Page 167 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

The thermal equator is an isotherm connecting all points of highest mean temperature, roughly 27°C; it trends southward into the interior of South America and Africa, indicating higher temperatures over the interiors of landmasses. In the Northern Hemisphere, isotherms shift toward the equator as cold air chills the continen- tal interiors. More moderate temperatures occur over oceans, with warmer conditions extending farther north than over land at comparable latitudes.
To see the temperature differences over land and water, follow along 50° N latitude (the 50th parallel) in Figure 5.12 and compare isotherms: 3°C to 6°C in the North Pacific and 3°C to 9°C in the North Atlantic, as contrasted with –18°C in the interior of North America and –24°C to –30°C in central Asia. Also, note the orien- tation of isotherms over areas with mountain ranges and how they illustrate the cooling effects of elevation— check the South American Andes as an example.
For a continental region other than Antarctica, Russia—and specifically, northeastern Siberia—is the coldest area (Figure 5.12). The intense cold results from winter conditions of consistently clear, dry, calm air; small insolation input; and an inland loca- tion far from moderating maritime effects. Prevailing global winds prevent moderating effects from the Pacific Ocean to the east.
Figure 5.13 maps average July global temperatures for comparison. The longer days of summer and higher Sun altitude are in the Northern Hemisphere. Winter domi- nates the Southern Hemisphere, although it is milder than winters north of the equator because continental land- masses, with their greater temperature ranges are smaller. The thermal equator shifts northward with the high summer Sun and reaches the Persian Gulf–Pakistan–Iran area. The Persian Gulf is the site of the highest recorded sea-surface temperature—an astounding 36°C.
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▲Figure 5.13 Global average temperatures
for July. Temperatures are in Celsius as taken
from separate air-temperature databases for ocean and land. note the inset map of north america and the poleward-trending isotherms in the interior. [adapted by Christopherson and redrawn from national Climatic Data Center, Monthly Climatic Data for the World, 47 (July 1994), and WMO and nOaa.]
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Chapter 5 global Temperatures 131
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