Glimpses home page • The Climate Wizard home page
By Dave Bryant, copyright 2002
Here's a quick run-down on the Köppen climate classification system and some bits of useful information for the prospective world-builder. First of all, of course, it helps a good deal to do some research, as wide-ranging as possible, on the various -ologies and -graphies that treat with how and why a life-bearing world works the way it does -- meteorology and oceanography, geology and geography, zoology and botany, and so on.
Köppen broke climates down into five rough categories, lettered A through E; a sixth "non-category", H, was added in later elaborations. The original categories proceed approximately from equator to pole, and H stands for "highlands". A, C, and D are humid climates, and B and E are arid climates. (Humid in this context doesn't mean the same thing it might in casual conversation; in this case it simply means "not dry".) To describe a given region's climate in greater detail, there are a dozen or so modifiers, expressed as lower-case letters appended to the capital letter of the overall category. In general -- and it's important to remember this is a merely a rule of thumb -- forests or woodlands tend to dominate humid climates and grass- or scrublands tend to dominate arid climates.
A climates are tropical, and usually are humid in the everyday sense, often stiflingly so when considering their typical temperatures. Rain forest and savanna are the typical habitats of A regions. There are four variations, based on the pattern of precipitation of an area.
B climates can be anywhere from the equator to moderately high latitudes and are anything from somewhat to very dry. These are the deserts and semideserts of the world, whether hot or cold, and typical habitats range from grasslands through scrub and sparse growth to bare rock or sand. There are two major variations, depending on whether an area is semiarid (steppe) or truly arid (desert), and two subvariations of each, based on temperature.
C climates are subtropical, and from the human point of view tend to be the most congenial. They can be found in some surprisingly high latitudes, thanks usually to warm ocean currents, and are defined primarily by the fact that no month of the year averages below freezing. Typical habitats can be as varied as heathers and broadleaf deciduous forests. There are three variations, based on how warm the summer is, and three subvariations, based on the pattern of precipitation.
D climates are continental and sometimes are called "boreal" (meaning "northerly") or "snow forest" climates. In the real world, they are found only on the broad expanses of the far northern hemisphere, where high latitudes, lack of proximity to wide oceans and their moderating effects, and large continents combine. The most basic element of such climates is the fact that at least one month of the year average below freezing. Forests overwhelmingly dominate these habitats, though they may be edged by heathers and other low-growth areas. There are four variations, based on how warm the summer is, and three subvariations, based on the pattern of precipitation.
E climates are found only at or near the poles. Most of these lands are covered by snow and ice and bare rock; what little vegetation there is tends to be low and stunted. There are two variations, based on whether the temperature averages below freezing year-round (icepack) or not (tundra).
H is a special case. Highland regions in much of the world -- aside from polar and sub-polar regions, which are universally cold anyway -- are so high and broken that it is impossible to determine any kind of coherent picture of the area's climate. Instead, small, extremely localized microclimates are the rule, and there can be wide variations among these microclimates, sometimes within amazingly short distances.
For those table freaks and detail hounds (like me) who are burning with curiosity, here are some tables summing up the actual criteria behind the Köppen categories and modifiers, including a list of what are considered valid climates in the Köppen system.
| Code | Description | Coldest Month Averages | Hottest Month Averages |
| A | Humid tropical | Hotter than 18° C (64.4° F) | n. a. |
| B | Dry | n. a. | n. a. |
| BS | Steppe (semiarid) | n. a. | n. a. |
| BW | Desert (arid) | n. a. | n. a. |
| C | Humid mesothermal | Between 0° C (32° F) and 18° C (64.4° F) | Hotter than 10° C (50° F) |
| D | Humid microthermal | Colder than 0° C (32° F) | Hotter than 10° C (50° F) |
| E | Polar | Colder than 0° C (32° F) | Colder than 10° C (50° F) |
| ET | Tundra | Colder than 0° C (32° F) | Between 0° C (32° F) and 10° C (50° F) |
| EF | Perpetual frost | Colder than 0° C (32° F) | Colder than 0° C (32° F) |
| H | Undifferentiated highlands | n. a. | n. a. |
| Mod. | Description | Climates |
| a | Hot summer: hottest month averages hotter than 22° C (71.6° F) | C, D |
| b | Warm summer: hottest month averages colder than 22° C (71.6° F) | C, D |
| c | Cool summer: fewer than four months of the year average hotter than 10° C (50° F) | C, D |
| d | Cool summer, very cold winter: as for c, but coldest month averages colder than –38° C (–36.4° F) | D only |
| f | Constantly moist: rainfall throughout year | A, C, D |
| h | Hot and dry: all months average hotter than 0° C (32° F) | B only |
| k | Cold and dry: at least one month averages colder than 0° C (32° F) | B only |
| m | Monsoon: short dry season, but enough annual rainfall to support rain forest | A only |
| n | Frequent fog | B only |
| n' | Infrequent fog, but humid with low rainfall | Not used? |
| s | Dry season in summer | A, C |
| w | Dry season in winter | A, C, D |
| Code | Climate | Description |
| Af | Tropical wet | Warm or hot year-round with no dry season |
| Am | Tropical monsoon | Warm or hot year-round with short dry season and heavy rains in other months |
| As | Tropical forest | Warm or hot year-round with dry summer |
| Aw | Tropical savanna | Warm or hot year-round with dry winter |
| BSh | Subtropical steppe | Warm or hot and semiarid |
| BSk | Mid-latitude steppe | Cool or cold and semiarid |
| BWh | Subtropical desert | Warm or hot and arid |
| BWk | Mid-latitude desert | Cool or cold and arid |
| BWn | Tropical fog desert | Hot and arid, with frequent fog |
| Csa | Mediterranean | Mild winter and hot dry summer |
| Csb | Mediterranean | Mild winter and warm dry summer |
| Caf | Humid subtropical | Mild winter and hot summer, with no dry season |
| Caw | Humid subtropical | Mild, dry winter and hot summer |
| Cbf | Marine west coast | Mild winter and warm summer, with no dry season |
| Cbw | Marine west coast | Mild, dry winter and warm summer |
| Ccf | Marine west coast | Mild winter and cool summer, with no dry season |
| Daf | Humid continental | Severe winter and hot summer, with no dry season |
| Daw | Humid continental | Severe dry winter and hot summer |
| Dbf | Humid continental | Severe winter and warm summer, with no dry season |
| Dbw | Humid continental | Severe dry winter and warm summer |
| Dcf | Subarctic | Severe winter and cool summer, with no dry season |
| Dcw | Subarctic | Severe dry winter and cool summer |
| Ddf | Subarctic | Severe, very cold winter and cool summer, with no dry season |
| Ddw | Subarctic | Severe, very cold, dry winter and cool summer |
| ET | Tundra | Polar tundra, no true summer |
| EF | Icecap | Perennial ice |
| H | Undifferentiated highlands |