Climate is the characteristic state of the atmosphere at a location or in a region. It is the entirety of weather conditions over the long term. It is composed of averages, extremes, and variability. The World Meteorological Organization specifies climate-length weather records to be 30 years and more. Climate is derived from the Greek word klima, signifying the angle of the sun above the horizon. The input of solar energy is the prime driver of climate. Solar angles vary by latitude with lower latitudes having the highest solar angles and the most solar radiation at the top of the atmosphere.
From the times of the classical Greeks it was known that Earth was spherical and that varying solar angles caused varying climates. So, two-and-a-half millennia ago climate was classified into torrid, temperate, and frigid zones. Although the Greek classification was largely correct, they had never traveled to the Equator to discover temperatures are not as hot as in other parts of the tropics where the sun is not as high in the sky. The erroneous Greek conclusion points to the existence of non-solar influences on climate. In this case, the presence of the clouds in the Intertropical Convergence Zone (see winds and pressure systems) moderates equatorial temperatures.
Latitude is clearly a key factor, but there are a number of factors that, when integrated together, determine the climatic nature of Earth’s regions. Six other factors are generally given as climatic controls. (1) The distribution of land and water is quite uneven over the planet and land heats and cools so much more rapidly than oceans that annual temperature ranges are significantly impacted. (2) The circulation of the atmosphere circulates large amounts of air. For example, in the middle latitudes, the surface and upper westerlies bring oceanic air to the western parts of continents thus moderating their temperatures. (3) There are pronounced ocean currents, and these are capable of modifying climate. For instance, the cold Benguela Current off of southwestern Africa stabilizes air passing over it to help create the exceptional dryness of the Namib Desert. (4) Storminess and lack of storminess play a considerable climatic role. Places prone to storminess are subject to much more cloudiness and precipitation than places lacking storminess. In that the various types of storms are mechanisms by which the atmosphere’s energy budget is maintained, storminess helps to moderate extreme surface temperatures by forcing horizontal and vertical mixing. (5) Elevation is well correlated with various weather elements like pressure, moisture and temperature which all decrease with altitude. Significant altitudinal differences make for different climates in adjacent, whether or not there are mountains present. (6) Topographic blockage is the effect of surface weather unable to bypass ranges of mountains and large hills. The climates can be quite different on the different sides of the topographic impediments. For instance, the frigid wintertime Siberian air is unable to pass over the Tibetan Plateau with an average elevation of 4,500 m (14,000 ft) and the Indian Subcontinent to its south enjoys a mild winter.
Vladimir Köppen (1846–1940) was first to numerically regionalize climate thus revolutionizing climate science. His work was inspired by the world vegetation maps then available; Köppen attempted to fit numerical boundaries. More subtly, his classification scheme of the early 20th century was made possible by the increasing availability of climate-length weather records. After the middle of the 19th century, weather instruments had become standardized. Numerous world cities boasted published climatological summaries. The summaries were mainly temperature and precipitation averages. Köppen used these two dimensions in constructing his classification system. In modified form, this classification has been well used by geographers and other scientists for the last century. The Köppen system is a relatively simple system with which to understand the large regionalities of Earth’s climate. It has continued appeal because of its correspondence with biomes and soil regions.
Below is a listing of modified Köppen climate zones that are also displayed on the accompanying Map. Köppen’s original work was modified several times as the boundaries of the world’s major vegetation regions became better known and small weaknesses in the original scheme became evident. Nonetheless, the popularity of Köppen’s classification has never been supplanted. Several other classifications exist for specialized uses.
Climate maps are intellectually appealing to geographers because they invite place-to-place comparisons. This is vital in the geographer’s understanding of similarities and differences on the physical Earth. In the broadest sense, climate is an explainer of other world patterns. Earth’s climate is the delivery system for energy and water. Thus, climatic knowledge is a potent tool in understanding the world distribution of biomes and soils. One can surmise that the soils are similar enough that a crop adapted to one of these climates would do well in broad zones of the analogous climate of another climate. However, the map user must exercise caution in using these maps in settings beyond the classroom. World climate regions are broad zones with climatic nuances that might be quite important for crops. For instance, the humid subtropical climate (Cfa) occurs from western Oklahoma to northern Florida. Oklahoma has significantly hotter, drier summers than Florida so that it does not follow that a successful crop in Oklahoma must also be so in Florida. We must remember that the attempts to classify the invisible entity of “climate” are based on similarities so that differences are downplayed.
