A-Tropical Humid Climate

Introduction – Where do we find Tropical Humid Climates?

When we talk about tropical humid climates, immediately two keywords should come to our mind — “tropics” and “humidity.”

• The tropics lie between the Tropic of Cancer (23½° N) and the Tropic of Capricorn (23½° S).
• In this region, the sun is almost overhead throughout the year.
• This is also the zone where the Inter Tropical Convergence Zone (ITCZ) dominates — meaning winds from both hemispheres converge, forcing the hot moist air upwards and causing heavy rainfall.

So the overall character becomes: hot + humid + high rainfall + low annual temperature range.

Now, Köppen divided the tropical climates into three types:

1. Af – Tropical Wet Climate (no dry season)
2. Am – Tropical Monsoon Climate
3. Aw – Tropical Wet and Dry Climate (Savanna)

First, we will first focus on Af (Tropical Wet / Equatorial Rainforest Climate).

Af (Tropical Wet / Equatorial Rainforest Climate).

Distribution – Where do we see Af Climate?

• It is confined to regions close to the equator (about 5° N to 5° S).
• Why only here? Because here the Coriolis force is very weak → no tropical cyclones, only uniform rising of moist air.
• Major regions:
  • Amazon Basin in South America
  • Congo Basin in Africa
  • Malaysia, Indonesia, Papua New Guinea in Southeast Asia

So, if you think of equatorial climate, always imagine dense evergreen forests like the Amazon or Congo.

Climate Characteristics

(a) Temperature

• Extremely uniform throughout the year → no winter.
• Annual average: around 27°C.
• Both annual range (difference between hottest and coldest months) and diurnal range (difference between day and night) are very small.
• Why? Because of cloudiness, high humidity, and daily sea–land breezes which moderate extremes.

👉 In short: Climate is equable.

(b) Rainfall

• Average annual rainfall: 150–300 cm, distributed throughout the year.
• Almost every afternoon, thunderstorms occur due to convectional rainfall (Cumulonimbus clouds).
• There is no dry season. Every month gets rainfall more than 6 cm.
• But still, there are two rainfall peaks (April & October) around the Equinoxes — when the sun (and ITCZ) passes directly overhead.
• Lowest rainfall is in June and December (solstices).

👉 This double rainfall maximum is a unique feature of equatorial climate. No other climate has this.

Equatorial Vegetation

Now, because of constant heat + abundant rainfall, nature produces the most luxuriant vegetation on earth: Tropical Rainforests (Selvas in Amazon).

Features of Equatorial Rainforests:

1. Multilayered structure:
   • Tallest trees (~50 m) form a canopy.
   • Below them → medium trees.
   • At the bottom → ferns, shrubs, shade-tolerant plants.
   • Because sunlight hardly reaches the ground, undergrowth is sparse.
2. Epiphytes: Plants that grow on trees to reach sunlight (e.g., orchids, mosses).
3. Evergreen hardwoods: Mahogany, ebony, rosewood, dyewoods.
4. Mangroves: In swampy coastal regions.

Why despite huge forests, timber is not exploited commercially?

• Because trees grow in mixed stands, not in pure blocks like pine or oak in temperate regions.
• Hardwood trees are too heavy to float on rivers, so transportation is costly.
• Hence paradox: Equatorial countries → dense forests but net timber importers.

Human Life and Economy

(a) Traditional Life

• Population is sparse.
• Many indigenous tribes survive as hunter-gatherers or by shifting cultivation (slash-and-burn).

(b) Early Commercial Activities

• Amazon tribes: collected wild rubber.
• Congo Pygmies: gathered nuts.
• Malaysia’s Orang Asli: made cane products.

(c) Plantations (after Europeans came)

• Huge forests cleared → large-scale plantations.
• Major crops:
  • Rubber (Malaysia, Indonesia)
  • Cocoa (Ghana, Nigeria)
  • Palm oil (Indonesia, West Africa)
  • Sugarcane & Coffee (Brazil)
  • Coconuts, tea, tobacco, spices

👉 Plantation crops are tropical cash crops, grown for industrial demand in the West.

Problems and Constraints

(a) Health Issues

• High humidity + heat → diseases like malaria, yellow fever.
• Insects, pests spread diseases and damage crops.

