Why this topic matters for UPSC
Prelims: NCERT-anchored MCQs on pressure units (millibar / hPa) · pressure belts (equatorial low, sub-tropical high, sub-polar low, polar high) · doldrums, horse latitudes, roaring forties · Coriolis force (zero at equator, max at poles) · Ferrel's Law · planetary winds (trades, westerlies, polar easterlies) · monsoons · local winds (Loo, Foehn, Chinook, Mistral, Sirocco, Bora, Khamsin, Harmattan) · jet streams (sub-tropical · polar front · easterly tropical jet over India).
Mains GS-1: "Explain the three-cell model of global atmospheric circulation", "Discuss the role of Coriolis force in shaping wind systems", "How do jet streams affect Indian monsoon?", "Compare land breeze and sea breeze", "Significance of local winds in regional climate".
Contents
- Atmospheric pressure — concept & measurement
- Vertical & horizontal pressure distribution; pressure belts
- Forces controlling winds (PGF · Coriolis · friction)
- Three-cell circulation model
- Planetary (permanent) winds
- Seasonal winds — monsoons & land-sea breezes
- Local winds of the world
- Jet streams
- Previous Year Questions (Prelims + Mains)
- 15 must-know facts
1 · Atmospheric pressure — concept & measurement NCERT XI Ch 10GS-1
Atmospheric pressure is the weight per unit area exerted by the column of air above a point. Though air feels weightless, the entire atmospheric column above 1 m² of surface weighs ~10,000 kg — equivalent to 10 tonnes pressing down. We do not feel it because the same pressure acts in all directions inside our bodies.
1.1 · Units and measurement
| Unit | Value at sea level | Used by |
|---|---|---|
| Atmosphere (atm) | 1.000 | Reference unit |
| Millibar (mb) | 1013.25 | Indian meteorology (IMD) |
| Hectopascal (hPa) | 1013.25 | WMO standard (1 hPa = 1 mb) |
| Millimetres of mercury (mm Hg) | 760 | Classical, medical |
| Inches of mercury (in Hg) | 29.92 | US aviation |
| Pascal (Pa, SI) | 101,325 | Scientific |
1.2 · Instruments
- Mercury barometer (Torricelli, 1643) — glass tube inverted in mercury bath; column rises to 760 mm at sea level.
- Aneroid barometer — sealed metal box ("Vidie cell"); flexes with pressure changes; portable, used in aircraft as altimeter.
- Barograph — aneroid + pen-on-rotating-drum for continuous trace.
- Modern: digital piezo-electric sensors — used in IMD AWS (Automatic Weather Stations) and radiosondes.
1.3 · An isobar
An isobar is a line joining points of equal atmospheric pressure on a weather map — values are reduced to sea level to remove altitude effect. Closely-spaced isobars = steep pressure gradient = strong winds; widely-spaced = weak gradient = calm.
2 · Vertical & horizontal pressure distribution; pressure belts NCERT XI Ch 10GS-1
2.1 · Vertical distribution
Pressure falls rapidly with altitude — but not linearly:
| Altitude | Pressure (mb) | % of sea-level | Remark |
|---|---|---|---|
| Sea level (0 km) | 1013 | 100% | Standard atmosphere |
| 1 km (Bengaluru ~ 0.9 km) | 898 | 89% | Hill cities |
| 3 km (Leh ~ 3.5 km) | 700 | 69% | Mountain sickness onset |
| 5.5 km | 506 | 50% | Half of mass below |
| 10 km (cruise alt.) | 264 | 26% | Cabin pressurised to 750 mb |
| 30 km (stratosphere mid) | 12 | 1.2% | Above 99% of mass |
Rate of fall: ~1 mb per 10 m near surface; halves every 5.5 km on average.
2.2 · Horizontal distribution — global pressure belts
Earth's surface is encircled by seven alternate low and high pressure belts caused by differential heating (thermal) and the Earth's rotation (dynamic).
2.3 · Thermal vs Dynamic belts
Thermal belts
Caused by heating / cooling of air at the surface.
- Equatorial Low — intense insolation → warm rising air
- Polar Highs — extreme cooling → cold dense air sinks
Dynamic belts
Caused by Earth's rotation and resulting air-mass behaviour.
- Subtropical Highs — descending limb of Hadley + Coriolis pile-up
- Sub-polar Lows — divergence due to upper-air dynamics; meeting of contrasting air masses
3 · Forces controlling winds NCERT XI Ch 10GS-1
A wind starts because of a pressure difference but its final direction and speed are sculpted by four forces: Pressure Gradient Force (PGF), Coriolis Force, Frictional Force, Centripetal Force.
1 · Pressure Gradient Force (PGF)
Force from high pressure → low pressure, perpendicular to isobars. Magnitude ∝ pressure gradient (Δp / Δd). Steep gradient = strong wind.
