Global Patterns of Air TemperatureGlobal patterns of air temperature are a reflection of the relative importance of the factors discussed in the preceding sections. Isotherms, or lines connecting points of equal air temperature are used to map the spatial pattern of temperature. Latitudinal Patterns (north to south patterns)The first thing to note on January and July temperature maps is the spacing and linearity (or lack thereof) of the isotherms. Overall, isotherms in the southern hemisphere tend to be more linear or run straight across the map. In the Northern Hemisphere the isotherms tend to be more wavy, dipping far to the south or north depending on season and location. This pattern reflects the impact of land-sea contrasts on the heating of the air. The southern hemisphere is a more uniform surface, mostly water. Where there is land, isotherms take sudden swings over the surface. The northern hemisphere has much more land surface and the configuration of the isotherms reflect it. Isotherms dip far to the south in January as cold temperatures occupy the continental interiors and coastal locations stay more mild being influenced by the ocean. The values of the isotherms validate our general conception of the distribution of world temperatures. Low latitudes that experience the highest sun angles throughout the year have the highest air temperatures. The highest temperatures occur in the tropical and subtropical deserts of North Africa, Australia, and the southwestern portion of the United States. Because the angle of the sun doesn't vary much during the year and day length is about the same, annual temperature ranges tend to be small, on the order of only a few degrees. However, daily temperature ranges can be quite large as pointed out earlier in the chapter. High latitude locations have much lower annual temperatures as the sun never gets directly overhead and sun angles are quite a bit lower than those in low latitudes. However, the seasonal temperature range is large. Cold air masses penetrate south dropping air temperatures during the winter months. During the summer, warm tropical air masses stream toward higher latitudes raising temperatures. The movement of air masses and more varied sun angles results in larger temperature ranges than one experiences in the low latitudes. Longitudinal Patterns (west to east patterns)Longitudinal patterns of temperature reflect the influence of continentality and ocean circulation. Let's examine the longitudinal temperature patterns one sees in the midlatitudes of North America. In the mid-latitudes the prevailing wind direction is from west to east. Places located along the coast receive a constant influx of oceanic air throughout the year. Because oceans don't change their temperature much during the year, the air above them doesn't change much either. When the oceanic air streams on to land, temperatures tend to be rather mild. The changes in air temperature one experiences as you travel from west to east across a midlatitude continent largely reflect the influence of continentality. The temperature range along the west coast of North America tends to be small due to the constant influx on oceanic air. Temperature extremes increase as distance from the coast increases. In the interior of the North American continent, warm air masses from the Gulf of Mexico work their way northward, especially during the summer. During the winter, cold continental polar air masses dominate. The great difference in the temperature of these two air masses results in a large temperature range. Air masses moving to the east from the interior tend to be warm, but the proximity to water keeps air temperatures mild giving east coast locations moderate ranges of temperatures. Table AT.1 West - East comparison of Annual Temperature Range
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