Encyclopedia > Albedo

  Article Content

Albedo

Albedo is a measure of the tendency of a surface to absorb and re-emit incoming radiation, and thus lower its own temperature. Its value ranges from 0% (for an object which absorbs all radiation) to 100% (for an object which reflects all radiation). Powdered charcoal has a low albedo; whilst, a mirror has a relatively high albedo. The albedo of an object can be used to help determine it's chemical composition. Every molecule of every substance has its own albedo, but albedo can also refer to a collection of very many objects, and can even be applied to objects of a very large scale, such as the planet Earth, which has an albedo of 37-39%.

Since albedo can greatly affect the temperature of an area, it is advantageous for people, especially in cold climates, to settle in areas where the albedo is very low, so that they can get as much warmth as possible from the sun. The reverse is true in desert and tropical[?] climates; people want to live in areas where as much solar radiation as possible will be emitted back into space. According to the National Climatic Data Center[?]'s GHCN 2 data, which is composed of 30-year smoothed climatic means for thousands of weather stations across the world, the college weather station at Fairbanks[?], Alaska, is about 3°C (5°F) warmer than the airport at Fairbanks, partly because of drainage patterns but also largely because of the lower albedo at the college resulting from a higher concentration of pine trees and therefore less open snowy ground to reflect the heat back into space. Neunke and Kukla have shown that this difference is especially marked during the late winter months, when solar radiation is greater. To give another example, the entire nation of Belgium, which is among the most urbanized in the world, is considerably warmer than the mostly open, unforested French countryside that lies to its immediate southwest, perhaps also because of albedo effects.

Although the albedo-temperature effect is most famous in colder regions of Earth, because more snow falls there, it is acually much stronger in tropical regions because in the tropics there is consistently more sunlight. When Brazilian ranchers cut down dark, tropical rainforest trees to replace them with even darker soil in order to grow crops, the average temperature of the area allegedly increases by an average of about 3°C (5°F) year-round. The difference would most likely be more than 17°C (30°F) if the landscape of the rainforest was white instead of dark green. As evidence for this theory, scientists have climbed to the top of Mount Kilimanjaro in Tanzania, which is located in an area well described as "steaming jungle". But Kilimanjaro's temperatures remain well below freezing all year round, despite the fact that its summit is only about 20,000 feet above sea level. Assuming that the average temperature at sea level around Kilimanjaro is about 27°C (81°F) (this is the GHCN's record of the average temperature at Dar Es Salaam, a nearby sea-level weather station), and that the temperature up the mountain slope decreases only 1°C (2°F) per 1,000 feet because of the semi-moist environment, it seems that the temperature at the summit should be about 5°C (41°F), which is much warmer than the approximately -7°C (20°F) that it has been estimated that Kilimanjaro actually receives. By comparison, the Hawaiian volcano Mauna Loa, which peaks at about 14,000 feet, has an annual average temperature in the GHCN of approximately 4°C (40°F), despite the fact that Hawaii is a cooler and drier environment than northern Tanzania and thus the lapse rate there would be expected to be higher, thus resulting in a colder temperature at the summit of Mauna Loa.

But there is no snow at the top of Mauna Loa to reflect back incoming solar radiation, and although the climate at the summit is very rainy, snow is relatively rare and the coldest recorded temperature is -10°C (14°F), which reduced to sea level at a rate of 1°C (2°F) gives 6°C (42°F), which is not much higher than the lowest temperatures recorded at other sea-level stations in Hawaii. And the average annual temperature at sea level in Hawaii is only about 23°C (74°F), and it is during the dry season a dry 25°C (77°F), so it can be seen that the lapse rate along the slope of Mauna Loa is not significantly higher than 1°C (2°F) per 1000 feet, whereas on Kilimanjaro the lapse rate is almost doubled, for no logical reason.

The only possible explanation is that on Kilimanjaro, a glacier built up when the climate was colder, and that this glacier never completely melted because its extremely high albedo reflected away enough heat to keep itself cold enough not to melt, and to keep the air temperature at the summit much lower than that atop Mauna Loa. This very wide gap in temperatures, of nearly 11°C (20°F), occurs despite the fact that the skies above both mountains are very cloudy and the climate of both is very wet, so not much solar radiation reaches either summit. If Mauna Loa were completely dry, it could be expected that its temperature would be even warmer, perhaps as warm as that of Tibet at 14,000 feet in summer.

