The most commonly used system of classifying global climatic regions, invented in the early 20th century by Waldimar Köppen and since revised by others.

The Köppen system classifies climates by rather general considerations of temperature, precipitation, and seasonal variation. Climates are denoted by a string of letters, the first (always capitalized) specifying the most general features, and the second and third giving more specific criteria.

Below is an overview of all the commonly used climate types. Other letter combinations usually don't correspond to any climate pattern seen on Earth.

A: moist tropical climates
Climates of type A are characterized by average temperatures greater than 18 degrees Celsius throughout the year, and annual rainfall exceeding 150 centimeters. As the name implies, they're located near the globe's equator.

Af "tropical wet" - this is the climate of the tropical rainforest. In addition to the aforementioned type "A" requirements, Af climates may not have any month averaging less than 6 cm of precipitation. The lack of a dry season makes the rainforest's lush vegetation possible.

Aw "tropical wet-dry" - the climate of the savanna familiar to African tribesman and big game hunters. Although annual precipitation exceeds 100 cm, there is a pronounced dry season of more than 2 months during the winter, and the dominant vegetation consists of tall grass and hardier trees.

As is identical to Aw except that the dry season occurs in the summer. This climate is only seen in the extreme east of South America.

Am "tropical monsoon" - total precipitation is as as high as that of the Af climate, but there is a very distinct rainy season, balanced out by a relatively dry season. Think of the monsoons of south Asia, for example.

B: dry climates
There is no specific criterion for the quantity of a B climate's precipitation, but it must be low enough that the evaporation (or potential evaporation) in the area exceeds the precipitation itself. In other words, water doesn't accumulate on the surface. Temperature varies greatly over the course of a single day.

BWh "low-latitude desert" - defined as a desert due to average precipitation being less than half of potential evaporation. Annual mean temperature exceeds 18 degrees C, and there is usually no pronounced winter. Found in the Sahara, among other places.

BWk "mid-latitude desert" - increased distance from the equator makes these deserts colder, though no less dry than BWh. The yearly average temperature is less than 18 degrees, and temperatures commonly drop below freezing during the day in winter. The Gobi and Mojave deserts fall in this category.

BSh "low-latitude steppe" - precipitation is less than, but more than half, of potential evaporation. Average temperature is greater than 18 degrees. Steppes aren't as bleak as deserts, having considerable amounts of low-growing vegetation. These regions are typically found adjacent to low-latitude deserts.

BSk "mid-latitude steppe" - these steppes have average temperatures less than 18 degrees, and can get quite cold in winter. They occur in such locales as the Great Plains of North America and the Caspian Sea region in Asia.

C: moist mid-latitude "subtropical" climates
Type C climates are more temperate than type A, with enough precipitation to avoid classification as type B and milder than the later types. The general requisite for this classification is the average temperature of the coolest month: below 18 and above -3 degrees Celsius. Winters are clearly defined, but not severe.

Cfa "moist subtropical" - these climates average over 22 degrees Celsius in the hottest month. Precipitation is evenly distributed year-round. Southeastern regions of North America and Asia fit these criteria.

Cwa moist subtropical like Cfa, but there is a severe winter drought, with less than one-tenth the precipitation of the wettest summer month falling in the driest winter month.

Csa "mediterranean" - a warm climate with a dry summer. The warmest month averages over 22 degrees, with at least 4 months greater than 10 degrees. The driest summer month has less than 4 cm precipitation, and the wettest month has at least three times that amount. As its name would suggest, this climate is prevalent on the shores of the Mediterranean Sea, as well as the coast of California and, as StrawberryFrog mentions, the southern tip of Africa.

Csb - a cooler mediterranean climate, with the hottest month not exceeding 22 degree Celsius average temperature.

Cfb - "marine" - a mild climate found on the west coasts of continents, farther from the equator than mediterranean regions. All months average below 22 degrees Celsius, and rainfall is evenly distributed throughout the year. This is the drizzly climate of the United Kingdom and North America's Pacific Northwest coast.

Cfc - a cooler marine climate, in which no month reaches an average temperature of 22 degrees; characteristic of the inhabited portion of Iceland.

D: moist mid-latitude "continental" climates
Type D climates occur at similar latitudes to type C, but have more severe winters, usually as a result of being located in the continental interior rather than near the coast. The coldest month in a type D region averages below -3 degrees C, while the warmest month averages above 50 degrees.

all "humid continental" types:

Dfa - humid continental with hot summers. The warmest month averages over 22 degrees C, and precipitation occurs year-round. This climate is seen in much of the American Midwest.

Dwa - hot summers and dry winters. The driest month seeing less than one-tenth the precipitation of the wettest. It's found in much of Korea and the surrounding area.

Dfb - mild summmers. No month averages warmer than 22 degrees C, and precipitation is distributed throughout the year. It occurs at higher latitudes than Dfa, covering wide swaths of Russia and Canada.

Dwb mild summers and dry winters. Identical temperature criteria to Dfb, with minimum monthly precipitation less than one-tenth maximum. This is seen on the Japanese island of Hokkaido and the Russian territory bordering the Sea of Japan.

all "subarctic" types:

Dfc - short, cool summers. Only one to three months have average temperatures higher than 10 degrees C. Precipitation is year-round.

Dwc - same as Dfc, but with dry winters, the familiar "w" criterion of less than one-tenth the precipitation of the rainiest summer month in the driest winter month.

Dfd - short, cool summers with extreme winters. Similar to Dfc, but the coldest month averages below -38 degrees Celsius.

Dwd - winter conditions like those of Dfd, with the winter drought of the "w" type. (how many times can I say the same thing over again?)

These subarctic climates collectively cover most of northern North America and Eurasia, before merging into tundra at approximately the latitude of the Arctic Circle. Think Siberia and the Yukon. They're warm enough in the short summer for trees to grow, but cold enough in the long winter to kill mercilessly. If you're living in one of these places and haven't been banished to a penal colony or something similarly cruel, I might suggest moving.

E: polar climates
Brr! Type E climates are the chilliest places on Earth, with the warmest month averaging below 10 degrees Celsius.

ET "tundra" - at least it thaws, eventually. The warmest month, and possibly another month or two, averages above 0 degrees Celsius. The upper meter or so of tundra soil is all that thaws, though, with permafrost remaining below. Precipitation is low, usually less than 20 cm annually, but very little of it evaporates due to the temperature being so low. Tundra conditions are predominant along the shores of the Arctic Ocean, and on the Antarctic Peninsula.

EF "ice cap" - no month attains an average temperature above freezing. The ground never thaws. Precipitation (in the form of snow, of course) is very meager, but it builds up gradually, and the ground is covered with a thick layer of ice. Ice caps cover the mainland of Antarctica and the interior of Greenland.

H: highland climates
Classification of type H is basically a cop-out by climatologists who can't figure out what to do with mountainous regions, where climate and vegetation change rapidly with elevation. It wasn't included in Köppen's original work, but became a necessity, as steep slopes change climate so rapidly that mountains can't be resolved in detail on global climatic maps.

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