(b) Jungle as a Barrier

• Road/rail construction faces snakes, insects, wild animals and diseases.
• Maintenance costs are high.

(c) Livestock Limitation

• No meadow grass → only tall coarse grasses, unfit for cattle.
• In Africa, tsetse fly spreads ngana disease → prevents cattle rearing.

(d) Resource Curse

• Region is rich in gold, copper, cobalt, oil, diamonds.
• But mining damages rainforest ecology.
• Examples:
  • Gold mining in Brazilian Amazon
  • Cobalt mining in Congo
  • Oil in Ecuador, Venezuela, Peru

Summary Thought

So, the equatorial climate appears to be a “paradise” of nature — uniform warmth, abundant rain, thick forests, biodiversity. But for human development, it has been more of a challenge: health hazards, transportation problems, agricultural limitations. That’s why, despite its riches, the equatorial belt is economically less developed compared to temperate regions.

👉 This paradox — rich natural resources, yet poor human development — is a recurring theme in world geography and very important for UPSC answers.

Excellent! Let’s now move from the Af – Equatorial Wet Climate to the second important type of Tropical Humid Climate under Köppen’s system: Am – Tropical Monsoon Climate.

Am – Tropical Monsoon Climate

Introduction – What makes Monsoon Climate different?

The equatorial climate (Af) is uniform: rain every month, no dry season. But the monsoon climate (Am) is completely different.

👉 The hallmark of monsoon climate is the seasonal reversal of winds, leading to distinct wet and dry seasons.

• In summer → On-shore winds (sea → land) bring heavy rain.
• In winter → Off-shore winds (land → sea) bring dryness.

Thus, unlike the equatorial climate, here people experience 3 well-marked seasons:

1. Summer (Hot and dry)
2. Rainy season (South-West Monsoon, heavy rainfall)
3. Winter (Cool and dry)

Floods in rainy season, droughts in dry season — both are common.

Distribution – Where do we find this climate?

• Latitudinal range: 5° to 30° N and S of equator.
• Best developed in:
  • Indian subcontinent (classic monsoon land)
  • Myanmar, Thailand, Laos, Cambodia, Vietnam, South China
  • Northern Australia
• Beyond this zone, in coastal margins, it merges into Tropical Marine Climate.

Temperature Characteristics

• Monthly means: above 18°C (since it is tropical).
• Summer: 30–45°C (mean ~30°C, peaks up to 44°C in Sindh and peninsular India).
• Winter: 15–30°C (mean ~20–25°C).
  👉 Thus, annual range is larger than equatorial climate because of the seasonal rhythm.

Rainfall Characteristics

• Annual mean rainfall: 200–250 cm, but in some places 350 cm.
• Extreme examples: Cherrapunji & Mawsynram in Meghalaya (wettest places on Earth, ~1150 cm per year).
  • Why so much rain?
    • They lie on the windward side of Meghalaya hills (Orographic rainfall).
    • Surrounded by hills → funneling effect concentrates clouds.

👉 Rain is concentrated mainly in the rainy season (June–Sept), unlike the equatorial regions where it rains all year.

Seasonal Rhythm

(a) Cool Dry Season (Oct–Feb)

• Dominated by North-East Monsoon (off-shore dry winds).
• Most of India remains dry.
• Exceptions:
  • Punjab → gets winter rain from Western Disturbances (frontal cyclones from Mediterranean). Important for wheat.
  • Tamil Nadu coast → gets rain because NE monsoon picks up moisture from Bay of Bengal.

(b) Hot Dry Season (March–mid-June)

• Sun shifts northward, heating land.
• Temperatures: 35–44°C common.
• Loo winds (hot, dry winds) blow in northern India.
• Occasional hailstorms in April–May.
• Very little rain.

(c) Rainy Season (Mid-June–Sept)

• The South-West Monsoon bursts with full force.
• Torrential downpours occur — bulk of the annual rainfall concentrated here.
• Crops, rivers, agriculture — all depend on this rainfall.

(d) Retreating Monsoon (Sept–Oct)

• Monsoon weakens and withdraws southward.
• Skies clear, humidity drops.
• North-East Monsoon sets in again.

