Definitions

temperate climate

Köppen climate classification

The Köppen climate classification is one of the most widely used climate classification systems. It was developed by Wladimir Köppen, a German climatologist, around 1900 (with several further modifications by Köppen himself, notably in 1918 and 1936). It is based on the concept that native vegetation is the best expression of climate; thus, climate zone boundaries have been selected with vegetation distribution in mind. It combines average annual and monthly temperatures and precipitation, and the seasonality of precipitation.

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: Tropical/megathermal climates

Tropical climates are characterized by constant high temperature (at sea level and low elevations) — all twelve months of the year have average temperatures of 18 °C (64.4 °F) or higher. They are subdivided as follows:

  • Tropical rain forest climate (Af): All twelve months have average precipitation of at least 60 mm (2.36 inches). These climates usually occur within 5-10° latitude of the equator. In some eastern-coast areas, they may extend to as much as 25° away from the equator. This climate is dominated by the Doldrums Low Pressure System all year round, and therefore has no natural seasons.
    Examples:
    Singapore
    Belém, Brazil.
    • Some of the places that have this climate are indeed uniformly and monotonously wet throughout the year (e.g., the northwest Pacific coast of South and Central America, from Ecuador to Costa Rica, see for instance, 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.
      Note. The term aseasonal refers to the lack in the tropical zone of large differences in day light hours and mean monthly (or daily) temperature throughout the year. There are annual cyclic changes in the tropics, not as predictable as those in the temperate zone, albeit unrelated to temperature but to water availability whether as rain, mist, soil, or ground water. Plant response (e. g., phenology), animal (feeding, migration, reproduction, et cetera), and human activities (plant sowing, harvesting, hunting, fishing, et cetera) are tuned to this seasonality. Indeed, in tropical South America and Central America, the rainy season (and the high water season) is called Invierno or Inverno, even though it could occur in the northern hemisphere summer; likewise, the dry season (and low water season) is called Verano or Verão and can occur in the northern hemisphere winter.
  • Tropical monsoon climate (Am): 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 Nassau, Bahamas are included among these locations.
  • Tropical wet and dry or savanna climate (Aw): 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
    Darwin, 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, Antioquia, Colombia) also qualifies. Actually, the Caribbean coast, eastward from Urabá gulf on the ColombiaPanamá border to the Orinoco river delta, on the Atlantic ocean (ca. 4,000 km), have long dry periods (the extreme is the BSh climate (see below), characterised by very low, unreliable precipitation, present, for instance, in extensive areas in the Guajira Península, and Coro, western Venezuela, the northernmost peninsulas in South America, which receive <300 mm total annual precipitation, practically all in two or three months). This condition extends to the Lesser Antilles and Greater Antilles forming the Circumcaribbean dry belt. The length and severity of the dry season diminishes inland (southward); at the latitude of the Amazon river — which flows eastward, just south of the equatorial line — the climate is Af. East from the Andes, between the dry, arid Caribbean and the ever-wet Amazon are the Orinoco river' Llanos or Savannas, from where this climate takes its name.
    • Sometimes As is used in place 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 because of rainshadow effects during the 'high-sun' part of the year.

GROUP B: Dry (arid and semiarid) climates

These climates are characterized by the fact that precipitation is less than potential evapotranspiration. The threshold is determined as follows:

  • To find the precipitation threshold (in millimeters), multiply the average annual temperature in °C by 20, then add 280 if 70% or more of the total precipitation is in the high-sun half of the year (April through September in the Northern Hemisphere, or October through March in the Southern), or 140 if 30%-70% of the total precipitation is received during the applicable period, or 0 if less than 30% of the total precipitation is so received.
  • 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 [64.4 °F]), 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:
    Yuma, Arizona (BWh)
    Almería, Spain (BWh)
    Cobar, New South Wales, Australia (BSh)
    Murcia, Spain (BSh)
    Medicine Hat, Alberta, Canada (BSk),
    Enna, Italy (BSh)
    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, Asbi'ah, Libya as BShs, Ömnögovi Province, Mongolia as BWkw, and Xining, China as BSkw (BWks and BSks do not exist if 0 °C in the coldest month is recognized as the h/k boundary). If the standards for neither w nor s are met, no fourth letter is added.

