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THE NATURAL ENVIRONMENTGeography 101 |
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ToCHEATAlbedoGreenhouseBalanceGlobalLocal |
Global Temperature
Temperature readings from different places are compiled into full-coverge thematic maps, as shown in the global temperature map below. On the map, lines of equal temperature are called isotherms; analogous to lines of equal atmospheric pressure being called isobars, or lines of equal rainfall called isohyets.
Major differences in air temperature from place to place depend on the local energy balance and proximity to large bodies of water. Energy Balance VariationOn a global scale, sunlight energy causes most variation in temperature between places. Annual differences in sunlight due to Earth's orbit around the sun produce seasonal changes and latitude differences explain the north-south pattern obvious in global temperature maps. Seasonally, sunlight intensity changes in a predictable way. Simply put, a place receives maximum sunlight energy at its summer solstice when the daylight hours are longest and the sun's rays most perpendicular to the ground. Compare the solar beam angles between the Northern and Southern Hemisphere in the December Solstice diagram for example; on average, angles are much more vertical in the Southern Hemisphere. Sunlight energy is more concentrated when the sun is higher in the sky producing more surface heating. The pattern flip-flops at June solstice, when the Northern Hemisphere experiences its highest sun angles and surface temperatures.
Global Temperature MapThe change of sunlight intensity with latitude determines the most obvious pattern in annual temperature maps: tropics are warm and high latitudes cool. This should seem familiar as we discussed the same topic in the previous section under Planetary Energy Balance. Compare the diagram from that section with the map below. They are different ways of looking at the same phenomenon, with the Planetary Energy Balance showing why polar areas are cold and tropics warm, and the map below showing the actual temperature distribution. The air temperature map gives much more detail, however, as it shows the entire Earth surface. Remember the solar hot water heater analogy used to explain heat transfer in the Balance section? Look at the South Pacific Ocean between Australia and South America. On the Australia side warmer water flows poleward, while on the South America side, cooler water moves north toward the equator. The South Pacific has a huge circular ocean current that sends warm water to higher latitudes on the Australia side and returns cold water to be re-heated in the tropics on the South America side. The net result is a transfer of heat from tropical areas to polar areas through ocean currents. Also notice that the temperature change between equator and Poles is fairly uniform over the oceans, but varies widely over the continents. The reason for this is discussed below. Effect of Water BodiesLarge bodies of water hold enormous amounts of heat, much more than land surfaces. With a very high heat capacity, ocean surfaces heat and cool slowly, while land surfaces heat and cool much more quickly. As a result, air temperatures over or near oceans or large lakes remains fairly stable, while inland areas tend to have large temperature ranges. (Range refers to the temperature difference between the warmest and coldest months.) This stabilizing affect can be see at all time scales, from annual temperature ranges to the daily temperature cycle.
Also note that the annual temperature varies little in the tropics (always warm) and polar areas (always cold) but a great deal in the midlatitudes, which is the region of heat transfer from the tropics to polar latitudes. In the midlatitude zone, temperatures can vary quickly and widely to produce Earth's most chaotic weather. Average Annual Temperature Range (°C) |
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ToC | HEAT | Albedo | Greenhouse | Balance | Global | Local |