Classifying climates

To study world climate, data are organised into smaller units. There are three approaches to classifying climate:

• Empirical: based on observed data, especially temperature and precipitation.
• Genetic: organises climates by their causes.
• Applied: made for a specific purpose.

Koeppen's scheme (first developed by V. Koeppen in 1918 and modified since) is the most widely used. It is empirical: Koeppen saw a close link between vegetation and climate, chose values of temperature and precipitation (mean annual and mean monthly) that match vegetation boundaries, and used capital and small letters as codes.

Five major groups (four by temperature, one by precipitation):

• A - Tropical: coldest month averages 18°C or more.
• B - Dry: potential evaporation exceeds precipitation.
• C - Warm temperate (mid-latitude): coldest month above -3°C but below 18°C.
• D - Cold snow forest: coldest month -3°C or below.
• E - Cold (polar): every month below 10°C.
• A, C, D and E are humid; B is dry.

Small letters give the type:

• Dry seasons: f = no dry season; m = monsoon; w = winter dry season; s = summer dry season.
• a, b, c, d show how severe the temperature is.
• Group B is split by capitals: S = steppe (semi-arid) and W = desert.
• The main types: Af, Am, Aw; BSh, BWh, BSk, BWk; Cfa, Cs, Cfb (and Cwa); Df, Dw; ET, EF.

Example (new): a place coded Aw is tropical (coldest month 18°C or more) with a dry winter; one coded Df has a coldest month at or below -3°C and no dry season.