2.3 - The effect of the atmosphere

So interaction of light with objects allows us to see them. But what affects radiation reaching a sensor at the top-of-atmosphere? What is the contribution from the surface? How does radiation interact with the atmosphere?

Exercise: Please look at next figure showing the spectral distribution of the solar radiation both at the top of the atmosphere (TOA) and at ground level. Can you select the proper legend to the figure, from the two options below?

Next figure illustrates what happens to solar radiation in its transmission through the atmosphere:

As you can see, solar radiation is attenuated by:

  • Reflection (by clouds and atmospheric constituents);
  • Absortion (by atmospheric constituents)

What are the atmospheric constituents causing this attenuation of solar radiation? Do they act equally for the all regions of the electromagnetic spectrum?

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 LOST DUE TO FLASH DISCONTINUATION

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.