2 · Coriolis Force
Apparent force from Earth's rotation. Deflects winds RIGHT in NH, LEFT in SH (Ferrel's Law). Zero at Equator, maximum at Poles. F = 2 ω v sin φ.
3 · Frictional Force
Opposes motion at the surface. Strong over land (rough — forests, cities) up to ~1 km; weak over oceans. Reduces speed and Coriolis effect.
4 · Centripetal Force
Inward force needed for curved (cyclonic) flow. Significant in tight low-pressure systems & cyclones; small in straight isobars.
3.1 · Coriolis force in detail
As Earth rotates from west to east, a parcel of air moving across latitudes appears to be deflected from a stationary observer's viewpoint. Mathematically, the Coriolis acceleration is 2 ω v sin φ, where ω = Earth's angular velocity (7.29 × 10⁻⁵ rad/s), v = wind speed, φ = latitude.
- At the Equator (φ = 0) → sin 0 = 0 → no Coriolis. Cyclones cannot form within 5° of equator.
- At the Poles (φ = 90°) → sin 90 = 1 → maximum Coriolis.
- The faster the wind, the stronger the deflection (jet streams curve dramatically).
4 · Three-cell circulation model NCERT XI Ch 10GS-1
Heat surplus at the equator and deficit at the poles drives a meridional overturning. Earth's rotation breaks the single hemispheric Hadley loop (proposed by George Hadley 1735) into three cells per hemisphere: Hadley, Ferrel, Polar. This is the tri-cellular model (Palmen 1951).
| Cell | Latitudes | Type | Surface wind | Upper-air return |
|---|---|---|---|---|
| Hadley | 0° – 30° | Direct (warm air rises, cold sinks) | Trade winds (NE / SE) → equator | Anti-trades poleward at ~12 km |
| Ferrel | 30° – 60° | Indirect (driven by Hadley + Polar; mechanical) | Westerlies (SW/NW) → poleward | Easterlies equatorward at altitude |
| Polar | 60° – 90° | Direct (cold sinks at pole, warm rises at 60°) | Polar easterlies (NE/SE) → equatorward | Westerlies poleward at altitude |
Why three cells, not one?
George Hadley (1735) proposed a single equator-to-pole cell. But Earth's rapid rotation creates so much Coriolis deflection that air cannot travel all the way from equator to pole as one loop. Instead it sinks at ~30° (sub-tropical high), forming the Hadley cell. The Ferrel cell (30°–60°) is "mechanically" driven — sandwiched between Hadley and Polar, it rotates opposite to expected (indirect cell). Polar cell (60°–90°) is a true thermal cell. Three cells = consequence of Earth's rotation rate.
5 · Planetary (permanent) winds NCERT XI Ch 10GS-1
Winds that blow throughout the year in the same direction at fixed latitudes are called planetary or permanent winds. There are three pairs:
5.1 · Detailed comparison
| Wind | Direction (NH / SH) | Source belt → destination | Properties & significance |
|---|---|---|---|
| Trade Winds | NE / SE | Sub-tropical High (30°) → Equatorial Low (0°) | Steady; warm & dry over land (deserts on west coasts of continents) but moist after crossing oceans (east coasts get rainfall — Caribbean, Coromandel). Aided Columbus to America. |
| Westerlies | SW / NW | Sub-tropical High (30°) → Sub-polar Low (60°) | Variable; strong in SH where unobstructed by land — Roaring Forties (40°S), Furious Fifties (50°S), Shrieking Sixties (60°S). Bring rain to W coasts of continents (UK, NW USA, Chile). |
| Polar Easterlies | NE / SE | Polar High (90°) → Sub-polar Low (60°) | Cold, dry, irregular; meet warm westerlies at the Polar Front → genesis of temperate cyclones. |
6 · Seasonal Winds — Monsoons, Land & Sea Breezes, Valley Winds
Seasonal winds reverse direction with season due to differential heating of land vs sea. Three scales — planetary (monsoons, thousands of km, half-year cycle), regional (land-sea breeze, ~50 km, diurnal), local (mountain-valley, ~5 km, diurnal).
