Altermatt Lecture:   The Solar Spectrum


4.4:  Scattering of sunlight in the atmosphere

What is scattering? Scattering happens when incoming light causes charge to oscillate, which – in turn – radiates light as well, leading to a superposition of the incoming and this secondary radiation. The properties of the superposed radiation depends on the nature of the scatterers:

  • If a smooth metal film is used as a scatterer, the secondary waves have a defined phase angle to each other, causing reflection at a mirror, also called specular reflection.
  • If the metal film is not smooth, the secondary waves do not superimpose so nicely, causing diffuse reflection; this may be called scattering.
  • If, instead of a metal film, there are small particles dispersed in a medium, it is always called scattering, and the scattering properties depend on the size, distribution, and possibly on the shape of the particles. This type of scattering is present in the atmosphere.

Scattering of sunlight in the atmosphere can be categorised into two types:

  • If sunlight scatters at molecules or particles that are smaller than the wavelength, it is called Rayleigh scattering. Such particles scatter blue light stronger than red light and are the main cause for the blue sky. Accordingly, blue sunlight cannot penetrate the atmosphere as easily as red light. See the left figure below generated from the SMARTS software [1].
  • If the sunlight scatters at particles that are larger than the wavelength, it is called Mie scattering. It is not as selective to wavelength as Rayleigh scattering, because the secondary radiation can be thought of as being superimposed of many tiny scatterers contained in the relatively large particle. This causes the whitish haze, fog, and the white of the clouds.

Now consider: Why does the sky near the sun appear whitish on a clear day whereas the rest of the sky appears deep blue?

Diagram of scattering from molecules

Figure: The larger the scattering particles are relative to the wavelength of the light, the more is the light scattered in forward direction.

Atmospheric scattering in AM1-5d

Figure: Atmospheric transmittance of sunlight due to Mie scattering at large particles in the air under standard conditions (grey line), or on a very clear or on a hazy day (dotted grey lines). The Rayleigh scattering at air molecules is stronger towards the blue range of the spectrum and is rather invariable as it depends only very weakly on air pressure.


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