The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- Climate is the aggregate of weather conditions for a place or region over a long period of time. Earth's climate system involves the exchanges of energy and moisture that occur among the atmosphere, hydrosphere, solid Earth, biosphere, and cryosphere (the ice and snow that exist at Earth's surface).
- Climate classification brings order to large quantities of information, which aids comprehension and understanding, and facilitates analysis and explanation. Temperature and precipitation are the most important elements in a climatic description. Many climate classifications have been devised, with the value of each determined by its intended use. The Köppen classification, which uses mean monthly and annual values of temperature and precipitation, is a widely used system. The boundaries Köppen chose were largely based on the limits of certain plant associations. Five principal climate groups, each with subdivisions, were recognized. Each group is designated by a capital letter. Four of the climate groups (A, C, D, and E) are defined on the basis of temperature characteristics, and the fifth, the B group, has precipitation as its primary criterion.
- Humid tropical (A) climates are winterless, with all months having a mean temperature above 18°C. Wet tropical climates (Af and Am), which lie near the equator, have constantly high temperatures and enough rainfall to support the most luxuriant vegetation (tropical rain forest) found in any climatic realm. Tropical wet and dry climates (Aw) are found poleward of the wet tropics and equatorward of the subtropical deserts, where the rain forest gives way to the tropical grasslands and scattered drought-tolerant trees of the savanna. The most distinctive feature of this climate is the seasonal character of the rainfall.
- Dry (B) climates, in which the yearly precipitation is less than the potential loss of water by evaporation, are subdivided into two types: arid or desert (BW) and semiarid or steppe (BS). Their differences are primarily a matter of degree, with semiarid being a marginal and more humid variant of arid. Low-latitude deserts and steppes coincide with the clear skies caused by subsiding air beneath the subtropical high-pressure belts. Middle-latitude deserts and steppes exist principally because of their position in the deep interiors of large landmasses far removed from the ocean. Because many middle-latitude deserts occupy sites on the leeward sides of mountains, they can also be classified as rain shadow deserts.
- Middle-latitude climates with mild winters (C climates) occur where the average temperature of the coldest month is below 18°C but above -3°C. Several C climate subgroups exist. Humid subtropical climates (Cfa) are located on the eastern sides of the continents, in the 25- to 40-degree latitude range. Summer weather is hot and sultry, and winters are mild. In North America, the marine west coast climate (Cfb, Cfc) extends from near the U.S.–Canada border northward as a narrow belt into southern Alaska. The prevalence of maritime air masses means that mild winters and cool summers are the rule. Dry-summer subtropical climates (Csa, Csb) are typically located along the west sides of continents between latitudes 30 and 45 degrees. In summer, the regions are dominated by stable, dry conditions associated with the oceanic subtropical highs. In winter they are within range of the cyclonic storms of the polar front.
- Humid middle-latitude climates with severe winters (D climates) are land-controlled climates that are absent in the Southern Hemisphere. The D climates have severe winters. The average temperature of the coldest month is -3°C or below, and the warmest monthly mean exceeds 10°C. Humid continental climates (Dfa, Dfb, Dwa, Dwb) are confined to the eastern portions of North America and Eurasia in the latitude range between approximately 40 and 50 degrees north latitude. Both winter and summer temperatures can be characterized as relatively severe. Precipitation is generally greater in summer than in winter. Subarctic climates (Dfc, Dfd, Dwc, Dwd) 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 highest annual temperature ranges on Earth occur here.
- Polar (E) climates are summerless, with the average temperature of the warmest month below 10°C. Two types of polar climates are recognized. The tundra climate (ET) is a treeless climate found almost exclusively in the Northern Hemisphere. The ice cap climate (EF) does not have a single monthly mean above 0°C. As a consequence, the growth of vegetation is prohibited, and the landscape is one of permanent ice and snow.
- Compared to nearby places of lower elevation, highland climates are cooler and usually wetter. Because atmospheric conditions fluctuate rapidly with changes in altitude and exposure, these climates are best described by their variety and changeability.
- Humans have been modifying the environment for thousands of years. By altering ground cover with the use of fire and the overgrazing of land, people have modified such important climatological factors as surface albedo, evaporation rates, and surface winds.
- By adding carbon dioxide and other trace gases (methane, nitrous oxide, and chlorofluorocarbons) to the atmosphere, humans may be contributing significantly to global warming.
- When any component of the climate system is altered, scientists must consider the many possible outcomes, called climate-feedback mechanisms. Changes that reinforce the initial change are called positive-feedback mechanisms. On the other hand, negative-feedback mechanisms produce results that are the opposite of the initial change and tend to offset it.
- Because the climate system is very complex, predicting specific regional changes that may occur as the result of increased levels of carbon dioxide in the atmosphere is highly speculative. However, some potential consequences of global warming include: (1) altering the distribution of the world's water resources and therefore the productivity of agricultural regions that depend on rivers for irrigation, (2) a probable rise in sea level, and (3) a change in weather patterns, such as a higher frequency and greater intensity of hurricanes and shifts in the paths of large-scale cyclonic storms.
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