Climate

Climate begins with a brief description of Earth's climate system, followed by an overview of world climates. The Köppen system of climate classification, used throughout the chapter, and its five principal groups are presented and discussed. Beginning with the humid tropical (A) climates, the location, characteristics, and subtypes of each of the climate groups is examined in detail.

Learning Objectives

After reading, studying, and discussing this chapter you should be able to:

•Explain what is meant by Earth's climate system.

•Discuss the factors that give every location a distinctive climate.

•Describe the Köppen system of climate classification.

•List the five principal climate groups of the Köppen system and describe the criteria used to define each group.

•Describe the location and general characteristics of the principal climate groups.

Chapter Summary

•Climate is an aggregate of the weather conditions of 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). The varied nature of Earth's surface and the interactions that occur between atmospheric processes give every location a distinct climate.

•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-classification schemes 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, has been the best-known and most used system for more than70 years. 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, year-round rainfall, and 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 tropical 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 not as great as the potential loss of water by evaporation, are subdivided into two climatic 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 oceans. Because many middle-latitude deserts occupy sites on the leeward sides of mountains, they can also be classified as rainshadow 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° 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 United States-Canadian 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°. 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. D climates have severe winters. The average temperature of the coldest month is -3°C 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°N latitude. Both winter and summer temperatures may 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 lifeless 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 had an impact on the global climate by their use of fire as well as the overgrazing of marginal lands by domesticated animals. The consumption of fossil fuels (coal, natural gas, and petroleum) has added great quantities of the gas carbon dioxide, an important heat absorber, to the atmosphere. If the use of fossil fuels continues to increase at projected rates, models predict an increase in the mean global surface temperature of between 1.5° and 4.5°C. The atmospheric buildup of trace gases such as methane, nitrous oxide, and certain chlorofluorocarbons will also contribute to a future global increase in temperature. Some possible consequences of a greenhouse warming of the atmosphere include 1) global mean surface warming (very probable), 2) global mean precipitation increase (very probable), 3) reduction of sea ice (very probable), 4) polar winter surface warming (very probable), 5) summer continental dryness/warming (likely in the long term), 6) high latitude precipitation increase (probable), and 7) a rise in global mean sea level (probable).