Another good example of albedo in action is Greenland. Greenland is an island covered almost completely with ice. In the winter, very little sunlight reaches Greenland, so albedo is not as important. But in the summertime, it makes a very big difference, making summertime temperatures even colder than they would normally be for a given latitude and elevation. For example, Eismitte is a weather station situated a bit south of the geographical center of the island (near 70°N) at an altitude of 9843 feet. The average temperature inland at 0 feet at 70°N in Eurasia and North America in July is about 13°C (55°F). Given a lapse rate of 1°C (2°F) per 1000 feet, we could expect the July average at Eismitte to be 2°C (35°F). In fact, 1°C (2°F) per 1000 feet is often an overestimate near the poles because of the unusual convective activity that causes inversions to arise there. But the actual average July temperature at Eismitte is only -12°C (10°F), and furthermore, the next warmest month is June, at only -17°C (2°F). June being warmer than August is a clear sign that there are some outside influences on the climate, in particular that sunlight plays an unusually large role in regulating temperature, because June is the sunniest month of the year in most of the northern part of the Northern Hemisphere. But we cannot ignore the fact that the temperatures in all summer months are much colder than the predicted values using the 1°C (2°F) lapse rate. This is obviously due to the very high albedo that Greenland's ice cap provides. Further evidence for this theory can be seen in the fact that other stations in Greenland have very low summer temperatures as well, and in fact are relatively consistent all year round with a 1°C (2°F) per 1000 feet lapse rate.

Svalbard is a group of islands extending from about 75°N to 80°N in the very warmest part of the Arctic Ocean, the part that is bathed by the long stream of warm water coming from the Caribbean. In the wintertime, the coastline of most of the island remains free from ice, which gives towns on the most prominent headlands incredibly warm winters, with temperatures averaging as high as -7°C (19°F) at one weather station. But just a very short distance away, the northeastern part of the island often does freeze over. And here, the winter climate is a beautifully unstable oscillation between the very furthest reaches of the Gulf Stream penetration and the featureless cold Arctic winter. Temperatures in the northeast average about -29°C (-20°F) but have fallen as low as -57°C (-70°F). This would be unheard of in the ice-free areas of the islands.

Some parts of the Arctic Ocean are frozen over with ice all year round. These areas have the unusual distinction of experiencing heat convection coming from the ocean below as well as the air above. The temperature of the purest Arctic Ocean water is roughly -2°C (28°F) at all depths, in all seasons. The temperature of the ice above, however, is much colder because it reflects away some of the heat of the water below it. During the summertime, the upward face of the ice reflects away solar heat and prevents itself from melting completely. The ice that does melt helps keep the average July temperatures at the North Pole at about -1°C (31°F).

Albedo works on a smaller scale, too. People who wear dark clothes in the summertime put themselves at a greater risk of heatstroke than if they wear white clothes. Ironically, dark skin also serves to aggravate heat loss during the hot summer months, although the human body's blood circulation is much more powerful than the albedo effect of its skin. Thus children, much skinnier than adults, need more clothes in the wintertime to keep them warm, even if they are dark-skinned.

There is a simple relationship between the albedo of an area of the earth's surface and its color. The lighter the color, the higher the albedo. A white substance would have an albedo of 1.00, or 100%, while a black substance would have an albedo of 0.00 or 0%. The human eye perceives green light especially well, so green substances have albedos somewhat lower than one might think. For example, the albedo of a pine forest at 45°N in the winter in which the trees cover the land surface completely is only about 9%, among the lowest of any naturally occurring land environment. The ocean, because it is so deep, is even lower, at about 3.5% (the surface temperature of the ocean at the equator is allegedly over 38°C (100°F)). Dense swampland averages between 9% and 14%. Deciduous trees average about 13%. A grassy field usually comes in at about 20%. A barren field will depend on the color of the soil, and can be as low as 5% or as high as 40%, with 15% being about the average for farmland. A desert or large beach usually averages around 25% but varies depending on the color of the sand. All of these measurements were taken from Edward Walker's study in the Great Plains in the winter around 45°N, but with different values for latitude and day, they would be different. The reason for this is not because of the amount of sunlight coming in, which does not change a substance's albedo, but because the actual environments would be different in summer. For example, a cultivated field may come in as high as the low 20s, depending on the crop being cultivated. Because of increased photosynthesis, all of the values for the forests and swamp lands are nearly doubled in the summer. The grassy field, however, remains at 20% regardless of season or latitude. Near the equator, particularly in rainforest areas, the soil is very black, and its albedo is very close to 0. Some shallow lakes may have albedos as high as 45% during times when the sun is striking the lake from a very acute angle, and more of its light is reflected. Alpine areas often have strange albedo values because of the unusual land formations.

Urban areas in particular have very unnatural values for albedo because of the many human-built structures which absorb light before the light can reach the surface. In the northern part of the world, cities are relatively dark, and Walker has shown that their average albedo is about 7%, with only a slight increase during the summer. In most tropical countries, cities average around 12%. This is similar to the values found in northern suburban transitional zones. Part of the reason for this is the different natural environment of cities in tropical regions, e.g., there are more very dark trees around; another reason is that the tropics are very poor, and city buildings must be built with different materials.

