The scheme

Köppen climate classification scheme divides the climates into five main groups and several types and subtypes. Each particular climate type is represented by a 2 to 4 letter symbol:

• GROUP A - Megathermal or Tropical climates: Tropical climates (see tropics) are characterized by constant high temperature - all twelve months of the year have average temperatures of 18 °C (64.4 °F) or higher. They are subdivided as follows:
  • Af (Tropical rain forest) climate: All twelve months have average precipitation of at least 60 mm (2.36 inches). These climates, usually occurring within 5° latitude of the equator, are dominated by the Doldrums Low Pressure System all year round, and thus have no real seasons.
    Examples: Singapore
    Belem, Brazil.
    Some of the places that have this climate are indeed uniformly and monotonously wet throughout the year (e.g., Andagoya, Colombia), but in many cases the period of higher sun and longer days is distinctly wettest (as at Palembang, Indonesia) or the time of lower sun and shorter days may have more rain (as at Sitiawan, Malaysia).
    A few places with this climate are found at the outer edge of the tropics, almost exclusively in the Southern Hemisphere; one example is Santos, Brazil.
  • Am (Tropical monsoon) climate: This type of climate, most common in southern Asia and West Africa, results from the monsoon winds which change direction according to the seasons. This climate has a driest month (which nearly always occurs at or soon after the "winter" solstice for that side of the equator) with rainfall less than 60 mm, but more than (100 - [total annual precipitation {mm}/25]):
    Examples: Conakry, Guinea
    Chittagong, Bangladesh.
    There is also another scenario under which some places fit into this category; this is referred to as the trade-wind littoral climate because easterly winds bring enough precipitation during the "winter" months to prevent the climate from becoming a tropical wet-and-dry climate. Jakarta, Indonesia and Miami, Florida are included among these locations.
  • Aw (Tropical wet and dry or tropical savanna) climate: These climates have a pronounced dry season, with the driest month having precipitation less than 60 mm and also less than (100 - [total annual precipitation {mm}/25]):
    Examples: Bangalore, India
    Veracruz, Mexico
    Townsville, Australia.
    Most places that have this climate are found at the outer margins of the tropical zone, but occasionally an inner-tropical location (e.g., San Marcos, Colombia) also qualifies.
    Sometimes As is used in place of of Aw if the dry season occurs during the time of higher sun and longer days. This is the case in parts of Hawaii (Honolulu), East Africa (Mombasa, Kenya) and Sri Lanka (Trincomalee), for instance. In most places that have tropical wet and dry climates, however, the dry season occurs during the time of lower sun and shorter days.

• GROUP B - Dry climates: These climates are characterized by the fact that precipitation is less than evaporation. The threshold is determined as follows:
  • To find the precipitation threshold (in millimetres), multiply the average annual temperature in °C by 20, then add 280 (if 70%+ precipitation is in April through September in the Northern Hemisphere, or October through March in the Southern) or 140 (if 30%-70% precipitation is in April through September north of the equator or October through March south of it).
  • If the annual precipitation is less than half the threshold for Group B, it is classified as BW (desert climate) - if it is less than the threshold but more than half the threshold, it is classified as BS (steppe climate).
  • A third letter can be included to indicate temperature. Originally, h signified low latitude climate (average annual temperature above 18 °C) while k signified middle latitude climate (average annual temperature below 18 °C), but the more common practice today (especially in the United States) is to use h to mean that the coldest month has an average temperature that is above 0 °C (32 °F), with k denoting that at least one month averages below 0 °C.
  • Examples: In Salah, Algeria (BWh)
    Turpan, China (BWk)
    Cobar, Australia (BSh)
    Medicine Hat, Alberta (BSk).
    Some desert areas, situated along the west coasts of continents at tropical or near-tropical locations, are characterized by cooler temperatures than encountered elsewhere at comparable latitudes (due to the nearby presence of cold ocean currents) and frequent fog and low clouds, despite the fact that these places rank among the driest on earth in terms of actual precipitation received. This climate is sometimes labelled BWn and examples can be found at Lima, Peru and Walvis Bay, Namibia.
  • On occasion, a fourth letter is added to indicate if either the winter or summer is "wetter" than the other half of the year. To qualify, the wettest month must have at least 60 mm of average precipitation if all twelve months are above 18 °C, or 30 mm (1.18 inches) if not; plus at least 70% of the total precipitation must be in the same half of the year as the wettest month - but the letter used indicates when the dry season occurs, not the "wet" one. This would result in Khartoum, Sudan being reckoned as BWhw, Niamey, Niger as BShw, El Arish, Egypt as BWhs, Asbiah, Libya as BShs, Umnugobi, Mongolia as BWkw, and Xining, China as BSkw (BWks and BSks do not exist). If the standards for neither w nor s are met, no fourth letter is added.

