 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 2 pi times the molecule of particle radius divided by the radiation wavelength. The graph of the particle radius and wavelength with different size parameters begins to make more sense when we talk about how different size parameters affect scattering. This figure on scattering patterns for different size parameters illustrates the importance of the size parameter. For size parameters 1 and above, that is, the particle radius more than 1-6 to 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 wavelength is much larger than the particle radius, then more of the radiation is scattered back toward the radiator source and to the side. This should give you some idea how the radiation wavelength which shows up for radar, which looks at the back-scattered radiation from 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 strong. Last, we should look at the dependence of the scattering cross-section on 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 of 6 power and the inverse of the radiation wavelength to the 4th 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 and back. 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.