The Köppen climate classification system divides Earth’s climates into five major groups based on temperature, precipitation, and the vegetation those conditions support. Developed by German botanist and climatologist Wladimir Köppen at the end of the 19th century, it remains the most widely used climate classification framework in the world, appearing in everything from geography textbooks to agricultural planning tools.
Köppen built his system on a straightforward insight: vegetation patterns reflect climate. By studying which plants grew where, he could draw meaningful boundaries between climate zones using measurable thresholds for temperature and rainfall. He published his first climate map in the early 1900s and continued refining it until his death in 1940. Later climatologists, most notably Rudolf Geiger, updated the system further, which is why you’ll often see it called the Köppen-Geiger classification.
The Five Major Climate Groups
Each of the five groups is assigned a capital letter: A, B, C, D, and E. The boundaries between them are defined by specific temperature and precipitation cutoffs.
- A (Tropical): Every month of the year has an average temperature at or above 18 °C (64 °F). These are the equatorial zones dominated by rainforests, monsoon forests, and savannas.
- B (Dry): Evaporation exceeds precipitation on an annual average. This group captures both true deserts and semi-arid steppes, regardless of temperature. It’s the only group defined primarily by moisture rather than heat.
- C (Temperate): The coldest month averages between 0 °C and 18 °C (32–64 °F), while at least one month averages above 10 °C (50 °F). This covers mild climates like the Mediterranean, the southeastern United States, and much of Western Europe.
- D (Continental): The coldest month drops at or below 0 °C (32 °F), but the warmest month still exceeds 10 °C. These are the climates with harsh winters and warm or hot summers, spanning large portions of Russia, Canada, and the northern United States.
- E (Polar): No month averages above 10 °C. This includes both tundra landscapes, where the warmest month stays between 0 °C and 10 °C, and ice caps, where temperatures never rise above freezing.
How the Letter Code Works
A Köppen classification is a two- or three-letter code. The first letter identifies the major group. The second and third letters add detail about seasonal precipitation patterns and temperature characteristics, giving you a surprisingly precise climate profile in just a few characters.
For tropical climates (A), the second letter describes rainfall. An “f” means rain falls year-round (think equatorial rainforest), “m” means monsoon patterns with a brief dry season, and “w” or “s” indicates a dry winter or dry summer, respectively. Tropical savanna climates, for example, carry the code Aw.
For dry climates (B), the second letter distinguishes deserts (W, for “Wüste,” the German word for desert) from semi-arid steppes (S). A third letter indicates whether the dry climate is hot (h) or cold (k). The Sahara, for instance, is BWh: a hot desert. The dry plains of Mongolia are BWk: a cold desert.
Temperate (C) and continental (D) climates use similar second letters for precipitation. An “f” means no dry season, “s” means dry summers, and “w” means dry winters. The third letter describes summer heat: “a” for hot summers (warmest month above 22 °C), “b” for warm summers, “c” for cool and short summers, and “d” (in continental climates only) for extremely cold winters where the coldest month drops below −38 °C. A city like Atlanta, Georgia, is Cfa: temperate, no dry season, hot summer. Moscow is Dfb: continental, no dry season, warm summer.
Polar climates (E) use a simpler second letter. ET marks tundra, while EF marks permanent ice cap.
Temperature Thresholds Have Shifted Over Time
The system isn’t frozen in its original form. One notable change involves the boundary between temperate (C) and continental (D) climates. Köppen originally set this at −3 °C for the coldest month, reasoning that this temperature aligned with the persistence of snow cover in mid-latitude regions. But later researchers argued that 0 °C, the freezing point of water, was a more meaningful dividing line because it better reflects the transition from climates dominated by rain to those dominated by snow. Most modern versions of the classification now use 0 °C as the C/D boundary.
Why It Still Matters
For something developed over a century ago, the Köppen system has remarkable staying power. Soil scientists use it to establish expectations about soil temperature and moisture. Agricultural researchers rely on it to predict which crops will thrive in a given region. Ecologists use it as shorthand for the vegetative communities they expect to find, from steppe grasses to boreal forests. The classification also appears frequently in climate change research, where shifting zone boundaries serve as a visual, intuitive way to show how warming temperatures are redrawing the planet’s climate map.
Smartphone apps now let field researchers identify their Köppen zone in seconds, linking it to soil and environmental data. That kind of practical, portable utility is a testament to how well the system translates complex climate data into something immediately usable.
The Trewartha Modification
The most significant revision of the Köppen system came from American geographer Glenn Trewartha, whose modified version is often called the Köppen-Trewartha classification. Trewartha adjusted several of the original thresholds to produce boundaries that better match natural landscapes, particularly in transitional zones where the original system drew lines that didn’t quite align with what was happening on the ground. The Trewartha version uses the same raw temperature and precipitation data as the original but applies revised criteria, making it a popular alternative in regional climate studies, especially across East Asia.
Despite the Trewartha modification and other updates, the core logic of Köppen’s system remains intact: measure temperature and precipitation, apply consistent thresholds, and the resulting letter code tells you what kind of environment to expect. That simplicity is exactly why it has outlasted more complex alternatives.