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The Köppen climate classification scheme divides climates into five main groups (A, B, C, D, E), each having several types and subtypes. Each particular climate type is represented by a two- to four-letter symbol.

GROUP A: Tropical/megathermal climates

Tropical climates are characterized by constant high temperatures (at sea level and low elevations) — all 12 months of the year have average temperatures of 18 °C (64 °F) or higher. They are subdivided as follows:

  • Tropical rainforest climate (Af): All 12 months have average precipitation of at least 60 mm (2.4 in). These climates usually occur within 5–10° latitude of theequator. 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, so has no natural seasons.

Some of the places with 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 daylight hours and mean monthly (or daily) temperature throughout the year. Annual cyclic changes occur in the tropics, but 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, etc.), and human activities (plant sowing, harvesting, hunting, fishing, etc.) are tuned to this ‘seasonality’. Indeed, in tropical South America and Central America, the ‘rainy season’ (and the ‘high water season’) is called invierno orinverno, 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 South America, 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 1/25 the total annual precipitation.

Also, another scenario exists 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. Nassau, Bahamas, is included among these locations.

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 the Gulf of Urabá on the Colombia–Panamá border to the Orinoco River delta, on the Atlantic Ocean (about 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, 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 circum-Caribbean 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’s 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, northwestern Dominican Republic (Monte Cristi, Villa Vásquez, Luperón), 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 actual precipitation less than a threshold value set equal to the potential evapotranspiration. The threshold value (in millimeters) is determined as:

Multiply the average annual temperature in °C by 20, then add (a) 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 (b) 140 if 30%–70% of the total precipitation is received during the applicable period, or (c) 0 if less than 30% of the total precipitation is so received.

If the annual precipitation is less than 50% of this threshold, the classification is BW (desert climate); if it is in the range of 50%–100% of the threshold, the classification isBS (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 the coldest month has an average temperature above 0 °C (32 °F), with k denoting that at least one month averages below 0 °C.

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. The BSn category can be found in foggy coastal steppes.

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 (2.4 in) of average precipitation if all 12 months are above 18 °C (64 °F), or 30 mm (1.2 in) 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; Alexandria, Egypt, as BWhs; Asbi’ah, Libya, as BShs; Ömnögovi Province, Mongolia, as BWkw; and Xining, Qinghai, 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 monthly temperature above 10 °C (50 °F) in their warmest months (April to September in northern hemisphere), and an average monthly temperature above −3 °C (27 °F) in their coldest months. (Some climatologists prefer to observe 0 °C rather than −3 °C) in the coldest month as the boundary between this group and the colder group D (humid continental).

In Asia, this includes areas from South Korea, to east- China from Beijing southward, to northern Japan . In Europe this includes areas from coastal Norway south to southern France, In the USA, areas from near 40 latitude in the central and eastern states (the NYC/NJ/CT area southward, the lower Ohio Valley, lower Midwest and southern Plains, are located in the Köppen C group.

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 40 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 (72 °F) with at least four months averaging above 10 °C, b indicates warmest month averaging below 22 °C, but with at least four months averaging above 10 °C, while c means three 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.

C climates (or mesothermal) are broken down into two types: 1) A dry-summer or Mediterranean climate and a temperate climate:

Under the Köppen–Geiger classification, dry-summer (Csb) extends to additional areas not typically associated with a typical Mediterranean climate, such as much of thePacific Northwest, much of southern Chile, parts of west-central Argentina, parts of the Atlantic coastline of Morocco, and areas of northern Spain and Portugal. Many of these areas would be oceanic (Cfb), except dry-summer patterns meet Köppen’s Cs minimum thresholds. Additional highland areas in the subtropics also meet Csrequirements, although they, too, are not normally associated with Mediterranean climates.

