Interpret the patterns depicted on world temperature maps.

Take a minute to study the two world maps in Figures 3.18 and 3.19. From warm colors near the equator to cool colors toward the poles, these maps portray sea-level temperatures in the seasonally extreme months of January and July. On these maps, you can study global temperature patterns and the effects of the controls of temperature, especially latitude, the distribution of land and water, and ocean currents.

On both maps, the isotherms generally trend east and west and show a decrease in temperatures poleward from the tropics. This illustrates one of the most fundamental aspects of world temperature distribution: The amount of incoming solar radiation available to heat Earth’s surface and the atmosphere above it is largely a function of latitude. Moreover, there is a latitudinal shifting of temperatures caused by the seasonal migration of the Sun’s vertical rays. To see this, compare the color bands by latitude on the two maps.

The effect of the differential heating of land and water is clearly reflected on the January and July temperature maps—the coldest and warmest temperatures are found over land. Consequently, because temperatures do not fluctuate as much over water as over land, the north–south migration of isotherms from January to July is greater over the continents than over the oceans. In addition, the isotherms in the Southern Hemisphere, where there is less land compared to water, are much straighter than in the Northern Hemisphere, where they bend sharply northward in July and southward in January over the continents.

Isotherms also reveal the presence of ocean currents. The poleward transport of water warms the overlying air and results in air temperatures that are higher than would otherwise be expected for the latitude. Conversely, currents moving toward the equator produce cooler-than-expected air temperatures. Thus, warm currents cause isotherms to be deflected toward the poles, whereas cold currents cause an equatorward bending.

Figures 3.18 and 3.19 also show the seasonal extremes of temperature, and identifying the extremes enables us to see the annual range of temperature from place to place. Comparing the two maps shows that a station near the equator has a small annual range because it experiences little variation in the length of daylight and always has a relatively high Sun angle. A station in the middle latitudes, however, experiences wide variations in Sun angle and length of daylight and, hence, large variations in temperature. Therefore, we can state that the annual temperature range increases as we move from the equator toward the poles.

Moreover, land and water also affect seasonal temperature variations, especially outside the tropics. Continental locations must endure hotter summers and colder winters than coastal locations, where seasonal temperatures are not as extreme.

Figure 3.20, which shows the global distribution of annual temperature ranges, serves to illustrate the preceding two paragraphs. The tropics clearly experience small annual temperature variations. As expected, the highest ranges occur in the middle of large landmasses in the subpolar latitudes. It is also worth noting that annual temperature ranges in the ocean-dominated Southern Hemisphere are much smaller than in the Northern Hemisphere, with its abundance of large continents.

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• On world maps showing January and July mean temperatures, isotherms generally trend east–west and show a decrease in temperature moving from the tropics toward the poles. When the two maps are compared, a seasonal latitudinal shift of temperatures is easily seen.

• Bending isotherms often reveal the locations of ocean currents.

• Annual temperature range is small near the equator and increases with an increase in latitude. Outside the tropics, annual temperature range also increases as marine influence diminishes.