GROUP C: Temperate/mesothermal climates

These climates have an average temperature above 10 °C (50 °F) in their warmest months, and a coldest month average between −3 °C (27°F) and 18 °C (64 °F).

Some climatologists, particularly in the United States, however, prefer to observe 0 °C (32 °F) rather than −3 °C (27 °F) in the coldest month as the boundary between this group and Group D; this is done to prevent certain headland locations in New England — principally Cape Cod — and such nearby islands as Nantucket and Martha's Vineyard, from fitting into the Maritime Temperate category noted below; this category is alternately known as the Marine West Coast climate, and eliminating the aforementioned locations indeed confines it exclusively to places found along the western margins of the continents, at least in the Northern Hemisphere. This also moves some mid-latitude areas - such as parts of the Ohio Valley and some areas in the Mid-Atlantic States, plus parts of east-central Asia - from humid subtropical to humid continental.

  • 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 indicates dry summers (driest summer month less than 30 mm average precipitation and less than one-third wettest winter month precipitation) and f means significant precipitation 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) with at least 4 months averaging above 10 °C (50 °F), b indicates warmest month averaging below 22 °C, but 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, rainy 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 milder due to the nearby presence of cold ocean currents that may bring fog but prevent rain.
Examples:
Split, Croatia (Csa)
Gaziantep, Turkey (Csa)
Madrid, Spain (Csa)
Marseille, France (Csa)
Los Angeles, California, USA (Csa)
Barcelona, Spain (Csa)
Santiago, Chile (Csb)
Perth, Australia (Csa)
Risan, Montenegro (Csb)
Porto, Portugal (Csb)
San Francisco, California, USA (Csb)
Sanremo, Italy (Csa)

* 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° (46°N in Europe). 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:
Dallas, Texas, USA (Cfa — uniform precipitation distribution)
Milan, Italy (Cfa — uniform precipitation distribution)
Buenos Aires, Argentina (Cfa — uniform precipitation distribution)
Brisbane, Queensland, Australia (Cfa — uniform precipitation distribution)
Atlanta, Georgia, USA (Cfa — uniform precipitation distribution)
Porto Alegre, Brazil (Cfa — uniform precipitation distribution)
Luodian, Guizhou, China (Cwa — summer wetter than winter).
Guadalajara, Mexico (Cwa — summer wetter than winter)

* Maritime Temperate climates or Oceanic 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. In western Europe, this climate occur in coastal areas up to 63° 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 (Cfbuniform precipitation distribution)
Langebaanweg, South Africa (Cfbuniform precipitation distribution)
Curitiba, Brazil (Cfbuniform precipitation distribution)
Prince Rupert, British Columbia, Canada (Cfbuniform precipitation distribution)
Bergen, Norway (Cfbuniform precipitation distribution)
Cfb climates are also encountered at high elevations in certain subtropical and tropical areas, where the climate would be that of a subtropical/tropical rain forest if not for the altitude. These climates are called "Highlands" Bogotá, Colombia and Crkvice, Montenegro (Cfsb —perhumid Mediterranean mountain climate without summer dryness, Crkvice on Orjen holds also Europe's precipitation record - averaging 4927 mm/m² 1931-1960) are perhaps the best examples.
Cwb is found only at higher altitudes, without which the climate would be tropical wet and dry.
Examples:
Addis Ababa, Ethiopia
Mexico City, Mexico
Campos do Jordão, Brazil
In parts of the Pacific Northwest of North America and parts of south-western South America, Cfb climates are also somewhat similar to Mediterranean climates in that their summers are relatively dry. Examples include:
Seattle, Washington (Cfb, sometimes Csb)
Victoria, British Columbia, Canada (Cfb, sometimes Csb)
Puerto Montt, Chile (Cfb, sometimes Csb).

