents.

2. Harry Hess (1960s) — Sea-Floor Spreading. Hess, an American geologist, proposed that new oceanic crust forms at mid-ocean ridges and spreads outward like a conveyor belt. Old crust is destroyed at trenches. This gave continents a mechanism — they were not "ploughing" through ocean floor; they were riding on top of it.

3. Palaeomagnetism evidence (1960s) — Studies of magnetic stripes parallel to mid-ocean ridges showed that the ocean floor records Earth's reversing magnetic field as it forms. The symmetric pattern of stripes either side of the ridge confirmed sea-floor spreading.

4. Plate Tectonics theory (1967–68) — Jason Morgan, Dan McKenzie, and Robert Parker synthesised these ideas into the modern plate tectonics model: Earth's surface is divided into rigid plates that move because of mantle convection, and all major geological activity happens at plate boundaries.

For UPSC Prelims, remember the sequence: Wegener → Hess → Plate Tectonics, with palaeomagnetism as the supporting evidence.

Why it matters

Plate tectonics is a foundational theme cutting across UPSC GS Paper 1 (Geography), Prelims (Physical Geography MCQs), and even disaster-management questions in Mains. The 2001 Bhuj earthquake, the 2005 Kashmir earthquake, the 2015 Nepal earthquake — all explained by the continuing Indo-Eurasian convergence. The Andaman volcanic activity at Barren Island is explained by the Indian plate subducting under the Burma plate. Every Indian geographic hazard story starts with plate boundaries.

Real-world example

The Indo-Australian Plate is moving north-eastward at about 5 cm per year. As it pushes into the Eurasian Plate, it lifts the Himalayas (continent-continent collision, no volcanism) and triggers earthquakes along the Main Central Thrust and Main Boundary Thrust. Meanwhile, on its eastern margin, the same plate dives under the Burma micro-plate — producing the Sumatra-Andaman subduction zone (ocean-continent), the active Barren Island volcano, and the 2004 Indian Ocean tsunami. Both stories are different sub-cases of the same convergent boundary classification.

Common misconception

The single most common error in UPSC Geography is conflating continental drift (Wegener's 1912 theory) with plate tectonics (Morgan-McKenzie-Parker 1960s theory). They are not synonyms. Continental drift had no mechanism and was rejected by the scientific community for fifty years. Plate tectonics succeeded because it explained how the movement happens, using mantle convection and sea-floor spreading. UPSC frequently asks: "Who proposed the theory of plate tectonics?" — the answer is NOT Wegener.

A second misconception: that the Himalayas have active volcanoes. They do not — continent-continent collisions do not produce volcanism. The closest volcanic activity to India is in the Andaman region (subduction zone), not in the Himalayan range.

:::compare

Boundary Type	Plate Motion	Crust Effect	Key Features	Indian/Global Example
Divergent (Constructive)	Apart	Created	Mid-ocean ridges, rift valleys, shallow earthquakes, basaltic volcanism	Mid-Atlantic Ridge, East African Rift, Iceland, Red Sea
Convergent — Ocean-Continent	Together	Destroyed	Trench + volcanic arc on continent	Andes; Sunda Trench (India)
Convergent — Ocean-Ocean	Together	Destroyed	Trench + island arc	Japan, Philippines, Mariana Trench
Convergent — Continent-Continent	Together	Crumpled	Fold mountains, no volcanism	Himalayas, Alps, Urals
Transform (Conservative)	Sideways past	Neither	Strike-slip faults, shallow strong earthquakes	San Andreas Fault, USA
:::

:::keypoints

• Three boundary types: Divergent (apart, create), Convergent (together, destroy), Transform (slide, conserve).
• Mid-Atlantic Ridge, East African Rift, and Iceland are divergent.
• Andes = ocean-continent convergence; Japan / Philippines = ocean-ocean; Himalayas = continent-continent.
• Continent-continent collisions form fold mountains without volcanism — Himalayas are not volcanic.
• San Andreas Fault is the textbook transform boundary.
• Wegener (1912) proposed continental drift with Pangaea + Panthalassa; lacked a mechanism.
• Harry Hess (1960s) proposed sea-floor spreading; palaeomagnetism confirmed it.
• Plate Tectonics theory (1967–68): Morgan, McKenzie, Parker.
  :::

:::memory
DCT — Divergent Creates, Convergent Consumes, Transform Conserves.
Pangaea-Panthalassa for Wegener; Hess for sea-floor spreading; Morgan-McKenzie-Parker for plate tectonics.
:::

:::recap

• Plates can move apart (divergent), together (convergent), or past each other (transform).
• Divergent → new ocean floor; convergent → trenches, volcanoes or fold mountains; transform → earthquakes only.
• The Himalayas formed by continent-continent collision and have no volcanism.
• Wegener's continental drift evolved into the modern plate tectonics theory via sea-floor spreading and palaeomagnetism.
  :::

The Earth's outer shell is broken into a jigsaw of moving plates, and almost every dramatic landform — Mount Everest, the Andes, the Japanese trench, the Deccan Trap basalts — is a souvenir of those plates pushing against, sliding past, or pulling apart from each other. UPSC Prelims loves this topic because a single matching-type question can sweep across plate names, examples and landforms in one go.

Definition: A lithospheric plate (tectonic plate) is a large, rigid slab of the Earth's lithosphere (crust + uppermost mantle) that floats and moves slowly over the plastic asthenosphere below.

Definition: A convergent boundary is where two plates move towards each other; a divergent boundary is where they move apart; a transform boundary is where they slide past each other.

