[Next] [Previous] [Up] [Top]

The Atmospheric Window
CHAPTER 3 Energy Sources, Transfers and Transformations

Terrestrial Radiation


The Stephan-Boltzman Law indicates that all mass with a temperature above absolute zero (0 K or -273oC) produces radiation in proportion to the fourth power of the temperature in degrees Kelvin. The sun radiates at about 6000 K, and Wein's Displacement Law indicates that the greatest amount of solar energy is produced with a wavelength of 0.5 mm.The average temperature of the surface of the Earth is 288 K. Using the Wien's Displacement Law we find that the wavelength of light at which the greatest amount of radiation is produced is about 10 mm.



Terrestrial radiation is absorbed by the gases of the atmosphere (Figure 34). All gases, except diatomic gases, absorb radiation in the terrestrial wavelengths. The two important diatomic gases in our atmosphere are oxygen and nitrogen. Thus, it is the least abundant gases that serve as absorbers of terrestrial radiation, i.e. the greenhouse gases (Table 7). If our planet had no greenhouse gases in the atmosphere, then surface temperatures would be 255 K. As it is, global average surface temperature is about 288 K. The difference, 33 K, is the magnitude of the planetary greenhouse

FIGURE 34 Absorption of Terrestrial Radiation by Greenhouse Gases

The Atmospheric Window

Inspection of Figure 34 reveals that there is little absorption by any of the atmospheric gases between 9 and 13 mm. As the earth produces much of its terrestrial radiation at around 10 microns, this window permits radiation loss to space. The atmospheric window is shown as an enlargement in Figure 35. In addition to ozone, which absorbs in the window, hydrocarbons produced by terrestrial vegetation, i.e. terpenes, and evidenced by the hazy character of the atmosphere over the continents absorb radiation in the atmospheric window. Little work has been done on the role of hydrocarbons in the global energy budget. Studies in Hawaii indicate that at least 1% of the energy from the sun is absorbed by these hydrocarbons. Terrestrial radiation leaving the earth can be divided into 3 regions:

FIGURE 35 The Atmospheric Window


Climate Dynamics - 05 FEB 96
[Next] [Previous] [Up] [Top]

Generated with WebMaker