The world's favourite climate map was drawn by a botanist
When Wladimir Köppen sorted Earth's climates in 1884, he started from plants. His five big groups, labelled with letters from A to E, track the kinds of vegetation that thrive under different patterns of heat and rain. More than a century later, his scheme remains the most widely used climate classification in the world.
Köppen, a German-Russian climatologist, kept refining the system, notably in 1918 and 1936, and Rudolf Geiger revised it further in 1954 and 1961, which is why it is sometimes called Köppen–Geiger. Because it grew out of botany, it doubles as a guide to ecosystems and can help predict how plant life in a region may shift as climate changes.
The code works like a short address. The first letter gives the main group: A for tropical, B for arid, C for temperate, D for continental and E for polar. A second letter describes when rain falls, and a third, where used, describes how hot the summers get. Af, for instance, is tropical rainforest, where every month averages at least 18 °C and gets at least 60 millimetres of rain. Cs climates have dry summers, the Mediterranean pattern, while Cw climates get at least ten times more rain in the wettest summer month than in the driest winter month.
Some categories rely on clever formulas. Whether a place counts as desert or steppe depends on a threshold calculated from its average temperature, since heat drives more evaporation: below half that threshold it is desert, between half and the full amount it is semi-arid steppe. Continental climates need at least one month averaging below freezing, and the harshest subarctic type, labelled d, has a coldest month averaging below minus 38 °C.
Not everyone is satisfied. A common complaint was that group C lumped too many different middle-latitude climates together. In 1966 the Trewartha classification tried to fix that by refining the temperate zone, and it was revised again in 1980. Even so, Köppen's letters remain the shorthand that geographers, ecologists and atlases reach for first.
Source: Köppen climate classification