6.1 Monsoon — classical (thermal) vs modern (dynamic) theory
| Theory | Proponent | Mechanism | Limitations |
|---|---|---|---|
| Thermal / Classical | Edmund Halley (1686) | Summer: Asian landmass heats → low pressure → draws moist air from Indian Ocean (SW monsoon). Winter: Land cools faster → high pressure → wind blows from land to sea (NE monsoon). | Cannot explain burst, breaks, vagaries; ignores upper-air circulation & jet streams. |
| Dynamic / Modern | Flohn (1951), Koteswaram | ITCZ migration (Inter-Tropical Convergence Zone) shifts north to 25°N in July (over Indo-Gangetic plain). Upper-air Tropical Easterly Jet (TEJ over 15°N) + retreat of Subtropical Westerly Jet north of Himalayas trigger SW monsoon burst. Tibetan Plateau acts as elevated heat source. | Complex; needs upper-air data; explains burst but ENSO/IOD modulation still debated. |
| ENSO / IOD modulation | Sikka, Gadgil, Webster | El Niño (warm equatorial Pacific) → weak monsoon. La Niña → strong monsoon. Positive IOD (warm W Indian Ocean) → strong monsoon. | Statistical correlation; not deterministic. |
Fig 8.4 — Indian monsoon dual-panel. SW monsoon brings ~75 % of annual rain (Jun–Sep, Arabian + BoB branches). NE monsoon is dry over most of India but drenches Tamil Nadu & coastal Andhra (Oct–Dec) by picking up Bay of Bengal moisture.
6.2 SW Monsoon onset, branches & withdrawal
- Onset: Kerala 1 June (± 7 d). Travels north — Mumbai 10 Jun, Delhi 29 Jun, all-India by 15 Jul.
- Arabian Sea branch (65 %): Western Ghats (orographic) → Konkan, Karnataka, Kerala. Crosses Ghats, dries → rain-shadow Deccan.
- Bay of Bengal branch (35 %): Bends at Arakan Yoma, deflects west by Himalayas → Mawsynram/Cherrapunji (~11 000 mm), Brahmaputra valley, Indo-Gangetic plain.
- Break in monsoon: Days–weeks of dry weather caused by ITCZ shifting south of usual position, or strong westerly jet returning.
- Withdrawal: Begins NW India early September; complete by mid-October. Reverses to NE monsoon by November.
6.3 Land breeze & sea breeze — diurnal coastal winds
Fig 8.5 — Sea breeze vs land breeze cells. Day: land heats > sea, low pressure over land, wind blows from sea (high) to land. Night: reverse. Vertical cell ~1 km tall, extends ~30–50 km inland.
| Aspect | Sea breeze (day) | Land breeze (night) |
|---|---|---|
| Direction | Sea → Land | Land → Sea |
| Peak time | 2–4 pm | 2–4 am |
| Strength | Stronger (4–7 m/s) | Weaker (1–3 m/s) |
| Extent inland | 30–50 km | 5–10 km |
| Effect | Cools coast, brings humidity, may cause sea-breeze front thunderstorms (Florida, Konkan) | Carries land air offshore; helps traditional fishing boats sail out at dawn |
6.4 Mountain & Valley breeze (diurnal)
- Valley breeze (day, anabatic): Slope warms by Sun → air rises up slope, valley sucks in air → wind blows up-valley. Forms convective clouds over peaks by afternoon (Shimla afternoon storms).
- Mountain breeze (night, katabatic): Slope radiates heat fast → cold dense air drains down-slope. Can pool in valley floor → cold air drainage → frost pockets (Kashmir orchards damaged) & valley temperature inversion (recall Topic 7).
7 · Local Winds of the World
Local winds develop over small areas (~100s of km) due to local pressure/temperature/topographic anomalies. UPSC asks names ↔ regions ↔ hot/cold ↔ wet/dry. Use this two-axis classifier:
| Wind | Region | Type | Season | Origin / Mechanism | Effect |
|---|---|---|---|---|---|
| Loo | N India · Indo-Gangetic plain | Hot · Dry | May–June | From Thar & Iranian plateau, super-heated westerlies | Heat-stroke; ripens mangoes |
| Foehn | N Alps (Austria, Switzerland, S Germany) | Warm · Dry | Late winter / spring | Adiabatic descent on leeward side of Alps after SALR ascent on windward (orographic) | Melts snow → avalanches; ripens grapes; Foehn sickness (headaches) |
| Chinook | E slope of Rockies (USA, Canada) | Warm · Dry | Winter | Same as Foehn — leeward descent over Rockies (Pacific moisture lost) | "Snow-eater" — raises T 20 °C in hours; saves prairie cattle, melts pastures |
| Berg | S African plateau → coast | Warm · Dry | Winter | Katabatic descent from plateau (Foehn-like) | Withers vegetation on coast |
| Santa Ana | S California | Hot · Dry | Autumn | High pressure over Great Basin pushes air down San Bernardino mts → Pacific | Fans California wildfires |
| Zonda | E slope of Andes (Argentina) | Warm · Dry | Winter–spring | Foehn-type descent on lee of Andes | Melts snow, dries pampas |