* The temperate climate with dry winters (Cwb) is a type of climate charasteristic of the highlands inside the tropics of Mexico, Peru, Bolivia and South Africa but it is also found in the Argentine province of Córdoba Province (Argentina), outside the tropics. Winters are noticeable and dry and summers very rainy. In the tropics the rainy season is provoked by the tropical airmasses and the dry winters by subtropical high pressure. Temperate temperatures are the consequence of altitude which become cool in winter.
Examples:
Cuzco, Peru
La Paz, Bolivia
Mexico City, Mexico
Johannesburg, South Africa

* Maritime Subarctic climates or Subpolar Oceanic 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 (Cfcuniform precipitation distribution)
Monte Dinero, Argentina (Cfcuniform precipitation distribution)
Reykjavík, Iceland (Cfcuniform precipitation distribution)
Tórshavn, Faroe Islands (Cfcuniform precipitation distribution)
Harstad, Norway (Cfcuniform precipitation distribution).

GROUP D: Continental/microthermal climate

These climates have an average temperature above 10 °C (50 °F) in their warmest months, and a coldest month average below −3 °C (or 0 °C in some versions, as noted previously). These usually occur in the interiors of continents, or on their east coasts, north of 40° North latitude. In the Southern Hemisphere, Group D climates are extremely rare due to the smaller land masses in the middle latitudes and the almost complete absence of land south of 40° South latitude, existing only in some highland locations in New Zealand that have heavy winter snows.

  • 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 high 30s and low 40s in latitude, 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:
Chicago, Illinois (Dfa — summer wetter than winter)
Santaquin, Utah (Dfa — summer drier than winter)
Seoul, South Korea (Dwa).
Harbin, China (Dwa).
Dsa exists only at higher elevations adjacent to areas with hot summer Mediterranean (Csa) climates, such as Cambridge, Idaho and Saqqez in Iranian Kurdistan.

* Warm Summer Continental or Hemiboreal climates (Dfb, Dwb, Dsb): Dfb and Dwb climates are immediately north of Hot Summer Continental climates, generally in the high 40s and low 50s in latitude in North America and Asia, and also in central and eastern Europe and Russia, between the Maritime Temperate and Continental Subarctic climates, where it extends up to high 50s and even lowest 60 degrees latitude.
Examples:
Moncton, New Brunswick, Canada (Dfb — uniform precipitation distribution)
Minsk, Belarus (Dfb — summer wetter than winter)
Revelstoke, British Columbia, Canada (Dfb — summer drier than winter)
Fargo, North Dakota, (Dfb — winter drier than summer)
Vladivostok, Russia (Dwb).
Stockholm, Sweden (Dfb — winter somewhat drier than summer)
Dsb arises from the same scenario as Dsa, but at even higher altitudes or higher latitudes, 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 Boreal (taiga) climates (Dfc, Dwc, Dsc): Dfc and Dwc climates occur poleward of the other Group D climates, mostly in the 50s and low 60s North latitude, although it might occur as far north as 70° latitude.
Examples:
Sept-Îles, Quebec, Canada (Dfc — uniform precipitation distribution)
Anchorage, Alaska (Dfc — summer wetter than winter)
Mount Robson, British Columbia, Canada (Dfc — summer drier than winter)
Irkutsk, Russia (Dwc).
Kirkenes, Finnmark, Norway (Dfc - summer wetter than winter)
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. Two examples are Zubački kabao, Montenegro (Dfsc perhumid Mediterranean snow climate) and Galena Summit, Idaho.

* Continental Subarctic climates with extremely 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 Oymyakon — have become veritable synonyms for extreme, severe winter cold.

