CHAPTER 18 (Moran and Morgan, 1997) The climate of a locale represents the collection of all statistical weather information that aids in the description of the weather at that place for a given time interval. Typically, the climate of a region is often defined in terms of long-term arithmetic averages, but should also include such descriptors as extremes and other measures of variability. Certain physical principles govern the climate of essentially any place on earth, to include particular climate controls. Various schemes have been employed to describe global climatic patterns, including the classification of climate. STUDY NOTES CHAPTER 18 Figure 18.4 -- Compare the match between the seasonal positions of the arctic front (appearing as a blue band) with the northern and southern borders of the boreal forest. Figures 18.5, 18.6 and 18.7 -- Take several minutes to compare and contrast the isotherm patterns in the January and July maps of world mean monthly sea-level temperature. In these two figures, the isotherms are primarily zonal, or running primarily in an east-west direction. The summer hemisphere (in Figure 18.5 the Southern and in Figure 18.6, the Northern Hemisphere) has a weaker north-south gradient than in the corresponding winter hemisphere, as evident by the tighter packing of isotherms (more isotherms per given distance) in the winter hemisphere. The highest summer temperatures as well as the lowest winter temperatures are found in the interior of the continents. The map of the mean annual range in temperature appearing in Figure 18.7, produced from the arithmetic difference between the summer and winter temperatures on the previous two figures, shows that the largest temperature range appears in the interior of subpolar continents, such as Siberia or the Yukon Territories. Across the tropics the annual temperature range is small. Figure 18.6 -- Inspect the world map of mean annual precipitation. Notice that the tropics near the equator have the most precipitation, reflecting the position of the ITCZ. On the other hand, the subtropics, including the Saharan desert and desert in Australia are dry, since the region is under the influence of the subtropical high pressure belt. The polar regions of Greenland, northern Siberia and Canada are also dry. Note - The Climate Classification Scheme presented in the textbook, with accompanying Table 18.1 and Figures 18.9 through 18.16 is just one scheme used by geographers specializing in climatology to classify the planet's climate using a scheme predicated upon combinations of temperature and precipitation. Memorization of this classification scheme is not required in this course. However, you should realize that the large scale atmospheric and oceanic circulation patterns together with geographic considerations such as altitude, proximity to oceans and mountain barriers will combine to produce a climate regime for a particular region. Such a climate may have a profound influence upon the region's vegetation. CHAPTER 18 (Moran and Morgan, 1997) WORLD CLIMATES In this first of three chapters on climatology, we cover some basic properties of climate, climate controls, some principles governing climatic behavior, and a general description of the climates of the continents - Climate is defined as average weather plus extremes in weather, and climate is viewed as specific to a particular location over some time interval. We focus on the meaning of the climatic normal and climatic anomalies plus some statistical descriptions of climate. Finally, we examine the basis for climate classification schemes and describe in some detail global patterns of climate. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: explain why climate encompasses both average and extreme weather conditions. discuss the meaning of the climatic normal. explain why climatic anomalies are geographically nonuniform in both sign and magnitude. discuss the significance of agroclimatic compensation. explain how climate is described in terms of air mass frequency. list the many interacting factors that shape global climates. describe the general global patterns of temperature and precipitation. explain how various climate types are classified. identify the causes of dry climates. explain why precipitation in some regions of the world is distinctly seasonal. 18 World Climates 424 Describing Climate 425 Climate Controls 431 Global Patterns of Climate 432 Climate Classification 438 Conclusions 447 Special Topic: 1816, The Year Without a Summer? 429 Special Topic: Agroclimatic Compensation: The Benefits and Limitations 437 Key Terms 448 Summary Statements 448 Review Questions 448 Questions for Critical Thinking 448 Selected Readings 449 2