World Climatic Regions

Syllabus Unit: Geography of India & World

Why it matters

Chapter overview

World climates form a predictable belt system from equator to poles, driven by insolation, pressure belts and ocean currents. This topic covers how geographers classify that system (Koppen, Thornthwaite), the major climate types it produces — equatorial, tropical, temperate, desert, Mediterranean, polar — and the large-scale ocean-atmosphere disruptions like El Nino and La Nina that periodically override the normal pattern, a recurring theme in GS-I world geography and current affairs.

General Studies Paper I Geography of India & World 12%–15% weightage

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Group A: Tropical (coldest month ≥18°C)

Group B: Dry (evaporation exceeds precipitation)

Group C/D: Temperate & Continental (seasonal contrast)

Group E: Polar (warmest month <10°C)

1Koppen & Thornthwaite Classifications

Koppen classifies climates using temperature and rainfall thresholds tuned to match natural vegetation limits; Thornthwaite refines this by measuring actual water balance.

Learning Objectives

State the five major Koppen climate groups (A, B, C, D, E) and what each broadly represents.
Explain how Koppen calibrated his temperature/precipitation thresholds against natural vegetation boundaries.
Define potential evapotranspiration (PE) and explain why Thornthwaite's scheme improves on Koppen for water-balance studies.
State the Thornthwaite moisture index formula and what a positive versus negative value indicates.

Key Terms

Koppen SystemEmpirical climate classification (Wladimir Koppen, early 1900s) dividing world climates into five major letter groups by temperature and precipitation.

Group A / B distinctionGroup A (Tropical) requires the coldest month to average 18°C or above; Group B (Dry) is defined by evaporation exceeding precipitation, regardless of temperature.

Potential Evapotranspiration (PE)The water that would evaporate and transpire from a vegetated surface if moisture supply were unlimited — the core variable in Thornthwaite's method.

Moisture Index (Im)Thornthwaite's index comparing water surplus to deficit against PE, classifying climates on a continuum from perhumid to arid.

The **Koppen classification**, devised by German climatologist Wladimir Koppen, remains the most widely used climate system because its temperature and precipitation thresholds were deliberately chosen to line up with the real-world boundaries of natural vegetation.

The five major Koppen groups

A — Tropical: coldest month averages 18°C or above (e.g. Amazon Basin, Congo Basin, Indonesia).
B — Dry: evaporation exceeds precipitation year-round (e.g. Sahara, Thar, Arabian deserts and steppes).
C — Temperate/Mesothermal: coldest month between -3°C and 18°C (e.g. Mediterranean, humid subtropical China-type).
D — Continental/Microthermal: large seasonal temperature range, cold snowy winters (e.g. Siberia, Canadian interior).
E — Polar: warmest month below 10°C (tundra and ice-cap regions).

Key PointA second lowercase letter refines each group — e.g. Af (tropical rainforest, no dry season) vs Aw (tropical savanna, dry winter); Cs (dry-summer Mediterranean) vs Cw (dry-winter subtropical).

Thornthwaite's refinement: measuring actual water balance

C. W. Thornthwaite (1948) argued that temperature alone cannot capture how much moisture is actually available to plants and soil. He introduced **Potential Evapotranspiration (PE)** — water loss under unlimited moisture supply — and compared it to actual precipitation to compute a moisture index, making his scheme far more useful for irrigation planning and agricultural water-balance studies than Koppen's purely descriptive letters.

Im = 100 × (Water Surplus − Water Deficit) / Potential Evapotranspiration (PE)

Common MistakeExam trap: Koppen's system is vegetation-boundary based and empirical; Thornthwaite's is a genetic/water-balance system built from PE — don't attribute the moisture index formula to Koppen.