| Sirocco | N Africa → S Italy, Sicily, Malta | Hot · Dusty (dry → humid after sea) | Spring | Sahara high pushes hot air N across Mediterranean; picks up moisture en route | Damages crops; "blood rain" (red dust); muggy heat |
| Khamsin | Egypt | Hot · Dusty | Mar–May (50 days) | Sahara depression pulls hot dusty air NE | Sand storms; agriculture damage |
| Harmattan | W Africa (Sahel → Guinea coast) | Cool · Dry · Dusty | Nov–Mar | NE trades from Sahara; "the doctor" — health relief from humid heat | Replaces humid air; lowers T; dust haze |
| Mistral | S France (Rhône valley) | Cold · Dry | Winter | Cold N air funneled down between Alps & Massif Central → Mediterranean | Frost on Riviera; damages olive/vine |
| Bora | Adriatic coast (Croatia, Slovenia, Italy) | Cold · Dry | Winter | Cold E European air spills over Dinaric Alps | Sudden T drop >20 °C; sea storms |
| Pampero | Argentina, Uruguay pampas | Cold · Dry | After cold-front passage | Cold polar S air sweeping N over pampas | Sharp T drop, thunderstorms |
| Buran / Purga | Siberia, Central Asia | Very cold · Dry | Winter | Siberian high pushes Arctic air south | Blizzards (Purga = snow-laden variant) |
| Blizzard | N USA & Canada prairies, Antarctica | Cold · Snowy | Winter | Polar continental air mass with strong winds + snow | Whiteout, deadly windchill |
| Norwester (Kalbaisakhi) | Bengal, Bihar, Assam, Odisha | Hot then thundery | Apr–May ("cherry blossom shower") | Convective storms from W; brings hail + rain | |
| Mango shower | Kerala, Karnataka coast | Warm · Wet | Apr–May (pre-monsoon) | Convective thunderstorms; helps mango ripening & coffee blossom | Welcomed by farmers |
- "Snow-eater" = Chinook · "Doctor wind" = Harmattan · "Blood rain" = Sirocco dust.
- Foehn ⇔ Chinook ⇔ Zonda ⇔ Berg — all leeward warm dry winds (same mechanism, different mts).
- Mistral · Bora · Pampero · Buran · Blizzard — all cold. (Mnemonic: "MBPBB are Bad-cold".)
- Loo · Sirocco · Khamsin · Santa Ana — all hot.
- Kalbaisakhi = "calamity of Vaisakha (month)" — destroys but also brings jute & rice pre-monsoon water.
8 · Jet Streams
Jet stream = narrow band of fast (100–400 km/h) westerly wind in the upper troposphere / lower stratosphere (9–12 km), thousands of km long but only ~100 km wide and 2–3 km thick. Discovered by WWII bomber pilots over Japan (1940s); explained by Rossby & Palmén.
8.1 Formation
- Thermal wind principle: Wherever a steep horizontal temperature gradient exists in the troposphere, a strong westerly jet forms aloft (geostrophic wind that scales with the temperature contrast).
- Two strong T-gradient zones globally: (a) sub-tropical (warm tropical air vs cool mid-latitude air, ~30°), (b) polar front (cool mid-latitude vs cold polar air, ~60°). Hence two main jets.
8.2 Types & UPSC relevance
Fig 8.6 — Jet stream cross-section, NH. Two permanent westerly jets (STJ 30°N, PFJ 60°N) sit at tropopause breaks where T-gradients are sharpest. TEJ is a summer-only easterly jet over India that drives the SW monsoon. Polar Front Jet meanders as Rossby waves — when it dips south, it brings cold spells (e.g. North American polar vortex outbreaks).
| Jet | Latitude | Altitude | Direction | Season | UPSC link |
|---|---|---|---|---|---|
| Subtropical Westerly Jet (STJ) | 25°–35° | 12 km | Westerly | All year (winter-stronger) | Bifurcates over Himalayas in winter — N branch over Tibet, S branch over Indo-Gangetic plain → drives Western Disturbances (winter rain over N India, snow over Himalayas). |
| Polar Front Jet (PFJ) | 55°–65° | 9 km | Westerly | All year | Meanders as Rossby waves; large meanders → polar vortex breakdowns & cold outbreaks; controls mid-latitude weather, aviation routes. |
| Tropical Easterly Jet (TEJ) | 15°N (over India, Africa) | 14 km | Easterly | Summer only (Jun–Sep) | Drives SW monsoon burst. Forms because of Tibetan Plateau heating creating upper-air anticyclone; air flows from this high → easterly jet aloft over Peninsula. |
| Polar Night Jet | 60°N (stratosphere) | 30 km | Westerly | Winter only | Polar stratospheric vortex; its collapse triggers Sudden Stratospheric Warming (SSW) → European cold waves. |
| Low-level jet (LLJ) | 10°–15°N off Somalia | 1.5 km | SW | Summer | Findlater jet — surface-level fast wind off Somalia coast; feeds Arabian Sea branch of SW monsoon. |
PYQs & Practice — Prelims and Mains kept separate
A · Prelims (MCQ) — UPSC past + practice
Direct UPSC CSE Prelims questions on pressure, winds, monsoon, jet streams — followed by practice MCQs.
