THE NATURAL ENVIRONMENT
Geography 101 Online
Air temperature is one of the most basic climate elements. It is reported in just about every corner of the world and seems fundamental to human comfort (or discomfort). For standard air temperature measurements, instruments are housed in an enclosure, such as the ubiquitous Stevenson screen seen at the right, to shield the sensor from sunlight. Note the thermometer is placed at about head height. If it were located elsewhere, such as at ground level, maximum and minimum temperatures would be different, although the overall average would be about the same.
Temperature readings from different places are compiled into continuous 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 energy balance (especially how sunlight, the energy source, varies across Earth's surface) and proximity to large bodies of water.
Energy Balance Variation
On a global scale, sunlight energy causes most differences in temperature from place to place. Annual differences in sunlight due to Earth's orbit around the Sun cause 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, they are much more vertical in the Southern Hemisphere. Sunlight energy is more concentrated with greater angles between beam and surface causing more surface heating. The pattern flip-flops at June solstice, when the Northern Hemisphere experiences its highest temperatures.
The angle of sunlight to the surface also depends on latitude. In general, at higher latitudes the sun is lower on the horizon weakening sunlight intensity and producing cooler temperatures.
Global Temperature Map
The 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 Bodies
Large 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. This stabilizing affect can be see at all time scales, from annual temperature ranges to the daily temperature cycle.
The average annual temperature range of the ocean surface near Hawai'i, for example is about 3° C (5° F). In contrast, the middle of the Sahara Desert, at the same latitude as the Hawaiian Islands, has an average annual temperature range of over 12° C (20° F) with scorching hot days and cool nights. Places where temperature is greatly moderated (meaning that extreme temperatures are uncommon) by nearby water are said to have a marine climate, while areas far from water influence (where temperature extremes are more likely) are said to have a continental climate. In addition to the effect on temperature, marine climates are typically humid because evaporation from the ocean surface adds water to the air and continental climates are typically dry because they lack substantial evaporation sources.
The difference between marine and continental climates is most obvious in a world map showing the average annual temperature range. Range refers to the warmest month minus the coldest month temperatures. Notice how quickly the marine climate influence weakens as you move inland over the major continental areas outside of the tropics.
Also note that the annual temperature varies little in the tropics (always warm) and the polar areas (always cold) but a great deal in the midlatitudes, which is the region of heat transfer from the tropics to polar areas. In the midlatitude zone, temperatures can vary widely and quickly producing Earth's most chaotic weather.
Average Annual Temperature Range (°C)
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