The Seven Major Plates

Geologists count seven major plates that together cover most of Earth's surface:

1. Pacific Plate — the largest, almost entirely oceanic.
2. North American Plate — carries North America and parts of the Atlantic and Arctic Oceans.
3. South American Plate — carries South America and the western South Atlantic.
4. Eurasian Plate — carries most of Europe and Asia.
5. African Plate — carries Africa and the surrounding ocean floor.
6. Indo-Australian Plate — carries India, Australia and parts of the Indian Ocean (some geologists now split it into separate Indian and Australian plates, but the combined term still dominates exam syllabi).
7. Antarctic Plate — surrounds the South Pole and carries Antarctica.

In addition there are several minor plates — Nazca, Cocos, Caribbean, Arabian, Philippine, Juan de Fuca, Scotia — which often turn up as distractors in UPSC options.

The Pacific Ring of Fire

Definition: The Pacific Ring of Fire is a horseshoe-shaped belt running around the rim of the Pacific Ocean, roughly 40,000 km long, along which most of the world's volcanic and earthquake activity is concentrated.

About 75% of the world's active volcanoes and roughly 90% of all earthquakes occur along this belt. Why? Because the rim of the Pacific is essentially one giant chain of convergent boundaries, where the Pacific (and minor) oceanic plates dive — subduct — beneath the lighter continental plates around them. The descending plate scrapes the overlying mantle, releases volatiles, and triggers melting. The magma then rises, building volcanic arcs like the Andes, Cascades, Aleutians, Kamchatka, Japan, Philippines and the Tonga–Kermadec arc. Friction along the subduction interface stores elastic strain, which is released as the great megathrust earthquakes (Chile 1960, Alaska 1964, Tohoku 2011).

Why the Himalayas Are Different

The Himalayas exist because the Indo-Australian Plate has been ramming northward into the Eurasian Plate for roughly 50 million years, after the slow closure of an ancient ocean called the Tethys Sea that once lay between India and Asia. As the two continents collided, the Tethyan sea floor was crumpled, scraped off and uplifted, producing the world's youngest and highest fold mountain range.

This collision is continent–continent, not oceanic–continental. There is no subduction of a dense oceanic slab into the mantle, and therefore no major volcanism in the Himalayas — only colossal compressional earthquakes (Nepal 2015, Uttarkashi 1991, Kangra 1905). The same collision continues today: the Himalayas are still rising at roughly 5 mm per year, and India is still pushing into Asia at about 5 cm per year.

A delightful proof: marine fossils — ammonites, foraminifera and shelled creatures that once lived on the floor of the Tethys — are found high in the Himalayan rocks, including around Spiti and Kailash. Rocks that once sat under a tropical sea now sit above 4,000 m. That single fact is one of UPSC's favourite "explain how plate tectonics works" hooks.

Why It Matters

Plate tectonics is the unifying theory of physical geography — it explains why earthquakes cluster where they do, why volcanoes occur in arcs rather than randomly, why the world's tallest mountains line a few specific belts, why oil and certain minerals concentrate in particular basins. For UPSC, a single conceptual understanding of "what plate, what boundary, what landform, what hazard" lets you answer multiple Prelims questions and supports half of Mains GS-1 physical geography.

Real-World Example

The 2004 Indian Ocean tsunami that devastated the Andaman and Nicobar Islands, Tamil Nadu coast and Sri Lanka was triggered by a magnitude 9.1 megathrust earthquake off Sumatra, where the Indo-Australian Plate subducts beneath the Sunda (Eurasian) Plate. The earthquake occurred along the same Ring-of-Fire-type convergent margin logic, just on the eastern flank of the Indian Ocean — confirming that the Ring of Fire's mechanics extend beyond the Pacific rim.

Common Misconception

Misconception: "The Himalayas are part of the Ring of Fire and so should have active volcanoes."

Correction: The Himalayas lie outside the Pacific Ring of Fire. They are produced by continent-continent collision, which crumples crust but does not melt it in the way subduction of an oceanic slab does. So the Himalayas are intensely seismic (frequent earthquakes) but not volcanic. India's only active volcano — Barren Island — sits in the Andaman Sea, where a different oceanic plate is subducting, not in the Himalayas.

:::compare

Boundary Type	Plate Motion	Result	Indian Example
Convergent (oceanic–continental)	Towards each other; ocean plate dives	Volcanic arc + trench + earthquakes	Andaman arc; Barren Island volcano
Convergent (continent–continent)	Towards each other; both buoyant	Fold mountains; earthquakes; no volcanism	Himalayas
Divergent	Pulling apart	Mid-ocean ridges; rift valleys	Carlsberg Ridge in Indian Ocean
Transform	Sliding past	Strike-slip faults; earthquakes	(San Andreas Fault is the classic non-Indian example)
:::

:::keypoints

• Seven major plates: Pacific, North American, South American, Eurasian, African, Indo-Australian, Antarctic.
• The Pacific Plate is the largest and almost wholly oceanic.
• The Ring of Fire is a horseshoe-shaped belt around the Pacific along convergent (subduction) margins.
• ~75% of active volcanoes and ~90% of earthquakes lie along the Ring of Fire.
• The Himalayas formed from the Indo-Australian Plate colliding with the Eurasian Plate, closing the Tethys Sea.
• Marine fossils high in the Himalayas confirm a former sea floor uplift.
• Continent–continent collision causes earthquakes but no major volcanism, so the Himalayas have no active volcanoes.
• India's only active volcano, Barren Island, lies in the Andaman Sea (subduction zone), not in the Himalayas.
  :::

