1. Climate is more than “the average state of the atmosphere” because a complete climate description should also include variations and extremes to accurately portray the total character of an area. The most important elements in climate descriptions are temperature and precipitation inasmuch as they have the greatest influence on people and their activities and also have an important impact on the distribution of vegetation and the development of soils.

2. Perhaps the first attempt at climate classification was made by the ancient Greeks, who divided each hemisphere into three zones: torrid, temperate, and frigid. Many climate-classification schemes have been devised. The classification of climates is the product of human ingenuity and its value is determined largely by its intended use.

3. For decades, a climate classification devised by Wladimir Köppen (1846–1940) has been the best-known and most used tool for presenting the world pattern of climates. The Köppen classification uses easily obtained data: mean monthly and annual values of temperature and precipitation. Furthermore, the criteria are unambiguous, simple to apply, and divided the world into climate regions in a realistic way. Köppen believed that the distribution of natural vegetation was the best expression of an overall climate. Consequently, the boundaries he chose were largely based on the limits of certain plant associations. Köppen recognized five principal climate groups, each designated with a capital letter: A (humid tropical), B (dry), C (humid middle-latitude, mild winters), D (humid middle-latitude, severe winters), and E (polar). Four groups (A, C, D, E) are defined by temperature. The fifth, the B group, has precipitation as its primary criterion.

4. Order exists in the distribution of climate elements and the pattern of climates is not by chance. The world’s climate pattern reflects a regular and dependable operation of the major climate controls. The major controls of climate are (1) latitude (variations in the receipt of solar energy and temperature differences are largely a function of latitude), (2) land/water influence (marine climates are generally mild, whereas continental climates are typically more extreme), (3) geographic position and prevailing winds (the moderating effect of water is more pronounced along the windward side of a continent), (4) mountains and highlands (mountain barriers prevent maritime air masses from reaching far inland, trigger orographic rainfall, and where they are extensive, create their own climatic regions), (5) ocean currents (polewardmoving currents cause air temperatures to be warmer then otherwise would be expected), and (6) pressure and wind systems (the world distribution of precipitation is closely related to the distribution of Earth’s major pressure and wind systems). Situated astride the equator, the wet tropics (Af, Am) exhibit constant high temperatures and year-round rainfall that combine to produce the most luxuriant vegetation in any climatic realm—the tropical rain forest. Temperatures in these regions usually average 25ºC (77ºF) or more each month and the daily temperature variations characteristically exceed seasonal differences. Precipitation in Af and Am climates is normally from 175 to 250 centimeters (68 to 98 inches) per year and is more variable than temperature, both seasonally and from place to place. Thermally induced convection coupled with convergence along the intertropical convergence zone (ITCZ) leads to widespread ascent of the warm, humid, unstable air and ideal conditions for cloud formation and precipitation.

5. The tropical wet and dry (Aw) climate region is a transitional zone between the rainy tropics and the subtropical steppes. Here, the rain forest gives way to the savanna, a tropical grassland with scattered deciduous trees. Only modest temperature differences exist between the wet tropics and the tropical wet and dry regions. The primary factor that distinguishes the Aw climate from Af and Am is precipitation. In Aw regions, the precipitation is typically between 100 and 150 centimeters (40 to 60 inches) per year and exhibits some seasonal character—wet summers followed by dry winters. In much of India, southeast Asia, and portions of Australia, these alternating periods of rainfall and drought are associated with the monsoon, wind systems with a pronounced seasonal reversal of direction. The Cw climate, which is subtropical instead of tropical, is a variant of Aw.

