An empirical scheme devised by Wladimir Koppen (1918) that divides world climates using mean monthly temperature, mean monthly rainfall, and natural vegetation, expressed in a code of capital and small letters.
- Five major groups by capital letter: A (tropical, no winter), B (dry, deserts and steppes), C (warm temperate, mild winter), D (cold snow forest, severe winter), and E (polar, no true summer); some add H for highland.
- Small letters add detail: f (no dry season), m (monsoon), w (dry winter), s (dry summer), W (desert), S (steppe).
- Examples relevant to India: Amw (monsoon with short dry winter, west coast), As (monsoon with dry summer, Coromandel coast), Aw (tropical savanna, peninsular interior), BWhw (hot desert, Thar), BShw (semi-arid steppe), Cwg (humid subtropical with dry winter, Ganga plain), and E (Himalayan high altitude).
- The first letter is always temperature based except B, which is defined by aridity (rainfall versus evaporation).
- Koppen's scheme is empirical (based on observed temperature and rainfall data), not genetic (based on the causes such as air masses).
The five letters A to E, what each stands for, and the Indian codes (especially Aw, Amw, As, BWhw, Cwg) are standard climatology questions.
A versus B versus C order is by temperature except B (defined by dryness); w means dry winter while W (capital) means desert; s is dry summer (Mediterranean type), not subtropical.
Five letters: A tropical, B dry, C warm temperate, D cold, E polar, with small letters for rainfall seasonality.