Snow is the greatest source of albedo in the world, with an albedo that can be as high as 98%. This is for the ideal example, however: deep snow over a treeless landscape with no obstacles around. In the real world, the maximum albedo depends on the environment. Thus, even in the middle of winter with a heavy snow cover, the albedo of weather stations in the pine-forested areas from Europe to Siberia and in the Americas rarely goes above 40%, and many stations in this area have averages around 25%. Deciduous trees lose their leaves, and their albedos may rise to the mid-50's in winter. Open prairie climates will average around 73%, and barren tundra averages about 80%. The icecap of Antarctica will average in the high 80s; this is essentially the maximum possible albedo in the world.

Because trees are such strong absorbers of heat, it seems logical that removing forests would tend to cool the earth as a whole. This is an extremely complex subject, and there are large amounts of evidence both for and against this view. Some reasons that it may not be true are the fact that in tropical parts of the world, the soil underneath trees often absorbs more heat than the trees themselves, that forested areas tend to have higher cloud cover, and thus reflect away more heat anyway, and the belief that the earth is regulating its own temperature, and thus that deforestation has essentially no lasting effect on global temperature. But the fact remains that tree-free environments where deep snow covers the ground during the winter have winter albedos 10% to 50% higher than nearby forested areas, and temperatures in the middle latitudes are as much as 11°C (20°F) colder over the barrens. Near the poles, the difference approaches zero because less sunlight is coming in in the first place.

If the theory of global warming is true, and the polar regions of the planet are going to be rapidly warming up, then the magnitude and path of the warming trend will depend largely on the albedo of the earth's polar regions, and to a lesser extent on the albedo of the whole planet. Because warm days in the Arctic mean heavier snowfalls, the extra snow will have a negative feedback, tending to bring temperatures down, masking the warming effect. But if the temperature rises year-round, then summers in some areas will be warm enough to melt the snow and ice completely, and thus the feedback effect in the summer will become strongly positive. For example, currently the most northern part of the Davis Strait between Canada and Greenland is blocked with ice year-round, and summer temperatures are very cold even for its latitude because of the albedo. If the ice were to melt, the very high-albedo ice would be replaced with some very low-albedo water, and temperatures would rise enormously, perhaps as much as 8°C (15°F), which would make the area warmer than any other area at that latitude. (This is what accounts for the unusually warm summers on the islands of Svalbard, in Norway. But Svalbard is surrounded by ocean, whereas the Davis Strait is surrounded by land, and thus has the potential to become even warmer than Svalbard.)

If the ice melted, then there would be a way for the warm waters of the Gulf Stream to penetrate deep into the Arctic, and warm spells would become increasingly frequent. This could accelerate the melting of the Arctic Ocean, a process which would feed on itself. A melted Arctic would help many people living in Arctic areas, but could possibly hurt many others living in other parts of the globe.

In fact, those few remaining supporters of the old Soviet plan to melt the Arctic Ocean rest their arguments largely on the fact that once the ocean is melted, temperatures there would never become cold enough for the ice to build up again. Their theory is probably false, as climate models have shown that temperatures would still be below freezing for most of the year, and then, because of albedo, would soon be below freezing all year because the ice would remain throughout the summer. (The average temperature at the North Pole in July is estimated to be about -1°C (31°F), and it is estimated that without an icecap that July would average around 6°C (42°F).)

Clouds are another source of albedo that play into the global warming equation. Different types of clouds have different albedo values, theoretically ranging from a minimum of near 0% to a maximum in the high 70s. Climate models have shown that if the whole earth were to be suddenly covered by white clouds, the surface temperatures would drop to a value of about -151°C (-240°F). This model, though it is far from perfect, also predicts that to offset a 5.0°C (9°F) temperature change due to an increase in the magnitude of the greenhouse effect, all we would need to do is increase the earth's overall albedo by about 12% by adding more white clouds.

Other scientists, noting the effect of volcanic eruptions on global temperatures, have suggested other ideas, some of which are actually practical, to stop global warming by increasing the earth's albedo. While all of these solutions are expensive and would require cooperation from powerful nations, they show that the problem of global warming is not much of a problem at all, that it can be easily controlled and eliminated by human technology.

It is unknown, however, whether injecting dark-colored particles into the atmosphere would have a conversely warming effect. During the Kuwaiti oil fires in 1991, temperatures in the usually hot desert kingdom cooled down by an average of 7°C (13°F) during the period of greatest obstruction. Thus it may be impossible to warm the planet in a controlled way by clouding up the atmosphere with dark-colored materials, and thereby reflecting less heat back into space.



All Wikipedia text is available under the terms of the GNU Free Documentation License

 
  Search Encyclopedia

Search over one million articles, find something about almost anything!
 
 
  
  Featured Article
List of people by name

... Eh[?] Ei[?] Ej[?] Ek[?] El[?] Em[?] En[?] Eo[?] Ep[?] Eq[?] Er[?] Es[?] Et[?] Eu[?] Ev[?] Ew[?] Ex[?] Ey[?] Ez[?] F -- Fa[?] Fb[?] Fc[?] Fd[?] Fe[?] Ff[?] Fg[?] Fh[?] ...