  • Maritime temperate climates or Oceanic climates (Cfb, Cfc, Cwb, Cwc): Cfb climates usually occur on the western sides of continents between the latitudes of 45° and 60°; they are typically situated immediately poleward of the Mediterranean climates, although in Australia and extreme southern Africa this climate is found immediately poleward of temperate climates, and at a somewhat lower latitude. In western Europe, this climate occurs in coastal areas up to 63°N in Norway. These climates are dominated all year round by the polar front, leading to changeable, often overcast weather. Summers are cool due to cool ocean currents, but winters are milder than other climates in similar latitudes, but usually very cloudy. 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”.
  • Temperate highland tropical climate with dry winters (Cwb, Cwc) is a type of climate characteristic of the highlands inside the tropics of Mexico, Peru, Bolivia,Madagascar, Zambia, Zimbabwe and South Africa, but it is also found in central Argentina, outside the tropics. Winters are noticeable and dry, and summers can be very rainy. In the tropics, the rainy season is provoked by the tropical air masses and the dry winters by subtropical high pressure.
  • 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.
  • Dry-summer maritime subalpine climates (Csc): This climate exists in high-elevation areas adjacent to coastal Csb climate areas, where the strong maritime influence prevent the average winter monthly temperature from dropping below −3 °C. This climate is extremely rare and is only found in isolated areas of the Cascades and Andes Mountains, as the dry-summer climate extends further poleward in the Americas than elsewhere.

GROUP D: Continental/microthermal climates

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 and on their upper east coasts, normally north of 40°N. 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 at 40–60°S, existing only in some highland locations.

Group D climates are subdivided as:

Lettering

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 and (for c and d) winter cold — a indicates warmest month average temperature above 22 °C (72 °F) with at least four months averaging above 10 °C, b indicates warmest month averaging below 22 °C, but with at least four months averaging above 10 °C, c indicates warmest month averaging below 22 °C and with three or fewer months with mean temperatures above 10 °C, but coldest month averaging above –38 °C, and d indicates warmest month averaging below 22 °C, three or fewer months averaging above 10 °C, and coldest month averaging below –38 °C.

Scheme

  • Hot summer continental climates (Dfa, Dwa, Dsa): Dfa climates usually occur in the high 40 and low 50 latitudes, with a qualifying average temperature in the warmest month of >22 °C/72 °F. In Europe, these climates tend to be much drier than in North America. In eastern Asia, Dwa climates extend further south due to the influence of the Siberian high pressure system, which also causes winters there to be dry, and summers can be very wet because of monsoon circulation. Dsaexists at higher elevations adjacent to areas with hot summer Mediterranean (Csa) climates.
  • Warm summer continental or hemiboreal climates (Dfb, Dwb, Dsb): Dfb and Dwb climates are immediately north of hot summer continental climates, generally between 45° and 58° 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 65 degrees latitude in places.
  • Dsb arises from the same scenario as Dsa, but at even higher altitudes or latitudes, and chiefly in North America, since the Mediterranean climates extend further poleward than in Eurasia.
  • Continental subarctic or boreal (taiga) climates (Dfc, Dwc, Dsc): Dfc and Dwc climates occur poleward of the other group D climates, mostly in the 50° and low 60° north latitude, although it might occur as far north as 70°N.
  • Continental subarctic climates with extremely severe winters (Dfd, Dwd, Dsd): Places with this climate have the temperature in their coldest month lower than −38 °C. These climates occur only in eastern Siberia. The names of some of the places with this climate have become veritable synonyms for extreme, severe winter cold.

GROUP E: Polar and alpine climates

Main articles: Polar climate and Alpine climate

These climates are characterized by average temperatures below 10 °C in all 12 months of the year:

  • Tundra climate (ET): Warmest month has an average temperature between 0 and 10 °C. These climates occur on the northern edges of the North American and Eurasian land masses, and on nearby islands. ET climates are also found on some islands near the Antarctic Convergence, and at high elevations outside the polar regions, above the tree line.
  • Ice cap climate (EF): All twelve months have average temperatures below 0 °C (32 °F). This climate is dominant in Antarctica and inner Greenland, but also occurs at extremely high altitudes on mountains, above even tundra.
  • Occasionally, a third, lower-case letter is added to ET climates (distinguishing between ETf, ETs, and ETw), if either the summer or winter is clearly drier than the other half of the year. 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.
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