• GROUP C - Mesothermal or temperate climates: These climates have an average temperature above 10 °C (50 °F) in their warmest months, and a coldest month average between -3 °C and 18 °C (some climatologists, particularly in the United States, prefer to observe 0 °C rather than -3 °C in the coldest month as the boundary between this group and Group D).
  • The second letter indicates the precipitation pattern - w indicates dry winters (driest winter month average precipitation less than one-tenth wettest summer month average precipitation; one variation also requires that the driest winter month have less than 30 mm average precipitation), s inidicates dry summers (driest summer month less than 30 mm average precipitation, and less than one-third wettest winter month precipitation; in one variant, it is also necessary for the average annual precipitation to be not more than 890 mm [35 inches]) and f means rain in all seasons (neither above mentioned set of conditions fulfilled).
  • The third letter indicates the degree of summer heat - a indicates warmest month average temperature above 22 °C (71.6 °F), b indicates warmest month average temperature below 22 °C, with at least 4 months averaging above 10 °C, while c means 3 or fewer months with mean temperatures above 10 °C.
  • The order of these two letters is sometimes reversed, especially by climatologists in the United States.
  • Group C climates are subdivided as follows:
    • Mediterranean climates (Csa, Csb): These climates usually occur on the western sides of continents between the latitudes of 30° and 45°. These climates are in the polar front region in winter, and thus have moderate temperatures and changeable weather. Summers are hot and dry, due to the domination of the subtropical high pressure systems, except in the immediate coastal areas, where summers are cooler due to the nearby presence of cold ocean currents.
      Examples: Palermo, Sicily (Csa)
      Gaziantep, Turkey (Csa)
      Santiago, Chile (Csb)
      Portland, Oregon (Csb).
    • Humid Subtropical climates (Cfa, Cwa): These climates usually occur in the interiors of continents, or on their east coasts, between the latitudes of 25° and 40°. Unlike the Mediterranean climates, the summers are humid due to unstable tropical air masses, or onshore Trade Winds. In eastern Asia, winters can be dry (and colder than other places at a corresponding latitude) because of the Siberian high pressure system, and summers very wet due to monsoonal influence.
      Examples: Houston, Texas (Cfa - uniform precipitation distribution)
      Brisbane, Australia (Cfa - summer wetter than winter)
      Yalta, Ukraine (Cfa - summer drier than winter)
      Luodian, China (Cwa).
    • Maritime Temperate climates (Cfb, Cwb) - Cfb climates usually occur on the western sides of continents between the latitudes of 45° and 55°; they are typically situated immediately poleward of the Mediterranean climates, although in Australia this climate is found immediately poleward of the Humid Subtropical climate, and at a somewhat lower latitude. These climates are dominated all year round by the polar front, leading to changeable, often overcast weather. Summers are cool due to cloud cover, but winters are milder than other climates in similar latitudes.
      Examples: Limoges, France (uniform precipitation distribution)
      Langebaanweg, South Africa (summer wetter than winter)
      Prince Rupert, British Columbia (summer drier than winter).
      Cfb climates are also encountered at high elevations in certain tropical areas, where the climate would be that of a tropical rain forest if not for the altitude. Bogota, Colombia is perhaps the best example.
      Cwb is found only at higher altitudes, without which the climate would be tropical wet and dry; examples include Addis Ababa, Ethiopia and Mexico City.
    • Maritime Subarctic climates (Cfc): These climates occur poleward of the Maritime Temperate climates, and are confined either to narrow coastal strips on the western poleward margins of the continents, or, especially in the Northern Hemisphere, to islands off such coasts.
      Examples: Punta Arenas, Chile (uniform precipitation distribution)
      Monte Dinero, Argentina (summer wetter than winter)
      Torshavn, Faroe Islands (summer drier than winter).

