Page 166 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

130 part I The energy–atmosphere System Earth’s Temperature
Patterns
Figures 5.12 through 5.16 are a series of maps to help us visualize Earth’s temperature patterns: global mean tem- peratures for January and July, polar region mean tempera- tures for January and July, and global annual temperature ranges (differences between averages of the coolest and warmest months). Maps are for January and July instead of the solstice months of December and June because of the lag that occurs between insolation received and maxi- mum or minimum temperatures experienced (explained in Chapter 4). Data used for these maps are representative of conditions since 1950, although some temperatures are from ship reports dating back to 1850 and land reports to 1890.
The lines on temperature maps are known as isotherms. An isotherm is an isoline—a line along which
there is a constant value—that connects points of equal temperature to portray the temperature pattern, just as a contour line on a topographic map illustrates points of equal elevation. Isotherms are useful for the spatial analysis of temperatures.
January and July Global
Temperature Maps
In January, high Sun altitudes and longer days in the Southern Hemisphere cause summer weather conditions; lower Sun altitudes and short days in the Northern Hemi- sphere are associated with winter. Isotherms on the January average-temperature map mark the general de- crease in insolation and net radiation with distance from the equator (Figure 5.12). Isotherms generally trend east– west, are parallel to the equator, and are interrupted by the presence of landmasses. This interruption is caused by the differential heating of land and water discussed earlier.
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▲Figure 5.12 Global average temperatures for January. Temperatures are in Celsius as taken from separate air-temperature databases for ocean and land. note the inset map of north america and the equatorward-trending isotherms in the interior. [adapted by Christopherson and redrawn from national Climatic Data Center, Monthly Climatic Data for the World, 47 (January 1994), and WMO and nOaa.]
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Satellite
Isotherms bend equatorward
Global Surface
Temperatures, Land and Ocean
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