PRESENTER: Scattering is messy. So we will focus on just a few concepts that you can use. The first is the concept of size parameter, which is just two pi times the molecule or particle radius divided by the radiation wavelength. The graph of the particle radius and wavelengths with different size parameters begins to make more sense when we talk about how different size parameters affect scatter.

This figure on scattering patterns for different size parameters illustrates the importance of the size parameter. For size parameters one and above, that is the particle radius more than 1/6 of the radiation wavelength, we see that the radiation is strongly scattered in the forward direction, with little radiation scattered to the back or side. When the radiation wavelengths is much larger than the particle radius, the more the radiation is scattered back toward the radiator source into the side.

This should give you some idea how the radiation wavelength which shows up on radar, which looks at the back scattered radiation for precipitation. Note that the most efficient scattering of radiation occurs when the particle radius is about equal to the radiation wavelength, and that for radiation with much shorter wavelengths, scattering is inefficient, while the scattering for radiation with longer wavelengths is almost as small.

Last, we should look at the [INAUDIBLE] of the scattering cross section of particle radius and radiation wavelength. For small size parameters, the scattering cross section, which is just a scattering strength, is proportional to the particle radius to the sixth power and the inverse of the radiation wavelength to the fourth power. Equation 619 tells us that blue radiation is scattered much better than red radiation. And quite a bit of the radiation is scattered to the side. In fact, this more efficient scattering of blue is part of the reason that the sky is blue, but that the sun appears to be yellow when high in the sky, and even red when setting.