Tropical Marine Climate (a Transition Zone)

Outside the core monsoon zone, coastal regions experience a Tropical Marine Climate.

• Rainfall is more evenly distributed (not so seasonal).
• Influenced by trade winds all year.
• Found in: Central America, West Indies, N.E. Australia, Philippines, East Africa, Madagascar, Guinea Coast, Eastern Brazil.
• But these regions are prone to tropical cyclones (typhoons, hurricanes).

Vegetation – Tropical Monsoon Forests

• Also called Tropical Deciduous Forests.
• Key difference from equatorial forests: trees are deciduous → shed leaves in dry season to conserve water.
• Forests are more open, less luxuriant, fewer species compared to equatorial jungle.
  • Luxuriant vegetation refers to dense, lush, multi-layered plant growth with rich biodiversity, sustained by abundant rainfall and favorable climatic conditions.
• Where rainfall is high → luxuriant forests (e.g., southern Myanmar, coastal India).
• Where rainfall decreases → thorny scrubland or savanna (e.g., Deccan Plateau, rain-shadow of Western Ghats).
• In driest areas (Telangana, Vidarbha) → semi-desert conditions.

Important timber species: Teak (Myanmar’s pride), Sal, Neem, Acacia, Mango, Eucalyptus, Bamboo thickets.

👉 Teak is highly valued because of strength, durability, resistance to fungus.
But teak logs are so heavy they don’t float → hence special methods like “poisoning” the tree years before felling (to dry and lighten the wood).

Population and Economy

(a) Agriculture

• Monsoon lands are densely populated (India, China, SE Asia).
• Farming is the dominant occupation.
• Rice is the staple crop (needs standing water, heavy rain).
• Other food crops: maize, millet, sorghum, wheat, gram, beans.
• Cash crops: sugarcane, jute, cotton, spices.

(b) Plantations in Highlands

• Europeans developed plantations in upland slopes with moderate climate.
• Tea → India (Assam, Darjeeling), Sri Lanka, Java, China (for local use).
• Coffee → Brazil (world leader), Central America, India, Java.

(c) Shifting Cultivation (Slash-and-Burn)

• Common in tribal belts, especially forested monsoon regions.
• Soil fertility is poor (tropical leaching), so land is cultivated for a few years, then abandoned.
• Known by different names:
  • Jhum – NE India
  • Taungya – Burma
  • Tamrai – Thailand
  • Caingin – Philippines
  • Chena – Sri Lanka
  • Milpa – Africa & Central America

Summary Thought

So, if we compare Af (Equatorial Wet) and Am (Tropical Monsoon):

• Equatorial: uniform rain, no dry season, evergreen jungle, sparse population, shifting cultivation.
• Monsoon: distinct wet/dry seasons, deciduous forests, dense population, agriculture-based economy, rice + cash crops, timber exploitation.

👉 In UPSC answers, always highlight:

• Monsoon = rhythm of life in Indian subcontinent.
• Its dual nature: a blessing (agriculture, forests) and a curse (floods, droughts).

Perfect, now we move to the third and final sub-type of Tropical Humid Climate in Köppen’s classification — the Savanna or Tropical Wet and Dry Climate (Aw).
This is an extremely important topic for UPSC because it is a transitional climate between the Equatorial Rainforest (Af) and the Hot Desert (BWh). Let’s try to understand this part:

Savanna or Tropical Wet and Dry Climate (Aw)

Introduction – Nature of Savanna Climate

• Symbol: Aw → A = Tropical; w = dry winter.
• The defining feature: alternate wet and dry seasons.
• Unlike the equatorial climate (rain all year) or monsoon climate (seasonal burst of rains), here rainfall is restricted to the summer months only.
• Dry winters are long and harsh, often leading to droughts; summers are wet but can bring floods.

👉 So, Savanna is a land of contrasts:

• Green pastures in rainy season → dry, fire-prone landscape in winter.

Distribution – Where is Savanna found?

Savanna is best developed in Sudan region of Africa, hence also called Sudan Climate.
Being transitional, it lies between equatorial forests and hot deserts.