Q. Consider the following statements:
- The winds which blow between 30°N and 60°N latitudes throughout the year are known as westerlies.
- The moist air masses that cause winter rains in north-western region of India are part of westerlies.
Which of the above statements is/are correct?
(a) 1 only · (b) 2 only · (c) Both · (d) Neither
Answer: (c) Both. Western Disturbances are extratropical lows embedded in the westerly flow steered by STJ.
Q. What explains the eastward flow of the equatorial counter-current?
(a) The Earth's rotation on its axis · (b) Convergence of the two equatorial currents · (c) Difference in salinity of water · (d) Occurrence of the belt of calm near the equator
Answer: (d). The doldrums / equatorial belt of calm + accumulation of water on western side pushes a return counter-current eastward.
Q. The seasonal reversal of winds is the typical characteristic of —
(a) Equatorial climate · (b) Mediterranean climate · (c) Monsoon climate · (d) All of the above
Answer: (c) Monsoon climate.
Q. La Niña is suspected to have caused recent floods in Australia. How is La Niña different from El Niño?
- La Niña is characterised by unusually cold ocean temperature in equatorial Indian Ocean whereas El Niño is characterised by unusually warm ocean temperature in equatorial Pacific Ocean.
- El Niño has adverse effect on south-west monsoon of India, but La Niña has no effect on monsoon climate.
Which of the above statements is/are correct?
(a) 1 only · (b) 2 only · (c) Both · (d) Neither
Answer: (d) Neither. Both anomalies are in the Pacific; La Niña actually enhances Indian SW monsoon.
Q. A geographic region has the following distinct characteristics: dry climate · low precipitation in winter, summer rainfall · high temperature throughout the year · subtropical high pressure cells. The above set of conditions are most likely to be found in —
(a) African Savanna · (b) Central Asian Steppe · (c) Indian Bhabar & Tarai · (d) Siberian Tundra
Answer: (a) African Savanna (Sudan-type, between trade winds and equatorial).
Q. Among the following, which one is the least water-efficient crop? (a) Sugarcane · (b) Sunflower · (c) Pearl millet · (d) Red gram
Answer: (a) Sugarcane. (Linked to monsoon irrigation policy.)
Q. "Mountain Pass" lies on the route connecting Kullu valley with Lahaul-Spiti — this pass is called —
(a) Banihal Pass · (b) Pir Panjal Pass · (c) Rohtang Pass · (d) Zoji La
Answer: (c) Rohtang. (Asked because mountain-pass winds funnel cold air, affect local circulation.)
Q. Variations in length of daytime and nighttime from season to season are due to —
(a) Earth's rotation on its axis · (b) Earth's revolution round the sun in elliptical manner · (c) Earth's revolution + axis tilted to its orbital plane · (d) Earth's revolution at an inclined angle
Answer: (c). (Drives insolation → pressure belts → monsoon.)
Practice MCQs (model-style)
Q. The normal atmospheric pressure at sea level is —
(a) 1013.25 mb · (b) 760 mm Hg · (c) 1013.25 hPa · (d) All of the above
Ans: (d) All are equivalent expressions.
Q. The Coriolis force is —
(a) maximum at equator · (b) maximum at poles · (c) uniform everywhere · (d) zero at poles
Ans: (b) Fc = 2Ω v sin φ → max at 90°, zero at 0°.
Q. The horse latitudes are —
(a) equatorial low · (b) subtropical high · (c) subpolar low · (d) polar high
Ans: (b) Subtropical high (~30°), so named because becalmed sailors threw horses overboard.
Q. "Doctor wind" of West Africa is —
(a) Sirocco · (b) Harmattan · (c) Khamsin · (d) Loo
Ans: (b) Harmattan; brings cool dry NE air = relief from humid heat.
Q. Chinook is a —
(a) cold dry wind in S France · (b) warm dry wind on the leeward side of the Rockies · (c) hot dusty wind in Egypt · (d) seasonal monsoon wind
Ans: (b). "Snow-eater."
Q. Which jet stream is responsible for the burst of SW monsoon over India?
(a) Polar Front Jet · (b) Subtropical Westerly Jet · (c) Tropical Easterly Jet · (d) Polar Night Jet
Ans: (c) TEJ (forms only in summer over Peninsula due to Tibetan heating).
