2.3.1 - Absorption bands and atmospheric windows

Figure 2.8 shows the absorption spectra for the various atmospheric gases between the top of the atmosphere and the earth’s surface, as a function of the wavelength, namely:

  • The tropospheric nitrous oxide (N2O) and methane (CH4), which have absorption bands in the infrared. Nevertheless, their concentrations are too low to have a significant impact.

Figure 2.8 – Absorption spectra for various atmospheric gases (adapted from Peixoto and Oort, 1992)

Exercise: Considering the absorption spectra for O2, O3, Water vapour and CO2 (Figure 2.8), choose the proper correspondence between the sentences below:

Exercise: Now considering the total effect of the absorbing gases, select from the bars on next Figure the regions of the electromagnetic spectrum where you expect to have less absorption of solar radiation by the atmosphere.

To answer to this question you should have mentally combined the individual effects of the atmospheric gases. This corresponds to Figure 2.9:

Figure 2.9 – Impact of the atmospheric gases on solar radiation (adapted from Peixoto and Oort, 1992)

The spots you selected are called atmospheric windows, where radiation is allowed to pass through the atmosphere with little attenuation (transmissivity is high and absortivity is low). It is through these windows that satellites observe the surface of the Earth on the visible and/or infrared regions of the electromagnetic spectrum.

The areas of the electromagnetic spectrum where radiation is most absorbed are known as absorption bands.

The solar energy that is able to pass through the atmosphere will reach the surface of the earth and interact with the surface features through the same mechanisms of reflection, absorption and transmission.