The Earth's surface longwave radiation emission peaks at about 10 ¡m, or 1000 cm-1, and is significant between 5 and 50 ¡m, as can be seen from Fig. 3.4. Thus, molecular bands that absorb significantly in this spectral region play an important role in atmospheric absorption and emission, and hence control the outgoing longwave radiation to space and hence the Earth's longwave radiation budget. This, in turn, determines the greenhouse effect of the atmospheric molecules as the balance between the net incoming solar radiation and the outgoing longwave radiation primarily determines the Earth's surface temperature.
In Table 4.2 we divide the terrestrial infra-red spectral region into intervals within which there is significant band absorption by the atmospheric molecules. We list spectral regions where there is absorption via rotational bands, vibrational-rotational bands, and continuum absorption. The importance of water vapour is clearly seen.
Water vapour is the most important atmospheric greenhouse gas. Its bands cover most of the spectral region of the Earth's surface thermal radiation emission. Water vapour has important rotational bands between 0 and 800 cm-1 (> 12 ¡m), vibrational-rotational bands between 1300 and 2125 cm-1 (the 6.3 ¡m bands), and continuum absorption in the region 769-1250 cm-1. It also has many weak bands in the spectral region between 1 and 10 ¡m, some of these we
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