Q. Buys-Ballot's Law states —
(a) in NH, low pressure lies to the left when wind blows behind you · (b) in NH, low pressure lies to the right when wind blows behind you · (c) in SH, low pressure lies to the left when wind blows behind you · (d) applies only at equator
Ans: (a) NH: wind behind → low on left, high on right. (Mirror in SH.)
Q. Western Disturbances are brought to N India by —
(a) Tropical Easterly Jet · (b) Southern branch of STJ · (c) Polar Front Jet · (d) Trade winds
Ans: (b) Southern branch of Subtropical Westerly Jet (in winter, when STJ bifurcates over Himalayas).
Q. The Hadley cell extends approximately between —
(a) 0°–30° · (b) 30°–60° · (c) 60°–90° · (d) 0°–60°
Ans: (a) 0°–30°.
Q. ITCZ shifts north of equator during —
(a) NH winter · (b) NH summer · (c) Equinoxes · (d) Never shifts
Ans: (b) NH summer (Jul: ~25°N over India).
Q. Sea breeze blows from —
(a) land to sea during day · (b) sea to land during day · (c) sea to land during night · (d) land to sea during night
Ans: (b) Sea to land during day (land heats faster → low pressure).
Q. The wind known as Loo blows over —
(a) Western Ghats · (b) Deccan plateau · (c) Indo-Gangetic plain · (d) Tamil Nadu coast
Ans: (c) N India / Indo-Gangetic plain (May–June).
Q. Mistral is —
(a) cold dry wind in Rhône valley, France · (b) hot wind from Sahara · (c) warm leeward Alpine wind · (d) Adriatic cold wind
Ans: (a). Bora (option d) is Adriatic.
Q. Findlater Jet is —
(a) upper-tropospheric easterly · (b) low-level (1.5 km) SW wind off Somalia · (c) polar stratospheric jet · (d) mid-latitude westerly
Ans: (b). Feeds Arabian Sea SW monsoon branch.
Q. Roaring Forties, Furious Fifties, Shrieking Sixties are names of —
(a) Trade wind belts in NH · (b) Westerlies belts in SH · (c) Polar easterlies · (d) Jet streams
Ans: (b) SH westerlies belts (no large landmass → unobstructed strong winds).
Q. Geostrophic wind blows —
(a) across isobars from high to low · (b) parallel to isobars · (c) perpendicular to coastline · (d) only at the surface
Ans: (b) Parallel to isobars (PGF balanced by Coriolis, no friction aloft).
Q. Mango shower occurs in —
(a) Tamil Nadu in October · (b) Kerala & coastal Karnataka in April–May · (c) Punjab in February · (d) Assam in June
Ans: (b) Pre-monsoon thunderstorms helping mango ripening & coffee blossom.
Q. The sub-tropical high pressure belt is —
(a) dynamically induced · (b) thermally induced · (c) both dynamic + thermal · (d) none
Ans: (a) Dynamically induced — descending limbs of Hadley + Ferrel cells (not heat-driven).
Q. A katabatic wind is —
(a) warm up-slope wind during day · (b) cold down-slope wind, usually night · (c) a tropical easterly · (d) a wind named for a Greek god
Ans: (b). Anabatic = up-slope (day); Katabatic = down-slope (night).
Q. Norwester / Kalbaisakhi affects —
(a) Punjab in winter · (b) Bengal & Assam in April–May · (c) Kerala in October · (d) Gujarat in summer
Ans: (b). Pre-monsoon convective storms; brings hail, damages crops, also moisture.
B · Mains (descriptive) — UPSC past + practice
GS Paper 1 Mains questions on planetary winds, monsoon mechanism, jet streams, local winds — plus model practice questions with diagram cues.
Q. Discuss the meaning of colour-coded weather warnings for cyclone prone areas given by India Meteorological Department. (10 marks · 150 words)
Approach: Define IMD's 4-tier colour code (Green = no warning, Yellow = watch, Orange = alert, Red = action). Tie to pressure-wind regime: cyclone = deep low; T-number → wind speed → category. Mention 12-hr, 24-hr, 48-hr lead times. Conclude with NDMA-IMD integration after Cyclone Amphan 2020.
Q. How are jet streams formed? What are their atmospheric implications? (15 marks · 250 words)
Diagram cue: Draw Fig 8.6 — NH cross-section with STJ at 30°N tropopause, PFJ at 60°N, TEJ at 15°N (summer). Approach: Formation via thermal wind principle (steep horizontal T-gradient → fast westerly aloft). Two permanent belts (sub-tropical, polar-front) + summer-only TEJ. Implications: (i) STJ steers Western Disturbances → Rabi crop; (ii) PFJ Rossby waves → mid-lat cyclones, polar vortex cold spells; (iii) TEJ → SW monsoon burst; (iv) aviation efficiency (east-bound flights save fuel); (v) climate change — Arctic warming weakens PFJ → more frequent extreme weather. Conclude: jet streams are atmosphere's "rivers in the sky," linking polar & tropical climates.
