Page 132 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

96 part I The Energy–atmosphere System
Albedo values (% reflected)
Fresh snow 80%–95%
Grass 25%–30%
Moon 6%–8%
Water bodies 10%–60% (varies with Sun altitude)
Forests 10%–20%
Crops, grasslands 10%–25%
Earth’s albedo (average) 31%
Dark roof 8%–18%
Light roof 35%–50%
Brick, stone 20%–40%
Asphalt (black top) 5%–10%
Concrete, dry 17%–27%
▲Figure 4.6 Various albedo values. in general, light surfaces are more reflective than dark surfaces and thus have higher albedo values.
Satellite
Global Albedo Values, Global Shortwave Radiation
sets, its refracted image is visible from over the hori- zon for about 4 minutes afterward. These extra min- utes vary with atmospheric temperature, moisture, and pollutants.
Reflection and Albedo A portion of arriving energy bounces directly back into space—this is reflection. The reflective quality, or intrinsic brightness, of a surface is albedo, an important control over the amount of insola- tion that reaches Earth. We report albedo as the percent- age of insolation that is reflected—0% is total absorption; 100% is total reflectance.
In terms of visible wavelengths, darker-coloured surfaces (such as asphalt) have lower albedos, and lighter-coloured surfaces (such as snow) have higher albedos (Figure 4.6). On water surfaces, the angle of the solar rays also affects albedo values: Lower angles produce more reflection than do higher angles. In addi- tion, smooth surfaces increase albedo, whereas rougher surfaces reduce it.
Individual locations can experience highly variable albedo values during the year in response to changes in cloud and ground cover. Satellite data reveal that albedos average 19%–38% for all surfaces between the tropics (23.5° N to 23.5° S), whereas albedos for the polar regions may be as high as 80% as a result of ice and snow. Tropi- cal forests with frequent cloud cover are characteristi- cally low in albedo (15%), whereas generally cloudless deserts have higher albedos (35%).
Earth and its atmosphere reflect 31% of all insolation when averaged over a year. The glow of Earth’s albedo, or the sunshine reflected off Earth, is called earthshine. By comparison, a full Moon, which is bright enough to read by under clear skies, has only a 6%–8% albedo value. Thus, with earthshine being four times brighter than moonlight (four times the albedo), and with Earth four times greater in diameter than the Moon, it is no surprise that astro- nauts report how startling our planet looks from space.
Absorption Insolation, both direct and diffuse, that is not part of the 31% reflected from Earth’s surface and atmo- sphere is absorbed, either in the atmosphere or by Earth’s surface. Absorption is the assimilation of radiation by mole- cules of matter, converting the radiation from one form of energy to another. Solar energy is absorbed by land and water surfaces (about 45% of incoming insolation), as well as by atmospheric gases, dust, clouds, and stratospheric ozone (together about 24% of incoming insolation). It is converted into either longwave radiation or chemical en- ergy (the latter by plants, in photosynthesis). The process of absorption raises the temperature of the absorbing surface.
The atmosphere does not absorb as much radiation as Earth’s surface because gases are selective about the wave- lengths they absorb. For example, oxygen and ozone effec- tively absorb ultraviolet radiation in the stratosphere. None of the atmospheric gases absorb the wavelengths of visible light, which pass through the atmosphere to Earth as direct radiation. Several gases—water vapour and carbon dioxide,