The Köppen-Geiger classification, letter by letter

Wladimir Köppen, a botanist and climatologist, proposed his classification from 1884 onwards on a simple idea: the great vegetation belts of the globe already trace the climatic boundaries. Rather than dividing arbitrarily, he calibrated his thresholds on what vegetation tolerates. Rudolf Geiger revised it in the mid-twentieth century, hence the compound name.

It is by far the most widely used climate classification, and it has the great merit of requiring only two monthly series — temperature and precipitation — which exist for almost every place on Earth.

A — tropical. Every month averages above 18 °C. No thermal winter.

B — arid. Defined not by temperature but by a deficit: annual precipitation falls below a threshold computed from mean temperature and the timing of the rainy season. It is the only letter where lack of water overrides everything else.

C — temperate. The coldest month lies between −3 °C and 18 °C: a winter exists, but a mild one.

D — continental. The coldest month is below −3 °C and the warmest above 10 °C. Pronounced winter, real summer.

E — polar. No month reaches an average of 10 °C.

One detail that matters as soon as you compare two sources: the boundary between C and D sits at −3 °C in Köppen's original, while several modern revisions place it at 0 °C. The same city can therefore be classed C here and D elsewhere with nobody being wrong — the convention differs, not the climate.

f — precipitation spread across the year, no dry season.

s — dry summer. The Mediterranean signature: rain falls in winter.

w — dry winter. A monsoon or savanna regime, rain falls in summer.

m — monsoon, reserved for the tropical zone: very wet overall, with a brief less-watered season.

In the arid B zone this position works differently: W marks a desert and S a steppe — a degree of aridity rather than a calendar.

a — hot summer, the warmest month exceeds 22 °C.

b — warm summer, below 22 °C but with at least four months above 10 °C.

c — short cool summer, fewer than four months above 10 °C.

d — very severe winter: the coldest month falls below −38 °C. Reserved for extreme continental climates.

For arid climates, h means hot and k means cold, according to the mean annual temperature.

Cfb — temperate, no dry season, warm summer. This is the oceanic climate of much of Western Europe: rain year-round, mild winter, unremarkable summer.

Csa — temperate, dry summer, hot summer. The classic Mediterranean climate: winter rain, hot dry summer.

Dfb — continental, no dry season, warm summer. Cold winters and moderate summers, typical of Central and Eastern Europe.

Köppen is a threshold classification, so it is discontinuous: two climatically very similar places can fall on either side of a boundary and receive different codes. The code says which side of the threshold you are on, not by how much.

It is also, by construction, computed over a reference period. In a changing climate a place can change class — and the code then only means something when the period it was established on is stated with it.

Finally, it ignores everything that is neither temperature nor precipitation: wind, sunshine, fog, humidity. Two climates sharing a code can feel very different to live in.