Q. Mention the global occurrence of volcanic eruptions in 2021 and their impact on weather conditions. (15 marks · 250 words)
Approach: List La Soufrière (St Vincent), Mt Nyiragongo (DRC), Cumbre Vieja (La Palma), Mt Semeru (Indonesia). Wind/pressure linkage: Stratospheric ash + SO₂ → spread by polar night jet & STJ; SO₂ → sulphate aerosol → global albedo↑ → "volcanic winter" (cooling 0.1–0.5 °C). Pinatubo 1991 historical parallel. Conclude: jet-stream transport explains why N-hem volcanoes cool N-hem more.
Q. "The Himalayas are highly prone to landslides." Discuss the causes and suggest suitable measures of mitigation. (15 marks · 250 words)
Wind/monsoon hook: Orographic uplift of SW monsoon → cloudbursts → saturation triggers slides; Western Disturbances + snowmelt = spring slides; valley breezes destabilise loose moraines. Mitigation: drainage, terracing, early-warning radar tied to IMD, afforestation. Show how seasonal-wind regime is the trigger, geology is the cause.
Q. Explain the factors responsible for the origin of the ocean currents. How do they influence regional climates, fishing and navigation? (12.5 marks · 200 words)
Wind link: Primary driver = planetary winds (trades push N&S Equatorial currents westward; westerlies push N Atlantic Drift / W Wind Drift eastward). Coriolis curls them into gyres. Climates: Gulf Stream warms NW Europe; Peru (Humboldt) cold current causes Atacama desert; California cold current causes fog. Fishing: Cold/warm meeting zones (Grand Banks, Hokkaido) = plankton bloom. Navigation: Ships still ride W Wind Drift, Equatorial Counter Current.
Q. Most of the unusual climatic happenings are explained as an outcome of the El-Nino effect. Do you agree? (12.5 marks · 200 words)
Approach: Define ENSO (Walker circulation reversal — warm Pacific E, weak trades). Effects: weak Indian monsoon, Australian drought, Peruvian floods, S African drought. But: not the only driver — IOD, MJO, NAO, AO also matter. Show: 1997-98, 2015-16 strong El Niño years had below-normal Indian monsoon; 2009 (El Niño + IOD−) was drought; 2019 (IOD+ even with El Niño) saved Indian monsoon. Partial agreement: dominant but not exclusive.
Practice Mains Questions (model)
Q. Explain the formation of the global pressure belts and account for their seasonal shifts. (15 marks)
Diagram cue: Fig 8.1 (pole-to-pole pressure cross-section). Cover: thermal vs dynamic origin (Eq Low, Pol High = thermal; STH, SPL = dynamic); 5° N/S seasonal shift following Sun's declination. End with monsoon implication.
Q. Compare the classical (thermal) and modern (dynamic) theories of Indian monsoon. (15 marks)
Approach: Halley vs Flohn/Koteswaram. Tabulate mechanism + limitations. Add ENSO/IOD modulation. Diagram cue: Fig 8.4 dual-panel + Fig 8.6 jets. Conclude: thermal explains what, dynamic explains why & when.
Q. Differentiate between geostrophic wind and surface wind with the help of a labelled diagram. Explain why surface winds cross isobars. (10 marks)
Diagram cue: Fig 8.2 dual-panel. Key: Geostrophic = PGF + Coriolis only (parallel to isobars, no friction aloft); Surface wind = PGF + Coriolis + Friction → 3-force balance → wind crosses isobars at angle 10°–30° (land) or 10° (sea) from high to low.
Q. Discuss the role of Tibetan Plateau in the Indian monsoon system. (10 marks)
Approach: Elevated heat source (4500 m) → mid-tropospheric warming → upper-air anticyclone → creates TEJ on its south flank. STJ retreat to N of plateau in summer = trigger for SW monsoon burst. Diagram cue: Fig 8.6 (TEJ position).
Q. "Local winds reflect local geography." Substantiate with examples from at least four continents. (15 marks)
Tabulate: Loo (Asia, Thar), Chinook (N America, Rockies), Mistral (Europe, Rhône), Harmattan (Africa, Sahara→Sahel), Berg (Africa, plateau→coast), Pampero (S America, pampas). Show 2-axis: hot/cold × wet/dry. Conclude: Topography (mt barriers, valleys) + adjacent thermal source (desert, plateau, sea) determine character.