:::memory
"PEN-A-SAI" — for the seven plates: Pacific, Eurasian, North American, Antarctic, South American, African, Indo-Australian. And for the Ring of Fire mechanics: "Sub-duct, smoke and shake" — subduction creates volcanoes (smoke) and earthquakes (shake).
:::

:::recap

• Seven major plates carry continents and ocean floors; the Pacific is the largest.
• The Ring of Fire = subduction-driven volcanic and seismic belt around the Pacific.
• The Himalayas are a product of continental collision (Indo-Australian into Eurasian) and lack volcanism.
• Marine fossils in the Himalayas are direct evidence of an uplifted Tethys sea floor.
  :::

Wegener's key evidences: (1) Jig-saw fit of South America's east coast and Africa's west coast. (2) Matching geological structures—Caledonian/Appalachian mountains across the Atlantic. (3) Fossil distribution—Mesosaurus (freshwater reptile) in Brazil and South Africa; Glossopteris flora across all southern continents. (4) Placer gold deposits in Ghana traced to Brazil. (5) Tillite deposits (glacial) indicate former Gondwana. Sea-floor spreading evidence: symmetrical magnetic stripes (palaeomagnetism) on either side of mid-ocean ridges, and youngest rocks at the ridge with age increasing away from it. Mnemonic for southern supercontinent: 'Gondwana = South (India, Australia, Antarctica, Africa, South America).' Laurasia = northern.

Why do polar foxes have stubby ears while desert foxes have huge ones? Why do you not find rats in Antarctica? These are not curiosities — they are textbook NEET illustrations of how organisms physically negotiate with temperature and water. The chapter "Organisms and Populations" frames these as responses to abiotic factors, and a handful of named rules and named examples reappear year after year.

Definition: Allen's Rule states that endotherms (warm-blooded animals) from colder climates tend to have shorter appendages — ears, limbs, tail, snout — to minimise heat loss, while those from hotter climates tend to have longer appendages to maximise heat dissipation.

Definition: A thermoregulator is an organism that maintains a constant body temperature despite environmental fluctuations (most birds and mammals). A thermoconformer lets its body temperature track the environment (most invertebrates, fish, amphibians, reptiles).

Allen's Rule — geometry doing biology's work

Heat is lost across the surface; heat is produced and stored in the volume. Long, thin appendages have a large surface-to-volume ratio, so they shed heat fast. That is wonderful in a hot habitat — think of the giant ears of the fennec fox in the Sahara, which work as living radiators. It is disastrous in the Arctic, which is why the Arctic fox has small, rounded ears tucked close to the body and the polar bear's ears barely poke out of its fur. Allen's Rule is the appendage cousin of Bergmann's Rule (colder climates favour larger body size, because a bigger body has a smaller surface-to-volume ratio overall).

Why small endotherms struggle in extreme cold

A mouse-sized animal has roughly the same metabolic machinery as a fox, but its skin area is enormous relative to its body mass. In bitterly cold environments, the heat lost across that big relative surface is so great that the animal would have to eat almost continuously to keep its core warm. The metabolic cost simply does not pencil out. That is the textbook reason small animals are rarely found in polar regions — and why most "small mammals of the cold" (like lemmings and pikas) cope by burrowing into snow tunnels where temperatures are far milder than the surface air.

Kangaroo rat — surviving without drinking

The kangaroo rat of the North American deserts is a NEET favourite because it shows two beautiful adaptations stacked together:

1. Metabolic water. When the body oxidises fat (and to a smaller extent carbohydrate), one of the chemical products is water. Fat is especially efficient — roughly 1.1 g of water is produced per gram of fat oxidised. The kangaroo rat exploits this so well that it can live its entire life without ever drinking liquid water; its needs are met internally.
2. Concentrated urine. Its kidneys have unusually long loops of Henle, producing a urine many times more concentrated than the blood. Very little water leaves with the waste.

Combine the two and you get an animal that "drinks" its own metabolism and "stores" water by refusing to throw it away.

Desert plants — four classic tricks

Plants cannot run from heat or drought; they re-engineer leaves and metabolism.

• Thick cuticle: a waxy waterproof layer on leaves and stems blocks evaporation.
• Sunken stomata: stomata sit in pits, often lined with hairs, that trap a humid micro-environment and slow water loss.
• CAM photosynthesis: in Crassulacean Acid Metabolism, stomata open at night (cool, humid) to take in CO₂, store it as malic acid in vacuoles, and use it during the day with stomata shut. The plant photosynthesises without bleeding water.
• Leaves as spines: in Opuntia (prickly pear cactus), the leaves are reduced to spines (tiny surface area, no transpiration, defence bonus). Photosynthesis is shifted to the flattened green stem called a phylloclade or cladode.

Why it matters: NEET routinely asks one or two MCQs that turn entirely on remembering the name of the rule (Allen's, Bergmann's), the named example (kangaroo rat, Opuntia), or a specific mechanism (CAM, metabolic water). These are gift marks if you have learnt them as a tight package and a wasted question if you blur them with general "desert animals save water" reasoning.

Real-world example: The fennec fox (Sahara) versus the Arctic fox is the cleanest visual proof of Allen's Rule. Both are foxes; the fennec's enormous ears and the Arctic fox's nub-like ears are the same species-group answering opposite climates. Closer home, the Thar Desert chinkara and the Himalayan tahr show the same body-plan logic in Indian fauna.