• GROUP D - Microthermal or Continental climates: These climates have an average temperature above 10 °C in their warmest months, and a coldest month average below -3 °C (or 0 °C in some versions). These usually occur in the interiors of continents, or on their east coasts, north of 40° North latitude. Group D climates do not exist at all in the Southern hemisphere due to the smaller land masses here.
  • The second and third letters are used as for Group C climates, while a third letter of d indicates 3 or fewer months with mean temperatures above 10 °C and a coldest month temperature below -38 °C (-36.4 °F).
  • Group D climates are subdivided as follows:
    • Hot Summer Continental climates (Dfa, Dwa, Dsa) - Dfa climates usually occur in the forties latitudes, and in eastern Asia Dwa climates extend further south due to the influence of the Siberian high pressure system, which also causes winters here to be dry, and summers can be very wet because of monsoon circulation.
      Examples: Lowell, Massachusetts (Dfa - uniform precipitation distribution)
      Peoria, Illinois (Dfa - summer wetter than winter)
      Santaquin, Utah (Dfa - summer drier than winter)
      Beijing, China (Dwa).
      Dsa exists only at higher elevations adjacent to areas with Mediterranean climates, such as Cambridge, Idaho and Saqqez in Iranian Kurdistan.
    • Warm Summer Continental climates (Dfb, Dwb, Dsb) - Dfb and Dwb climates are immediately north of Hot Summer Continental climates, and also in central and eastern Europe, between the Maritime Temperate and Continental Subarctic climates.
      Examples: Moncton, New Brunswick (Dfb - uniform precipitation distribution)
      Minsk, Belarus (Dfb - summer wetter than winter)
      Revelstoke, British Columbia (Dfb - summer drier than winter)
      Vladivostok, Russia (Dwb).
      Dsb arises from the same scenario as Dsa, but at even higher altitudes, and chiefly in North America since here the Mediterranean climates extend further poleward than in Eurasia; Mazama, Washington is one such location.
    • Continental Subarctic or Taiga climates (Dfc, Dwc, Dsc) - Dfc and Dwc climates occur poleward of the other Group D climates, mostly north of 50° North latitude.
      Examples: Sept-Iles, Quebec (Dfc - uniform precipitation distribution)
      Anchorage, Alaska (Dfc - summer wetter than winter)
      Mount Robson, British Columbia (Dfc - summer drier than winter)
      Irkutsk, Russia (Dwc).
      Dsc, like Dsa and Dsb, is confined exclusively to highland locations near areas that have Mediterranean climates, and is the rarest of the three as a still higher altitude is needed to produce this climate. Example: Galena Summit, Idaho.
    • Continental Subarctic climates with severe winters (Dfd, Dwd): These climates occur only in eastern Siberia. The names of some of the places that have this climate - most notably Verkhoyansk and Oimekon - have become veritable synonyms for extreme, severe winter cold.

• GROUP E - Polar climates: These climates are characterized by average temperatures below 10 °C in all twelve months of the year:
  • ET (Tundra) climate: Warmest month has an average temperature between 0 °C and 10 °C. These climates occur on the northern edges of the North American and Eurasian landmasses, and on nearby islands; they also exist along the outer fringes of Antarctica (especially the Palmer Peninsula) and on nearby islands.
    Examples: Iqaluit, Nunavut
    Providenya, Russia
    Deception Island, Antarctica.
    ET is also found at high elevations outside the polar regions, above the timber line - as at Mount Washington, New Hampshire.
  • EF (Ice Cap) climate: All twelve months have average temperatures below 0 °C. This climate is dominant in Antarctica (e.g., Scott Base) and in inner Greenland (e.g., Eismitte).
  • Occasionally, a third, lower-case letter is added to ET climates if either the summer or winter is clearly drier than the other half of the year; thus Qikiqtaruk, or Herschel Island, off the coast of Canada's Yukon Territory, becomes ETw, with Pic du Midi de Bigorre in the French Pyrenees acquiring an ETs designation. If the precipitation is more or less evenly spread throughout the year, ETf may be used, such as for Hebron, Labrador. When the option to include this letter is exercised, the same standards that are used for Groups C and D apply, with the additional requirement that the wettest month must have an average of at least 30 mm precipitation (Group E climates can be as dry or even drier than Group B climates based on actual precipitation received, but their rate of evaporation is much lower). Seasonal precipitation letters are almost never attached to EF climates, mainly due to the difficulty in distinguishing between falling and blowing snow, as snow is the sole source of moisture in these climates.

Some climatologists have argued that Köppen's system could be improved upon. One of the most frequently-raised objections concerns the temperate Group C category, regarded by many as overbroad (it includes both Tampa, Florida and Cape May, New Jersey, for example). In Applied Climatology (first edition published in 1966), John Griffiths proposed a new subtropical zone, encompassing those areas with a coldest month of between 6 °C (42.8 °F) and 18 °C, effectively subdividing Group C into two nearly equal parts (his scheme assigns the letter B to the new zone, and identifies dry climates with an additional letter immediately following the temperature-based letter).

Another point of contention involves the dry B climates; the argument here is that their separation by Köppen into only two thermal subsets is inadequate. Those who hold this view (including Griffiths) have suggested that the dry climates be placed on the same temperature continuum as other climates, followed by the substitution of a capital S for steppe or D for desert for the summer-heat letter (or the precipitation letter in the case of tropical climates) as applicable.

A third idea is to create a maritime polar or EM zone within Group E to separate relatively mild marine locations (such as Ushuaia, Argentina and the outer Aleutian Islands) from the colder, continental tundra climates. Specific proposals vary; some advocate setting a coldest-month parameter, such as -7 °C (19.4 °F), while others support assigning the new designation to areas with an average annual temperature of above 0 °C.