🌍 Physical Geography – GC03CDS Level★ High Priority
📌 CDS Focus: Extremely important, especially for the Indian subcontinent. Questions focus on: atmospheric layers (which layer, what happens there), pressure belts and wind systems (trade winds, westerlies, monsoon mechanism), cyclone comparisons (tropical vs temperate), humidity and precipitation types, and Koppen's climate classification. The monsoon mechanism is extremely important for CDS as it is uniquely relevant to India.
⚠️ Atmosphere Layer Traps: (1) Weather occurs in Troposphere — all clouds, rain, storms happen here. (2) Mesosphere is the coldest layer (−90°C), not the thermosphere (which is hot but very thin air). (3) Ozone layer is in Stratosphere (15–35 km). (4) Auroras occur in Thermosphere/Ionosphere. (5) Meteors burn up in Mesosphere (shooting stars). (6) Jet streams are in the Stratosphere/upper Troposphere.
INSOLATION & HEAT BUDGET
2. Insolation & Greenhouse Effect
Topic BHeat Budget of EarthKey Concept
Insolation
Incoming Solar Radiation (Insolation) — of 100 units received from Sun: ~35 units reflected back (albedo — by clouds, snow, ice); ~14 units absorbed by atmosphere; ~51 units absorbed by Earth's surface. The Earth re-radiates this energy as long-wave terrestrial radiation.
Lapse Rate
Normal Lapse Rate = 6.5°C per 1,000 m altitude (temperature decreases as you go higher in troposphere). Environmental lapse rate is actual measured change; adiabatic lapse rate is for rising air parcels (DALR = 10°C/1000m; SALR = 6°C/1000m for saturated air).
Greenhouse
Greenhouse Effect: Short-wave solar radiation passes through atmosphere easily; long-wave terrestrial radiation is trapped by greenhouse gases (CO₂, CH₄, N₂O, water vapour). Main greenhouse gas by volume: Water vapour (most abundant); main anthropogenic: CO₂. Without greenhouse effect, Earth's avg temp would be −18°C instead of +15°C.
PRESSURE BELTS & PLANETARY WINDS
3. Atmospheric Pressure Belts & Wind Systems
Fig. 3.1 — Global Pressure Belts & Planetary Wind Directions
Topic CPlanetary Winds — Key Facts
Trade Winds
Blow from subtropical high (30°) toward equatorial low (0°). North Hemisphere: NE Trade Winds. South Hemisphere: SE Trade Winds. Most consistent and reliable winds on Earth. Deflected by Coriolis effect. Important for navigation historically.
Westerlies
Blow from subtropical high (30°) toward sub-polar low (60°). From SW in Northern Hemisphere; from NW in Southern Hemisphere. Southern hemisphere westerlies are extremely strong — called Roaring Forties (40°S), Furious Fifties (50°S), Screaming Sixties (60°S).
Coriolis
Coriolis Effect (Ferrel's Law) — due to Earth's rotation, winds deflect to the right in Northern Hemisphere and left in Southern Hemisphere. Maximum at poles; zero at equator. Causes NE trade winds (from NE), not directly North.
Doldrums
The equatorial belt (~5°N–5°S) with very light, variable winds — called Doldrums. Hot, rising air creates low pressure. Also called the ITCZ (Inter-Tropical Convergence Zone). Sailing ships dreaded this zone as they were becalmed.
Horse Lat.
30°N and 30°S — areas of descending dry air, calm winds, and high pressure. Called Horse Latitudes. Great deserts of the world are found here (Sahara, Arabian, Thar, Atacama). Sailors allegedly threw horses overboard when ships were becalmed.
PERIODIC & LOCAL WINDS
4. Periodic & Local Winds
Topic DMonsoon & Periodic WindsHigh CDS Weightage
Monsoon
Seasonal reversal of wind direction caused by differential heating of land and sea. Summer monsoon (June–September) — winds blow from sea to land (SW monsoon in India), bringing rainfall. Winter monsoon (Oct–Feb) — winds blow from land to sea (NE monsoon), relatively dry. The ITCZ shifts northward in summer, allowing SW monsoon to penetrate India. Southwest monsoon enters India through two branches: Arabian Sea branch (first) and Bay of Bengal branch.
Sea Breeze
During day — land heats faster than sea → low pressure over land → wind blows from sea to land (sea breeze, cool). At night — land cools faster → high pressure over land → wind blows from land to sea (land breeze, warm). Same principle as monsoon but on daily cycle.