Common misconception: Students often say "kangaroo rats drink dew." They do not need to. Their water budget is balanced entirely by metabolic water and concentrated urine — that is the whole point of the example. Another mix-up: confusing CAM with C4 photosynthesis. C4 plants (like sugarcane, maize) separate CO₂ fixation in space (mesophyll vs bundle-sheath cells); CAM plants separate it in time (night vs day).

Question: Why is it harder for a small endotherm to live in the polar region than a large one?
Solution:
Step 1: Heat is lost across the body surface; heat is generated by metabolism within the volume.
Step 2: For a smaller body, the surface-to-volume ratio is larger.
Step 3: So per kilogram of body mass, the small animal loses heat faster than the large animal.
Step 4: To stay warm, the small animal must burn fuel at an unsustainable rate.
Conclusion: Extreme cold favours larger body size (Bergmann's Rule) and shorter appendages (Allen's Rule), and disfavours tiny endotherms.

:::compare

Adaptation	Where seen	What it does
Allen's Rule (short ears/limbs)	Cold-climate mammals	Reduces heat loss
Allen's Rule (long ears/limbs)	Hot-climate mammals	Increases heat loss
Metabolic water	Kangaroo rat	Generates water from fat oxidation
Concentrated urine	Kangaroo rat	Minimises water excretion
Thick cuticle, sunken stomata	Xerophytes (desert plants)	Reduces transpiration
CAM photosynthesis	Opuntia, pineapple, Agave	Stomata open at night to save water
Phylloclade / cladode	Opuntia	Photosynthesis shifted to green stem
:::

:::keypoints

• Allen's Rule: cold ⇒ short appendages; hot ⇒ long appendages.
• Bergmann's Rule (related): cold ⇒ larger body size.
• Small endotherms rarely live in polar regions because of unfavourable surface-to-volume ratio.
• Kangaroo rat = metabolic water + concentrated urine; no need to drink.
• CAM = stomata open at night to fix CO₂ as malic acid.
• Opuntia has leaves reduced to spines; photosynthesis shifts to flattened stem (phylloclade).
• Thermoregulators keep T constant; thermoconformers track environment.
  :::

:::memory
"Cold cuts the corners" — cold-climate animals "cut" their ears and limbs short (Allen's Rule).
"Kangaroo rat drinks fat" — its water comes from oxidising body fat.
"CAM = Closed At Midday" — stomata shut in the day, open at night.
:::

:::recap

• Animal adaptations to cold are geometric: change size (Bergmann) and shape (Allen).
• The desert problem is water, not heat — kangaroo rat and Opuntia solve it from inside.
• Always name the rule, name the example, name the mechanism — that is what NEET marks reward.
  :::

Look up on a clear morning and the sky looks empty. It isn't. A thin shell of gases — barely a hundredth of Earth's radius — is doing several jobs at once: shielding life from radiation, storing oxygen for every breath, holding heat at the surface, and redistributing it across latitudes. UPSC Geography wants you to know exactly what is in that shell and how energy flows through it.

Definition: The atmosphere is the gaseous envelope surrounding the Earth, held in place by gravity.

Definition: Insolation (short for Incoming Solar Radiation) is the solar energy received per unit area on a horizontal surface at the top of the atmosphere or at the Earth's surface.

Definition: Albedo is the fraction of incoming solar radiation that a surface or body reflects back into space, expressed as a decimal or percentage. Earth's average planetary albedo is about 30%.

What the air is made of

Dry air, by volume, is overwhelmingly two gases. The standard composition is:

• Nitrogen (N₂) — about 78%
• Oxygen (O₂) — about 21%
• Argon (Ar) — about 0.93%
• Carbon dioxide (CO₂) — about 0.04% (currently ~420 ppm and rising)
• Trace gases — neon, helium, krypton, methane, ozone, water vapour, plus dust and pollutants.

Water vapour is excluded from the "dry air" figure because it varies a lot — from near zero over the Sahara to nearly 4% over the equatorial belt. That variability is precisely why water vapour drives weather.

Although the atmosphere extends up to about 10,000 km thinly, 99% of its mass lies below 32 km, and about 75% within the troposphere (up to ~8 km at poles, ~18 km at the equator). Permanent gases (N₂, O₂, Ar) mix well up to about 80 km in a layer called the homosphere.

Why it matters: Examiners often ask the order of gases, the approximate percentages, or where a specific gas concentrates. Get the top three right — Nitrogen, Oxygen, Argon — and most MCQs become straightforward.

Ozone: a Class XI staple

Definition: Ozone (O₃) is a triatomic oxygen molecule that occurs naturally in trace amounts.

Ozone is concentrated in the stratosphere, roughly 15–35 km above the surface — the famous ozone layer that absorbs most of the Sun's harmful UV-B radiation. In the troposphere, ozone is a pollutant; in the stratosphere, it is a life-saver. The Montreal Protocol (1987) banned ozone-depleting CFCs; remembering it as the most successful environmental treaty is a high-yield UPSC factoid.

Insolation: how the Sun's energy reaches us

The Sun radiates as a near-perfect black body at about 6000 K, mostly in short waves (visible light, near-UV, near-infrared). This is the shortwave energy that reaches Earth's outer atmosphere as insolation.

Definition: Solar constant is the amount of solar energy received per unit area, per unit time on a surface perpendicular to the Sun's rays at the top of Earth's atmosphere when Earth is at its mean distance from the Sun. Its value is about 1.94 cal/cm²/min (or 1361 W/m² in SI units).