The Köppen classification is based on letter codes (Table 1). The code is composed of six major climate types: A (tropical), B (dry), C (mild middle latitudes), D (severe middle latitudes), E (polar), and H (highland). Individual climate types are designated with two and three letters. The secondary and tertiary letters are w (winter drought), s (summer drought), f (year-round precipitation), W (desert), S (steppe); a through d (increasingly cool summer temperatures), h (subtropical variety of desert), k (middle latitude variety of desert), T (tundra), and F (icecap).
|Köppen code||Climate name||Climate characteristics||Largest extent|
|Af||Tropical rainforest||Always warm; copious rainfall throughout the year||Brazil|
|Am||Tropical monsoon||Always warm; short dry season during low sun; very wet high-sun period||Southeastern Asia|
|Aw||Tropical wet-and-dry||Always warm to hot; low-sun season dry; overall drier than Am climate||Africa south and east of the Af climate|
|BWh||Subtropical desert||Hottest temperatures on Earth; precipitation scarce and irregular||Sahara of Africa|
|BWk||Middle latitude desert||Very hot in summer with cool winters; precipitation somewhat more than in BWh and includes winter snow||Central Asia|
|BSh||Subtropical steppe||Marginal to BWh climates with more moderate summer temperatures and greater precipitation||African Sahel|
|BSk||Middle latitude steppe||Marginal to BWk climates with more moderate summer temperatures and greater precipitation||Steppes of southern Russia|
|Csa||Mediterranean||Modest winter precipitation and dry summers; milder summer temperatures than inland climates with winters above freezing||Coastal lands fringing the Mediterranean Sea|
|Csb||Mediterranean||Summers not quite as hot as Csa||Coastal lands fringing the Mediterranean Sea|
|Cfa||Humid subtropical||Summers warm to hot with precipitation all year; winter temperatures sometimes below freezing||Southeastern United States|
|Cwa||Humid subtropical||Summers warm to hot with plentiful precipitation; winters dry with temperatures sometimes below freezing||Southeastern China-Northeastern India|
|Cwb||Humid subtropical||Same as Cwa except for a cooler summer||Southeastern China-Northeastern India|
|Cfb||Marine west coast||Many cloudy days with light precipitation year round; moderate summer and winter temperatures||Western Europe|
|Cfc||Marine west coast||Same as Cfb except for cooler summers||Northern Europe|
|Dfa||Humid Continental||Hot summer, cold winter, year-round precipitation||U.S. Midwest|
|Dfb||Humid continental||Summer briefer and winter colder than Dfa; significant length of snow cover||Western Russia|
|Dwa||Humid continental||Hot summer, cold winter, small amount of winter precipitation||Southwestern Russia|
|Dwb||Humid Continental||Summer not as hot as Dwa, small amount of winter precipitation; significant length of snow cover||Southwestern Russia|
|Dfc||Subarctic||Moderate precipitation; summer brief and cool||Northern Russia|
|Dfd||Subarctic||Moderate precipitation; summer brief and cold||Northern Russia|
|Dwc||Subarctic||Winter very dry; summer brief and cool||Eastern Siberia|
|Dwd||Subarctic||Winter very dry; summer brief and cold||Eastern Siberia|
|ET||Tundra||Up to four months average above freezing; modest precipitation||Northernmost Eurasia|
|EF||Polar ice cap||No month averages above freezing; very little precipitation||Antarctica|
|H||Highland||An accumulation of heterogeneous mountain climates present with all the other climate types; H climates are found at all latitudes and are cooler and moisture than their lowland surroundings||Tibetan Plateau|
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