6. Dry regions of the world cover about 30 percent of Earth’s land area. Other than their meager yearly rainfall, the most characteristic feature of dry climates is that precipitation is very unreliable. Climatologists define a dry climate as one in which the yearly precipitation is less than the potential water loss by evaporation. To define the boundary between dry and humid climates, the Köppen classification uses formulas that involve three variables: (1) average annual precipitation, (2) average annual temperature, and (3) seasonal distribution of precipitation. The two climates defined by a general water deficiency are (1) arid or desert (BW), and semiarid or steppe (BS). The differences between deserts and steppes are primarily a matter of degree. Semiarid climates are a marginal and more humid variant of arid climates that represent transitional zones that surround deserts and separate them from the bordering humid climates. Under the strong influence of the subtropical highs, the heart of the subtropical desert (BWh) and steppe (BSh) climates lies in the vicinity of the Tropic of Cancer and the Tropic of Capricorn. Within subtropical deserts, the scanty precipitation is both infrequent and erratic. In the semiarid transitional belts surrounding the desert, a seasonal rainfall pattern becomes better defined. Due to cloudless skies and low humidities, low-latitude deserts in the interiors of continents have the greatest daily temperature ranges on Earth. Where subtropical deserts are found along the west coasts of continents, cold ocean currents produce cool, humid conditions, often shrouded by low clouds or fog. Unlike their low-altitude counterparts, middle-latitude deserts (BWk) and steppes (BSk) are not controlled by the subsiding air masses of the subtropical highs. Instead, these lands exist principally because of their position in the deep interiors of large landmasses.

7. Humid middle-latitude climates with mild winters (C climates) occur where the average temperature of the coldest month is less than 18ºC (64ºF) but above (27ºF). Several C climate subgroups exist. Humid subtropical climates (Cfa) are on the eastern sides of the continents, in the 25- to 40-degree latitude range. Because of the dominating influence of maritime tropical air masses, summer weather within these regions is hot and sultry, and winters are mild. In North America, the marine west coast climate (Cfb) extends from near the United States–Canada border northward as a narrow belt into southern Alaska. The prevalence of maritime air masses means mild winters, cool summers, and ample rainfall throughout the year. Dry-summer subtropical (Mediterranean) climates (Csa, Csb) are typically found along the west sides of continents between latitudes 30º and 45º. In summer, the regions are dominated by the stable eastern sides of the oceanic subtropical highs. In winter, as the wind and pressure systems follow the Sun equatorward, they are within range of the cyclonic storms of the polar front.

8. Humid continental climates with severe winters (D climates) experience severe winters. The average temperature of the coldest month is (27ºF) or below, and the average temperature of the warmest month exceeds 10ºC (50ºF). Humid continental climates (Dfa) are landcontrolled and do not occur in the Southern Hemisphere. They are confined to central and eastern North America and Eurasia in the latitude range 40º to 50ºN. Both winter and summer temperatures in Dfa climates can be characterized as severe, and annual temperature ranges -3°C -3°C are large. Precipitation is generally greater in summer and generally decreases toward the continental interior and from south to north. Wintertime precipitation is chiefly associated with the passage of fronts connected with traveling middle-latitude cyclones. Subarctic climates (Dfc, Dfd), often called taiga climates because they correspond to the northern coniferous forests of the same name, are situated north of the humid continental climates and south of the polar tundras. The outstanding feature of subarctic climates is the dominance of winter. By contrast, summers in the subarctic are remarkably warm, despite their short duration. The greatest annual temperature ranges on Earth occur here.

9. Polar climates (ET, EF) are those in which the mean temperature of the warmest month is below 10ºC (50ºF). Annual temperature ranges are extreme, with the lowest annual means on the planet. Although polar climates are classified as humid, precipitation is generally meager, with many nonmarine stations receiving less than 25 centimeters (10 inches) annually. Two types of polar climates are recognized. Found almost exclusively in the Northern Hemisphere, the tundra climate (ET), marked by the 10ºC (50ºF) summer isotherm at its equatorward limit, is a treeless region of grasses, sedges, mosses, and lichens with permanently frozen subsoil, called permafrost. The ice-cap climate (EF) does not have a single monthly mean above 0ºC. Consequently, the growth of vegetation is prohibited, and the landscape is one of permanent ice and snow.

10. Highland climates are characterized by a great diversity of climatic conditions over a small area. In North America, highland climates characterize the Rockies, Sierra Nevada, Cascades, and the mountains and interior plateaus of Mexico. Although the best-known climatic effect of increased altitude is lower temperatures, greater precipitation due to orographic lifting is also common. Variety and changeability best describe highland climates. Because atmospheric conditions fluctuate with altitude and exposure to the Sun’s rays, a nearly limitless variety of local climates occur in mountainous regions.

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