GROUP E: Polar climates

These climates are characterized by average temperatures below 10 °C (50 °F) in all twelve months of the year:

  • Tundra climate (ET): Warmest month has an average temperature between 0 °C (32 °F) and 10 °C (50 °F). 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, Canada
    Provideniya, Russia
    Deception Island, Antarctica.
    Longyearbyen, Svalbard
    ET is also found at high elevations outside the polar regions, above the tree line — as at Mount Washington, New Hampshire.
  • Ice Cap climate (EF): All twelve months have average temperatures below 0 °C (32°F). This climate is dominant in Antarctica (e.g., Scott Base) and in inner Greenland (e.g., Eismitte or North Ice).
  • 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 Herschel Island ('Qikiqtaruk', in Inuvialuit) 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.

Trewartha climate classification scheme

The Trewartha climate classification scheme is a modified version of the Köppen system. It attempts to redefine the broad climatic groups in such a way as to be closer to vegetational zoning, especially in the United States. Under the standard Köppen system western Washington and Oregon are classed into the same climate as southern California, even though the two regions have strikingly different vegetation. It also classes southern New England into the same climate as the Gulf Coast. Trewartha's modifications sought to reclass the Pacific Northwest seaboard as a different climate from California, and New England from the Gulf Coast.

  • Group A: This the tropical climate group, defined the same as in Köppen's scheme (i.e., all 12 months average 18 °C or above). Climates with no more than two dry months (defined as having less than 60 mm average precipitation, same as per Köppen) are classified Ar (instead of Köppen's Af), while others are classified Aw if the dry season is at the time of low sun/short days or As if the dry season is at the time of high sun/long days. There was no specific monsoon climate identifier in the original scheme, but Am was added later, with the same parameters as Köppen's (except that at least three months, rather than one, must have less than 60 mm average precipitation).
  • Group B: BW and BS mean the same as in the Köppen scheme, with the Köppen BWn climate sometimes being designated BM (the M standing for "marine"). However, a different formula is used to quantify the aridity threshold: 10 X (T − 10) + 3P, with T equalling the mean annual temperature in degrees Celsius and P denoting the percentage of total precipitation received in the six high-sun months (April through September in the Northern Hemisphere and October through March in the Southern). If the precipition for a given location is less than the above formula, its climate is said to be that of a desert (BW); if it is equal to or greater than the above formula but less than twice that amount, the climate is classified as steppe (BS); and if the precipitation is more than double the value of the formula the climate is not in Group B. Unlike in Köppen's scheme, no thermal subsets exist within this group in Trewartha's, unless the Universal Thermal Scale (see below) is used.
  • Group C: In the Trewartha scheme this category encompasses subtropical climates only (8 or more months above 10 °C). Cs and Cw have the same meanings as they do in Köppen's scheme, but the subtropical climate with no distinct dry season is designated Cr instead of Köppen's Cf (and for Cs the average annual precipitation must be less than 890 mm [35 inches] in addition to the driest summer month having less than 30 mm precipitation and being less than one-third as wet as the wettest winter month).
  • Group D: This group represents temperate climates (4 to 7 months above 10 °C). Maritime temperate climates (most of Köppen's Cfb and Cwb climates, though some of these would fit into Trewartha's Cr and Cw respectively) are denoted DO in the Trewartha classification (although some places near the east coasts of both North America and Asia actually qualify as DO climates in Trewartha's scheme when they fit into Cfa/Cwa rather than Cfb/Cwb in Köppen's), while continental climates are represented as DCa (Köppen Dfa, Dwa, Dsa) and DCb (Köppen Dfb, Dwb, Dsb). For the continental climates, sometimes the third letter (a or b) is omitted and DC is simply used instead, and occasionally a precipitational seasonality letter is added to both the maritime and continental climates (r, w, or s, as applicable). The dividing point between the maritime and continental climates is 0 °C in the coldest month, rather than the Köppen value of −3 °C (as noted in the section on the Köppen scheme, however, some climatologists — particularly in the United States — now observe 0 °C in the coldest month as the equatorward limit of the continental climates in that scheme as well).
  • Group E: This represents subarctic climates, defined the same as in Köppen's scheme (1 to 3 months with average temperatures of 10 °C or above; Köppen Cfc, Dfc, Dwc, Dsc, Dfd, Dwd). In the original scheme, this group was not further divided; later, the designations EO and EC were created, with EO (maritime subarctic) signifying that the coldest month averages above −10 °C, while EC (continental subarctic or "boreal") means that at least one month has an average temperature of −10 °C or below. As in Group D, a third letter can be added to indicate seasonality of precipitation. There is no separate counterpart to the Köppen Dfd/Dwd climate in Trewartha's scheme.
  • Group F: This is the polar climate group, split into FT (Köppen ET) and FI (Köppen EF).
  • Group H: Highland climates, in which altitude plays a role in determining climate classification. Specifically, this would apply if correcting the average temperature of each month to a sea-level value using the formula of adding 5.6 °C for each 1,000 meters of elevation would result in the climate fitting into a different thermal group than that into which the actual monthly temperatures place it. Sometimes G is used instead of H if the above is true and the altitude is 500 meters or higher but lower than 2,500 meters; but the G or H is placed in front of the applicable thermal letter rather than replacing it — and the second letter used reflects the corrected monthly temperatures, not the actual monthly temperatures.
  • Universal Thermal Scale: An option exists to include information on both the warmest and coldest months for every climate by adding a third and fourth letter, respectively. The letters used conform to the following scale:
    i — severely hot: Mean monthly temperature 35 °C (95 °F) or higher
    h — very hot: 28 to 34.9 °C (82 to 95 °F)
    a — hot: 23 to 27.9 °C (73 to 82 °F)
    b — warm: 18 to 22.9 °C (64 to 73 °F)
    l — mild: 10 to 17.9 °C (50 to 64 °F)
    k — cool: 0.1 to 9.9 °C (32 to 50 °F)
    o — cold: −9.9 to 0 °C (14 to 32 °F)
    c — very cold: −24.9 to −10 °C (−13 to 14 °F)
    d — severely cold: −39.9 to −25 °C (−39.9 to −13 °F)
    e — excessively cold: −40 °C (−40 °F) or below.
    Examples of the resulting designations include Afaa for Kuala Lumpur, Malaysia, BWhl for Aswan, Egypt, Crhk for Dallas, Texas, DOlk for London, EClc for Arkhangelsk, Russia, and FTkd for Barrow, Alaska.