Local Wind
Region
Type
Key Feature
Loo
India (Northwest / Punjab, Haryana, UP)
Hot, dry
Blows in summer afternoons (May–June); can be fatal
Chinook
Eastern Rockies, USA/Canada
Hot, dry (Foehn type)
"Snow eater" — causes rapid snowmelt
Foehn
Alpine valleys (Switzerland)
Hot, dry
Orographic warm wind on leeward side of mountains
Sirocco
North Africa → Mediterranean
Hot, dusty
From Sahara; picks up dust; reaches Europe as warm rain
Mistral
France (Rhône valley)
Cold, dry
Flows from Alps toward Mediterranean; strong and cold
Bora
Adriatic coast
Cold, dry
Descends from mountains; very strong winter wind
Harmattan
West Africa
Hot, dry
Blows from Sahara westward; called "Doctor" for drying effect
Blizzard
Polar regions / USA
Cold, wet
Snowstorm with strong winds; near-zero visibility
💡 Foehn Effect: When moist air rises over a mountain (windward side), it cools at SALR (6°C/1000m) and loses moisture as rain. When it descends on the leeward side, it warms at DALR (10°C/1000m). Result: the leeward side gets warmer and drier air than the windward side received. This explains Chinook (warm, snow-melting) and Foehn winds.
CYCLONES & ANTICYCLONES
5. Cyclones & Anticyclones
Fig. 5.1 — Tropical vs Temperate Cyclone: Key Differences at a Glance
🌀 Cyclone vs Anticyclone
Cyclone — low pressure centre; winds blow IN; anti-clockwise in N.H.; clockwise in S.H.
Anticyclone — high pressure centre; winds blow OUT; clockwise in N.H.; anti-clockwise in S.H.
Anticyclones = fair, dry, settled weather
Cyclones = stormy, cloudy, rainy weather
🌊 Ocean Currents & Climate
Warm currents — bring warm, moist air; increase rainfall on coasts (e.g., Gulf Stream → W. Europe)
Cold currents — stabilise air, reduce rainfall; cause deserts on west coasts (e.g., Benguela → Namib Desert)
Cold upwelling currents → fog (California coast, Newfoundland)
HUMIDITY, CLOUDS & PRECIPITATION
6. Humidity, Condensation & Precipitation
Topic FForms of Precipitation
Humidity
Absolute humidity — actual water vapour in g/m³. Relative humidity — ratio of actual moisture to maximum possible at that temp (expressed as %). Dew Point — temperature at which air becomes saturated and condensation begins.
Rain Types
Convectional rainfall — intense local heating → rising air → cooling → heavy rain + thunderstorms; common in equatorial regions (daily afternoon). Orographic/Relief rainfall — moist air forced over mountains → rain on windward side; leeward side is rain shadow (dry). Cyclonic/Frontal rainfall — warm and cold air meet at front; moderate, widespread rain.
⚠️ Rainfall Type Traps: (1) Western Ghats windward side = heavy orographic rain; leeward side (Deccan Plateau) = rain shadow = dry. (2) Mawsynram / Cherrapunji (Meghalaya) = highest rainfall — orographic, not convectional. (3) Cumulonimbus = thunderstorm cloud. (4) Convectional rainfall is typical of Amazon/Congo/equatorial regions — heavy afternoon rain daily.
CLIMATE CLASSIFICATION
7. Koppen's Climate Classification
Symbol
Climate Type
Key Feature
Example Region
A
Tropical (Humid)
No cold season; all months above 18°C; high rainfall
Amazon, Congo, SE Asia (Af = rainforest; Aw = savanna)
B
Arid / Dry
Evaporation > Precipitation; deserts and steppes
Sahara (BWh), Thar (BWh), Asian steppes (BSk)
C
Temperate / Mesothermal
Mild winters; hot or warm summers
Mediterranean (Cs), China type (Cfa), W. Europe (Cfb)
D
Continental / Microthermal
Cold winters; warm/hot summers; 4 seasons
Russia, Canada, Northern USA (Dfa, Dfb)
E
Polar
All months below 10°C; no true summer
Greenland (EF = Ice cap), Northern Canada (ET = Tundra)
H
Highland
Climate varies with altitude; not Koppen's original
Himalayas, Andes, Alps
💡 Mediterranean Climate (Cs): Hot, dry summers + cool, wet winters. Found between 30°–40° on west coasts of continents. Regions: Mediterranean Sea shores, California, S. Australia, S. Africa (Cape region), central Chile. India does NOT have Mediterranean climate — a CDS trap. Cherrapunji has Am (tropical monsoon) climate.