Q. What are Western Disturbances? Why are they significant for Indian agriculture? (10 marks)
Approach: Extratropical low-pressure systems originating over Mediterranean, steered by S branch of STJ in winter. Bring 2–3 spells/month of light-to-moderate rain over Punjab, Haryana, W UP, J&K, HP, Uttarakhand (Dec–Feb). Agri link: ~10–20 % of rabi-season (wheat, mustard, barley) water needs; also recharges Himalayan snowpack → summer river flow.
Q. Examine how climate change is altering jet stream behaviour and its implications for India. (15 marks)
Approach: Arctic warming 4× global avg → reduced equator-pole T gradient → weak & meandering PFJ → larger Rossby waves → extreme weather (heat domes, polar vortex breakouts). For India: weaker STJ in winter → erratic Western Disturbances → cloudbursts (Uttarakhand 2013, Kedarnath; Kashmir 2014; Himachal 2023). Stronger TEJ variability → erratic monsoon onset/withdrawal.
Q. Distinguish between sea breeze and land breeze. How do they influence the coastal climate of India? (10 marks)
Diagram cue: Fig 8.5 dual-panel. Sea breeze: 2–4 pm, sea→land, 30–50 km inland — moderates Mumbai/Chennai afternoon T. Land breeze: 2–4 am, land→sea, weaker — aids early-dawn fishing sail-out. Sea-breeze front along Konkan triggers afternoon thunderstorms.
Q. The three-cell model of atmospheric circulation explains both pressure belts and surface wind systems. Examine. (12.5 marks)
Diagram cue: Fig 8.1. Show Hadley (direct, thermal), Ferrel (indirect, dynamic), Polar (direct, thermal) cells. Sinking limbs = high pressure belts; rising = low pressure. Surface winds = N-S branches deflected by Coriolis: trades, westerlies, polar easterlies. Conclude: it's the simplest workable framework but cannot capture monsoons, jets, ENSO.
Q. Describe the mechanism of the South-West monsoon. Why does Cherrapunji receive much more rainfall than Shillong, only 50 km away? (15 marks)
Mechanism: Differential heating + ITCZ shift + TEJ + STJ retreat + Tibetan plateau heating + Findlater jet → SW flow → orographic uplift over Western Ghats & Khasi hills. Cherrapunji vs Shillong: Cherrapunji on south-facing slope of Khasi hills (windward, funnel effect, ~11 000 mm); Shillong on plateau top in rain-shadow (~2 200 mm).
Revision — 15 Memory-Anchor Facts
- Normal sea-level pressure = 1013.25 mb = 1013.25 hPa = 760 mm Hg.
- Pressure decreases ~1 mb per 10 m rise near surface; half of atmospheric mass is below 5.5 km.
- Seven pressure belts: Eq Low (0°) — STH (30° N/S) — SPL (60° N/S) — Polar High (90° N/S). EL & PH = thermal; STH & SPL = dynamic.
- Pressure belts shift ~5° with seasonal march of Sun (north in NH summer).
- Coriolis Fc = 2 Ω v sin φ → zero at equator, max at poles. Deflects right in NH, left in SH (Ferrel's Law).
- Geostrophic wind = PGF balanced by Coriolis → parallel to isobars (aloft, no friction).
- Buys-Ballot's Law (NH): stand with wind behind → low pressure on left, high on right.
- Three-cell model: Hadley (0°–30°, direct), Ferrel (30°–60°, indirect), Polar (60°–90°, direct).
- Planetary winds: Trades (NE in NH, SE in SH) 30°→0°; Westerlies (SW in NH, NW in SH) 30°→60°; Polar easterlies (NE in NH, SE in SH) 90°→60°.
- Roaring Forties (40°S), Furious Fifties (50°S), Shrieking Sixties (60°S) — SH westerlies belts named for unobstructed strength.
- SW monsoon onset: Kerala ~1 June; covers all-India by mid-July; withdraws from NW India early September. Brings ~75 % of India's annual rainfall.
- NE monsoon (Oct–Dec) drenches Tamil Nadu & coastal Andhra; picks up Bay of Bengal moisture en route from N India high.
- Hot dry winds: Loo (N India), Sirocco (Sahara→Italy), Khamsin (Egypt), Santa Ana (California). Warm dry leeward: Foehn (Alps), Chinook (Rockies, "snow-eater"), Zonda (Andes), Berg (S Africa). Cold dry: Mistral (Rhône), Bora (Adriatic), Pampero (S America), Buran (Siberia).
- Three jet streams over India region: STJ (30°N, 12 km, westerly, all-year; bifurcates over Himalayas in winter → S branch steers Western Disturbances) · PFJ (60°N, 9 km, westerly, Rossby meanders) · TEJ (15°N, 14 km, easterly, summer-only — drives SW monsoon).
- Tibetan Plateau + TEJ + Findlater low-level jet = three-engine driver of the SW monsoon burst (modern dynamic theory).