Criticisms of the Köppen scheme

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 Philadelphia, for example). In Applied Climatology (first edition published in 1966), John F. Griffiths proposed a new subtropical zone, encompassing those areas with a coldest month of between 6 °C (42.8 °F) and 18 °C (64.4 °F), 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, with the thermal letter being followed by an additional capital letter — S for steppe or W (or D) for desert — as applicable (Griffiths also advances an alternate formula for use as an aridity threshold: R = 160 + 9T, with R equalling the threshold, in millimeters of mean annual precipitation, and T denoting the mean annual temperature in degrees Celsius).

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.

The accuracy of the 10 °C warmest-month line as the start of the polar climates has also been questioned; Otto Nordenskiöld, for example, devised an alternate formula: W = 9 − 0.1 C, with W representing the average temperature of the warmest month and C that of the coldest month, both in degrees Celsius (for instance, if the coldest month averaged −20 °C, a warmest-month average of 11 °C or higher would be necessary to prevent the climate from being polar). This boundary does appear to more closely follow the tree line, or the latitude poleward of which trees cannot grow, than the 10 °C warmest-month isotherm; the former tends to run poleward of the latter near the western margins of the continents, but at a lower latitide in the landmass interiors, the two lines crossing at or near the east coasts of both Asia and North America.

World Map of the Köppen-Geiger climate classification for the period 1951-2000

Based on recent data sets from the Climatic Research Unit (CRU) of the University of East Anglia and the Global Precipitation Climatology Centre (GPCC) at the German Weather Service, a new digital Köppen-Geiger world map on climate classification for the second half of the 20th century has been compiled.

Other maps

References

External links

Climate records

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