Not all the insolation reaches the ground. The atmosphere reflects, scatters and absorbs a significant share. Typical splits:

• Reflected back to space by clouds, ice, dust — about 30% (= albedo).
• Absorbed by the atmosphere (especially by O₃, water vapour, CO₂) — about 20%.
• Reaches the surface as direct + diffuse radiation — about 50%.

Four factors that decide how much insolation a place receives

The exam often asks why two cities at different latitudes receive different amounts of insolation. Four factors stand out:

1. Angle of incidence (latitude) — the lower the latitude, the more nearly vertical the Sun's rays, so energy per square metre is highest at the equator and lowest at the poles.
2. Length of day — longer daylight hours mean longer reception time. This is why polar regions, despite low Sun angles, briefly receive more daily insolation than the equator in midsummer.
3. Atmospheric transparency — clouds, water vapour and dust block insolation. A clear desert sky receives more direct radiation than a cloudy coast at the same latitude.
4. Solar constant — varies only slightly across the year (about 3.3%) due to Earth's elliptical orbit; for prelims, treat as a near-constant.

Terrestrial radiation: why the lower troposphere is warmer

Definition: Terrestrial radiation is the long-wave (infrared) radiation emitted by the Earth's surface after it has been heated by absorbed insolation.

Here is one of the most counter-intuitive but high-yield ideas in physical geography: the atmosphere is heated mainly from below, not from above. Air is largely transparent to incoming shortwave radiation but quite opaque to outgoing long-wave radiation. So the Sun's energy passes through air to warm the ground; the warm ground then re-radiates infrared, which the lower atmosphere absorbs and re-emits — including back to the ground. This is the greenhouse effect.

This is why temperature in the troposphere generally decreases with altitude (lapse rate ~6.5 °C per km): you are moving away from the hot heating element (the ground), not towards a hot ceiling. It also explains why mountain peaks are cold even though they are closer to the Sun.

Heat budget: incoming equals outgoing

Definition: Earth's heat budget is the energy balance between incoming solar radiation absorbed by the Earth-atmosphere system and the long-wave radiation it emits back to space.

Over long periods, incoming = outgoing. If not, the planet would steadily warm or cool. Currently, anthropogenic CO₂ has tilted the balance slightly: more long-wave radiation is being trapped than escaping, producing global warming of about +1.1 °C since pre-industrial times.

Real-world example: When the Indian Ocean Dipole (IOD) turns positive, the western Indian Ocean warms, increasing convection over East Africa and altering the south-west monsoon over India. At its root, this is a heat-budget redistribution — same total energy, different geographical share.

Albedo and Indian context

Different surfaces have different albedos:

• Fresh snow — 80–90%
• Sea ice — 50–70%
• Sand (desert) — 30–40%
• Forest — 10–20%
• Ocean (sun overhead) — about 6%

That is why glacier loss in the Himalayas is a self-reinforcing process: once snow melts and exposes darker rock, albedo falls, more insolation is absorbed, and melting accelerates. This ice-albedo feedback is a UPSC favourite under climate change.

Worked example

Question: A station on the equator and a station at 23.5°N both receive sunlight at noon on 21 June. Which receives more direct insolation per unit area, and why?

Solution:
Step 1: On 21 June (the summer solstice), the Sun is overhead at the Tropic of Cancer (23.5°N), not the equator.
Step 2: At 23.5°N, the Sun's rays strike vertically; angle of incidence ≈ 0° from the zenith. Energy per m² is maximum.
Step 3: At the equator, the Sun is about 23.5° away from the zenith, so the same beam spreads over a larger area; insolation per m² is lower.
Conclusion: The station at 23.5°N receives more direct insolation per unit area on 21 June, despite being farther from the equator.

Common misconception

Many students think higher altitude means closer to the Sun, so warmer. The opposite is true in the troposphere: heating comes from the ground via terrestrial radiation, so going up moves you away from the heat source. The Sun's distance change is negligible compared to this near-surface heating effect.

Common misconception 2: "Greenhouse effect is bad." Without the natural greenhouse effect, Earth's average temperature would be about −18 °C instead of +15 °C. The problem is the enhanced greenhouse effect caused by excess CO₂ and methane, not the greenhouse mechanism itself.

:::compare

Phenomenon	Wavelength	Direction	Acts on
Insolation	Short-wave (UV, visible, near-IR)	Sun → Earth	Atmosphere mostly transparent
Terrestrial radiation	Long-wave (infrared)	Earth → atmosphere/space	Atmosphere mostly absorbs
Greenhouse re-radiation	Long-wave	Atmosphere → surface	Warms the surface
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:::keypoints

• Dry air = 78% Nitrogen, 21% Oxygen, ~0.93% Argon, ~0.04% CO₂, traces.
• 99% of atmospheric mass lies below 32 km.
• Ozone concentrates in the stratosphere (~15–35 km) and absorbs UV-B.
• Solar constant ≈ 1.94 cal/cm²/min at the top of the atmosphere.
• Earth's mean albedo is ~30%; snow/ice raise it, oceans lower it.
• The atmosphere is heated more by terrestrial radiation than by direct sunlight.
• Insolation depends on angle of incidence, day length, transparency, and solar constant.
• Over the long run, Earth's incoming insolation balances outgoing terrestrial radiation.
  :::

:::memory
"NOAC — Nitrogen, Oxygen, Argon, CO₂" — the four headline components in order of abundance.
"Sun heats ground, ground heats air" — the rule that explains why lower air is warmer than upper air.
"30% reflected, 20% absorbed, 50% reaches us" — a quick split of incoming insolation.
:::