Major regions:

1. Africa:
   • Belt of West African Sudan → curves down into East Africa (Kenya, Tanzania, Uganda) → extends to Southern Africa (north of Tropic of Capricorn).
   • This is the classic Savanna belt.
2. South America:
   • Llanos (Orinoco Basin, Venezuela & Colombia).
   • Campos (Brazilian Highlands).
3. Australia:
   • Northern Australia (south of monsoon strip), especially Queensland savannas.
4. India:
   • Parts of North Karnataka, South Maharashtra, Telangana → semi-arid to savanna features (later modified by irrigation projects).

Climate Characteristics

(a) Rainfall

• Annual rainfall: 80–160 cm (much less than monsoon).
• Seasonal rhythm:
  • Northern Hemisphere Savanna → rains May–Sept.
  • Southern Hemisphere Savanna → rains Oct–Mar.
• Dry winters are long, often leading to drought conditions.
• Rainfall decreases with distance from equator.

(b) Temperature

• Mean annual temperature: above 18°C.
• Range: 20°C–32°C in lowlands.
• Hottest period: just before rains begin (April in N. Hemisphere; Oct in S. Hemisphere).
• Characteristic feature: Large diurnal range → very hot days, cold nights.

(c) Winds

• Controlled by trade winds (easterlies).
• Bring moisture to eastern coastal areas, but by the time they reach interiors or west coast, they become dry.
• In West Africa, dry dust-laden Harmattan winds blow from the Sahara towards the Guinea coast during winter.

👉 Reason for alternating wet/dry seasons:

• Summer → onshore moist trades (ITCZ overhead) → rain.
• Winter → offshore trades → dryness.

Vegetation – Savanna Landscape

• Typical vegetation: “tall grass + scattered trees.”
• Grass: coarse, tall (6–12 ft); elephant grass can reach 15 ft.
• Trees: deciduous (shed leaves in dry season), e.g., acacias.
• Trees have:
  • Broad trunks to store water.
  • Umbrella-shaped tops to reduce wind exposure.
• In drier margins → vegetation thins into thorny scrub or semi-arid bushland.

👉 This is the classic “parkland” landscape — grasslands dotted with trees.

Animal Life – “Big Game Country”

• Savanna supports the richest animal diversity after equatorial forests.
• Famous for wildlife migration in search of water and grazing.
• Home to lions, leopards, cheetahs, giraffes, zebras, elephants, rhinos, buffaloes, antelopes.
• That’s why most National Geographic & Animal Planet documentaries are shot here.
• Unfortunately, this is also the world’s biggest hunting ground (both legal & illegal).

Human Life and Economy

(a) Tribes

• Masai (East Africa): Nomadic pastoralists.
  • Keep cattle for milk (not slaughtered).
• Hausa (Nigeria): Settled cultivators.
  • Practice permanent agriculture (not shifting cultivation).
• Many tribal areas taken over by European settlers → plantation farming (coffee, cotton, tea).

(b) Farming Challenges

• Soils: Lateritic, infertile (nutrients leached during rains; water evaporates in dry season).
• Rainfall is unreliable → frequent droughts.
• Political instability in many African states worsens infrastructure development.

(c) Crops

• Savanna has potential for plantation agriculture:
  • Cotton, cane sugar, coffee, oil palm, groundnuts, fruits.
• Examples:
  • Queensland (Australia): sugarcane, cattle ranching.
  • West Africa: groundnuts, cocoa, oil palm.
  • Brazil: Campos used for coffee & cattle.

(d) Cattle Rearing

• Savanna is often called “natural cattle country.”
• But quality of grass is poor compared to temperate grasslands.
• Local cattle breeds are also less productive (low meat/milk yield).
• Thus, large-scale ranching is limited.
• Exception: Queensland (Australia) has developed modern ranching → exports meat & dairy.

Summary Thought

The Savanna (Aw) climate represents the great “middle ground” of the tropics.

• It is not as lush as the rainforest (Af),
• nor as productive as the monsoon lands (Am),
• but it supports some of the richest biodiversity on Earth and is home to iconic human adaptations like the Masai pastoralism.

👉 From the UPSC perspective, always remember:

• Equatorial (Af): Evergreen rainforest → hunter-gatherers & plantations.
• Monsoon (Am): Deciduous forests → rice farming, dense population.
• Savanna (Aw): Grasslands + scattered trees → pastoralism, lateritic soils, wildlife diversity.