📐 Formula Sheet & Key Facts — GC03
Lapse Rates
Normal Lapse Rate: 6.5°C/1000m Dry Adiabatic (DALR): 10°C/1000m Saturated Adiabatic (SALR): 6°C/1000m
Heat Budget (100 units)
Reflected (albedo): ~35 units Absorbed by atmosphere: ~14 units Absorbed by Earth surface: ~51 units
Pressure Belt Latitudes
Equatorial Low (ITCZ): 0° Sub-tropical High: 30°N & S Sub-polar Low: 60°N & S Polar High: 90°N & S
Cyclone Wind Rotation
N.Hemisphere cyclone: anti-clockwise S.Hemisphere cyclone: clockwise Anticyclone: opposite to cyclone
Atmosphere Layers (key)
Weather: Troposphere (0–12 km) Ozone: Stratosphere (15–35 km) Coldest: Mesosphere (−90°C) Auroras: Thermosphere
Koppen Key Symbols
A = Tropical · B = Arid C = Temperate · D = Continental E = Polar · H = Highland f = no dry season · w = dry winter · s = dry summer
📝 Topic-Wise PYQs & Tricky Questions — GC03
Q1. Which layer of the atmosphere contains the ozone layer? CDS PYQ
The ozone layer (ozonosphere) is located in the Stratosphere at approximately 15–35 km altitude. It absorbs most of the Sun's harmful ultraviolet (UV-B and UV-C) radiation. Depletion of the ozone layer (ozone hole) is most severe over Antarctica. The Troposphere contains all weather; Mesosphere is the coldest layer; Thermosphere contains the ionosphere and is where auroras occur.
Q2. Winds that blow from subtropical high pressure to the equatorial low are called: CDS PYQ
Trade winds blow from the subtropical high pressure belt (30°N and 30°S) toward the equatorial low (0°). In the Northern Hemisphere they blow from the NE (NE Trade Winds); in the Southern Hemisphere from the SE (SE Trade Winds) — deflected by Coriolis effect. Westerlies blow from subtropical high toward sub-polar low (30° → 60°). Trade winds are the most consistent winds and were historically vital for sailing.
Q3. In the Northern Hemisphere, winds in a cyclone rotate in which direction? CDS PYQ
In the Northern Hemisphere, winds rotate anti-clockwise (counter-clockwise) in a cyclone — due to the Coriolis effect (deflection to the right). In the Southern Hemisphere, cyclones rotate clockwise. Anticyclones rotate in the opposite direction: clockwise in N.H., anti-clockwise in S.H. This is Ferrel's Law — winds deflect right in N.H. and left in S.H. due to Earth's rotation.
Q4. Cherrapunji receives very high rainfall mainly due to which type? ⚡ Tricky
Cherrapunji (Mawsynram/Sohra) in Meghalaya receives very heavy rainfall because the Bay of Bengal monsoon winds are forced upward by the Khasi Hills — this is classic orographic (relief) rainfall. The windward southern slopes receive heavy rain while the northern leeward side is comparatively dry. It is NOT convectional (which is daily, thundery, equatorial). This is one of the most tested CDS geography questions.
Q5. Mediterranean climate is characterised by: CDS PYQ
(a) Hot, wet summers and cold, dry winters(b) Hot, dry summers and cool, wet winters(c) Uniform rainfall throughout the year(d) Very low rainfall year-round
✔ Answer: (b) Hot dry summers and cool wet winters
Mediterranean climate (Koppen Cs) is defined by hot, dry summers and cool, wet winters. This is because subtropical high pressure dominates in summer (blocking rain) while the westerlies bring rain in winter. Found between 30°–40° on west coasts: Mediterranean shores, California, parts of S. Africa, S. Australia, Chile. In India, Mediterranean climate does NOT occur — J&K and Himachal have cold semi-arid/alpine climates, not Mediterranean.
Q6. The "Loo" is a local wind that blows over: CDS PYQ
(a) Western Ghats(b) Thar Desert at night(c) Indo-Gangetic plains in summer(d) Coastal Tamil Nadu
✔ Answer: (c) Indo-Gangetic plains in summer
Loo is a hot, dry, dusty local wind that blows during the summer (May–June) over the north-western and central parts of India — particularly Punjab, Haryana, Rajasthan, and UP in the Indo-Gangetic plains. It can cause fatal heat strokes. It blows mainly in the afternoon. The Chinook is the equivalent in North America (warm, snow-melting wind in the Rockies).
Q7. The layer of the atmosphere where most meteors burn up is: ⚡ Tricky
Meteors (shooting stars) burn up due to friction when they enter the Mesosphere (50–80 km). The Mesosphere also has the lowest temperatures of any atmospheric layer (~−90°C at the mesopause). Auroras occur in the Thermosphere/Ionosphere (80–600 km). Weather occurs in the Troposphere. Radio wave reflection happens in the ionosphere (part of Thermosphere). This is a frequently tested tricky distinction.
Q8. What is the term for the calm area at the centre of a tropical cyclone? CDS PYQ
(a) Front(b) Eye Wall(c) Eye(d) Vortex
✔ Answer: (c) Eye
The Eye is the calm, clear centre of a tropical cyclone — typically 20–65 km in diameter. It has the lowest pressure and light winds. The Eye Wall (surrounding the eye) has the strongest winds and heaviest rainfall. Temperate cyclones do NOT have an eye — this is a key distinguishing feature. The term "Front" refers to the boundary between warm and cold air masses in temperate cyclones.
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