:::recap

• Atmosphere = thin shell, but where almost all weather and life-supporting chemistry happens.
• Insolation = shortwave Sun-to-Earth energy; terrestrial radiation = longwave Earth-to-space energy.
• Greenhouse effect is natural and beneficial; enhanced greenhouse effect drives climate change.
• Heat budget balance is the master idea that ties albedo, latitude, clouds, and CO₂ together.
  :::

Temperature controls: latitude, altitude (decreases ~6.5 degrees C/km), distance from sea (continentality—land heats/cools faster than water), ocean currents, prevailing winds, slope/aspect and cloud cover. Isotherms are lines joining equal temperature. Temperature Inversion: normally temperature decreases with height, but in inversion it INCREASES with height. Conditions favouring inversion: long winter nights, clear cloudless skies, dry air, calm/still air, and snow-covered ground. Valleys experience inversion as cold dense air drains downslope (air drainage)—frost forms in valley bottoms while slopes stay warmer (why orchards/tea are grown on slopes, not valley floors). Inversion traps pollutants, causing smog. Mnemonic for inversion conditions: 'CLeaN DCS — Clear, Calm, Night, Dry, Cold, Snow.'

Seven pressure belts (alternating, from equator to pole): Equatorial Low (doldrums), Sub-tropical Highs (horse latitudes 30 deg), Sub-polar Lows (60 deg), Polar Highs. Equatorial and polar belts are thermally induced; sub-tropical and sub-polar belts are dynamically induced. Planetary/permanent winds blow from high to low pressure: Trade winds (sub-tropical high → equatorial low; NE in N hemisphere, SE in S), Westerlies (sub-tropical high → sub-polar low; the 'Roaring Forties, Furious Fifties, Shrieking Sixties' in S hemisphere), and Polar Easterlies. Coriolis force deflects winds RIGHT in the Northern Hemisphere and LEFT in the Southern (Ferrel's Law), and is zero at the equator, maximum at poles. Winds are named after the direction they blow FROM.

NULL in SQL is not zero, not empty string, not "missing-but-treat-as-something". It is the database's honest admission that "we do not know." GATE examiners love this corner of SQL precisely because most students remember the syntax of aggregates but forget what they silently do to NULLs — and that silence is where marks slip away.

Definition: An aggregate function in SQL collapses a set of values from a column into a single summary value — common ones are COUNT, SUM, AVG, MIN, MAX.

Definition: NULL is SQL's marker for an unknown or inapplicable value. It is not equal to 0, not equal to '', and not even equal to another NULL (NULL = NULL evaluates to UNKNOWN).

The one rule that decides everything

All aggregate functions, except COUNT(*), ignore rows where the input column is NULL.

Read that again. The rule is small, but every trick GATE plays in this area is just a consequence of it.

• COUNT(*) counts rows, not column values, so it sees NULLs.
• COUNT(col) counts non-NULL values in col.
• SUM(col), AVG(col), MIN(col), MAX(col) operate only on non-NULL values of col.

Why it matters: this single asymmetry between COUNT(*) and the others is the entire reason AVG can disagree with SUM/total_rows, and the reason an "empty" aggregate sometimes returns NULL and sometimes returns 0.

AVG and the denominator trap

Here is the GATE-classic numerical setup. Suppose a column marks has values {2, 4, NULL} across three rows.

• SUM(marks) = 2 + 4 = 6. (NULLs ignored.)
• COUNT(*) = 3. (Counts all rows.)
• COUNT(marks) = 2. (Counts only non-NULL.)
• AVG(marks) = SUM(marks) / COUNT(marks) = 6 / 2 = 3.

A careless student computes 6 / 3 = 2. That is wrong. AVG divides by the count of non-null inputs, not by the total number of rows.

So if you ever see a question that gives you a table with NULLs and asks for AVG, your first move is to compute COUNT(non-null), not COUNT(*).

:::compare

Function	Behaviour on NULL inputs	All-NULL column returns
COUNT(*)	Counts the row anyway	0
COUNT(col)	Ignores NULL values of col	0
SUM(col)	Ignores NULLs	NULL
AVG(col)	Ignores NULLs; divides by COUNT(col)	NULL
MIN(col)	Ignores NULLs	NULL
MAX(col)	Ignores NULLs	NULL
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Notice the second oddity: when every value is NULL, COUNT returns 0 while SUM, AVG, MIN, MAX all return NULL. Why? Because COUNT is fundamentally about counting things, and you can sensibly count zero of something. But the sum, average, min or max of nothing is undefined — there is no value to report — so the language returns NULL.

Common misconception: students sometimes think SUM(col) over all NULLs gives 0. It does not — it gives NULL. The temptation to "add nothing and get zero" is mathematically reasonable but contradicts the SQL standard. If you need 0 in such cases, wrap the result: COALESCE(SUM(col), 0).

A short worked example

Question: Consider the table Sales(amount) with five rows holding values {100, NULL, 200, NULL, 300}. What do the following queries return?

SELECT COUNT(*), COUNT(amount), SUM(amount), AVG(amount) FROM Sales;

Solution:
Step 1: COUNT(*) counts all five rows → 5.
Step 2: COUNT(amount) counts only non-NULL values: {100, 200, 300} → 3.
Step 3: SUM(amount) adds the non-NULL values → 100 + 200 + 300 = 600.
Step 4: AVG(amount) = SUM / COUNT(amount) = 600 / 3 = 200.
Conclusion: The result row is (5, 3, 600, 200). If you had divided 600 by 5 (the row count), you would have got 120 — that is the classic GATE trap.

Real-world example

Imagine a college DBMS lab with the Students(student_id, attendance_percent) table. Some students haven't started attending yet, so their attendance is recorded as NULL — meaning "not applicable yet," not zero. If the placement cell asks for the average attendance with AVG(attendance_percent), it correctly ignores the NULL rows. If instead they treated NULLs as 0, the average would unfairly pull the cohort's number down — and a student who simply hasn't registered would damage everyone's report. SQL's "ignore NULL" behaviour is what keeps that statistic honest.

A subtler GATE trap: COUNT(*) vs COUNT(col)

This is the trap behind many GATE one-mark questions:

SELECT COUNT(*) - COUNT(col) FROM T;

This expression returns the number of NULLs in column col. Recognising this idiom on sight is worth a mark.

What DISTINCT does to the picture

When you write COUNT(DISTINCT col) or SUM(DISTINCT col), NULLs are still ignored, but duplicates among non-NULL values are also collapsed. So for a column {2, 2, 4, NULL, NULL}:

• COUNT(col) = 3, COUNT(DISTINCT col) = 2.
• SUM(col) = 8, SUM(DISTINCT col) = 6.

GATE has used this overlap of DISTINCT and NULL handling to build multi-step numerical-type questions. The safe procedure: first drop NULLs, then apply DISTINCT, then aggregate.

GROUP BY and NULL

When you GROUP BY col, all NULLs in col form a single group of their own — SQL treats "unknown = unknown" specially here, even though NULL = NULL is otherwise UNKNOWN. Many beginners expect each NULL to be its own group. It is not.

Why is the standard like this?

The philosophy of NULL is three-valued logic: TRUE, FALSE, UNKNOWN. Comparisons involving NULL return UNKNOWN, which is filtered out of WHERE clauses (only TRUE rows pass). Aggregates were designed to be conservative: an unknown value should not corrupt a sum or pretend to contribute a measurable amount, so it is excluded. COUNT(*) is the lone exception because counting rows is structural, not value-dependent.

:::keypoints

• All aggregates except COUNT(*) ignore NULL inputs.
• AVG = SUM ÷ COUNT(non-null), so it can differ from SUM ÷ total_rows.
• SUM of all NULLs is NULL, not 0. COUNT of all NULLs is 0.
• MIN/MAX ignore NULLs; if everything is NULL, they return NULL.
• COUNT(*) - COUNT(col) = number of NULLs in col.
• GROUP BY collapses all NULLs into a single group.
• Use COALESCE(SUM(col), 0) when you need 0 instead of NULL.
• NULL ≠ 0, NULL ≠ '', and even NULL = NULL is UNKNOWN.
  :::

:::memory
"Aggregates skip NULL; only COUNT(*) sees them. AVG divides by the count of non-null values." Or the one-line proverb: NULL is invisible to maths, visible to counting-of-rows.
:::

:::recap

• COUNT(*) is the only aggregate that counts NULL rows.
• AVG is the most common GATE trap — always divide by COUNT(col), not COUNT(*).
• Empty-set aggregates return NULL except COUNT, which returns 0.
• Use COALESCE to convert NULL results to a sensible default.
  :::

Indian summers carry the dry sting of the Loo, while a French farmer reaches for a wool coat when the Mistral rolls down the Rhone valley. The world's local winds are the planet's small-scale moods — and for UPSC Prelims, they are also some of the most reliably asked one-mark factual questions. A neat map of who blows where, hot or cold, dry or moist, can hand you several marks.

Definition — Local wind: A wind of limited regional extent caused by local relief, pressure or temperature differences, distinct from the global planetary winds.
Definition — Jet stream: A fast, narrow, meandering current of air in the upper troposphere, generally flowing west-to-east at altitudes of 9–12 km.
Definition — Sea breeze / Land breeze: Diurnal coastal winds caused by the differential heating and cooling of land and water.

Hot local winds

The Loo sweeps across the northern Indian plains in May and June. It is a hot, dry continental wind that pushes daytime temperatures over 45 °C in Delhi, Rajasthan and western Uttar Pradesh. Heatstroke deaths during a Loo are a public-health emergency every summer.

The Foehn (also spelled Föhn) descends the leeward side of the Alps in Switzerland, Austria and Bavaria. As moist air is forced up the windward slopes, it loses moisture and releases latent heat; on its way down the other side it is warmed adiabatically. The result is a warm, dry wind that can melt snow within hours — useful for ripening grapes in Alpine valleys.

The Chinook is the North American cousin of the Foehn, blowing down the leeward (eastern) side of the Rocky Mountains across the prairies of Alberta, Montana and the Dakotas. Native Americans nicknamed it the "snow-eater" because it can clear pastures of snow overnight — a blessing for cattle ranchers in winter.

The Sirocco is a hot, dusty, sometimes humid wind that originates over the Sahara and blows northward across the Mediterranean to Italy and southern France. When it picks up Saharan dust and crosses the sea, it can deposit reddish dust on European cities — the famous "blood rain".

The Harmattan of West Africa is dry and dust-laden, carrying Saharan dust toward the Gulf of Guinea between November and March. Despite the dust, it gives relief from the heavy tropical humidity, which is why locals call it "the Doctor."

Cold local winds

The Mistral is a cold, dry, gusty wind that pours down the Rhone valley between the Alps and the Massif Central, hitting the Gulf of Lion in southern France. It chills Provence in winter, but also keeps the skies famously clear — one reason painters loved the south of France.

The Bora is a cold, dry, gusty wind that drops from the Dinaric Alps onto the Adriatic coast of Croatia and Slovenia. Like the Mistral, it is a cold-air drainage wind, often violent enough to overturn small boats.

Jet streams

High above all this local drama, the jet streams steer the world's weather. They form where the Hadley, Ferrel and Polar cells meet, and where huge temperature contrasts exist near the tropopause. Two big ones matter for India:

• The Sub-tropical Westerly Jet (STWJ) sits around 25°–35° N. In winter it shifts south and lies over north India south of the Himalayas. It steers Western Disturbances — extra-tropical storms originating over the Mediterranean — across north-west India, giving winter rain to Punjab and snow to Kashmir.
• The Tropical Easterly Jet (TEJ) appears only in summer, around 10°–15° N, over peninsular India and Africa. Its appearance is intimately linked with the burst of the south-west monsoon.

The classical UPSC fact is: the northward shift / withdrawal of the Sub-tropical Westerly Jet and the onset of the Tropical Easterly Jet together "trigger" the monsoon's arrival over India. The Polar Front Jet, lying near 60° latitude, steers mid-latitude cyclones and aircraft routes — which is why an east-bound transatlantic flight is shorter than a west-bound one.

Sea breeze and land breeze

By day, land heats up faster than the sea. Air over the land rises, lowering surface pressure; the cooler, higher-pressure sea air flows in. That's the sea breeze — sea to land — felt every afternoon on a Goa beach. By night the equation reverses: the land cools faster than the water, surface pressure over land becomes higher, and a land breeze flows from land to sea, often felt by fishermen setting out before dawn.

Why it matters

Air masses and local winds feature in nearly every UPSC Prelims paper, often as match-the-following or assertion-reason questions. Knowing that the Chinook is North American, the Foehn Alpine, the Loo Indian and the Mistral French, and whether each is hot or cold, separates an average candidate from a strong one. The jet-stream–monsoon link also feeds into Mains GS-1 essays and into questions on climate variability.

Real-world example

When IMD says, "A western disturbance over Kashmir will bring rain and snow to north India by Friday," it is really tracking a meandering loop of the Sub-tropical Westerly Jet. When farmers in Punjab smile at a December shower over standing wheat, they are reaping the harvest of a jet stream sitting thousands of metres above them.

Common misconception

Many aspirants confuse the Foehn (Alps) with the Chinook (Rockies) — both are warm dry leeward winds, but in different continents. A second confusion is between the Loo and a "monsoon wind" — the Loo is pre-monsoon, dry and continental, not a monsoon current.

Worked example

Question: Which of the following correctly matches a local wind with its region and character?
(a) Mistral — France — Cold
(b) Sirocco — Sahara to Mediterranean — Hot
(c) Chinook — Rockies leeward — Warm dry
(d) Harmattan — West Africa — Dry dusty
Solution:
Step 1: Mistral pours down the Rhone valley in southern France in winter — cold, dry. ✔
Step 2: Sirocco rises from the Sahara, crosses the Mediterranean — hot, dusty, sometimes humid. ✔
Step 3: Chinook is the "snow-eater" of the Rockies' leeward side — warm, dry. ✔
Step 4: Harmattan blows from the Sahara across West Africa — dry, dusty. ✔
Conclusion: All four pairings are correct — a likely "All of the above" answer.

:::compare

Local Wind	Region	Character	Notable Nickname / Effect
Loo	North India	Hot, dry	Heat-stroke wind of May–June
Foehn	Alps (leeward)	Warm, dry	Melts snow, helps viticulture
Chinook	Rockies (leeward)	Warm, dry	"Snow-eater" — saves cattle
Mistral	Rhone valley, France	Cold, dry	Clear skies in Provence
Sirocco	Sahara to Mediterranean	Hot, dusty	"Blood rain" in Italy
Bora	Adriatic coast	Cold, gusty	Hits Croatia, Slovenia
Harmattan	West Africa	Dry, dusty	Called "the Doctor"
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:::keypoints

• Loo, Foehn, Chinook, Sirocco, Harmattan = HOT winds.
• Mistral, Bora = COLD winds.
• Chinook = Rockies leeward (snow-eater); Foehn = Alps leeward — same mechanism, different continents.
• Sub-tropical Westerly Jet steers Western Disturbances that bring winter rain to north-west India.
• Tropical Easterly Jet appears in summer and is linked with the burst of the SW monsoon.
• Sea breeze = day, sea→land; land breeze = night, land→sea.
• Local winds form due to differential heating of land/sea, orography (leeward warming) and pressure gradients.
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:::memory
"Cold Mistral, Cold Bora — rest are hot" — only the M and B in the list are cold. Everything else (Loo, Foehn, Chinook, Sirocco, Harmattan) is hot or warm.
"FoCh = warm dry leeward" — Foehn and Chinook are twins on different mountains.
"Doctor cures, Loo kills" — Harmattan brings relief, Loo brings heatstroke.
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:::recap

• Local winds are small-scale, named for their region; jet streams are global, high-altitude.
• The Loo dominates pre-monsoon north India; the Mistral dominates winter France.
• Chinook and Foehn warm by descent; Mistral and Bora chill by cold-air drainage.
• The withdrawal of the STWJ + onset of the TEJ = the green light for the Indian SW monsoon.
  :::