In this section we summarize how different types or classes of particles cause the polarization state to change. This is done geometrically using the Poincaré sphere to represent the polarization state of the signals (Scott, 1999). The Poincaré sphere provides a very useful way of characterizing the polarization state (e.g., Mott, 1986; McCormick and Hendry, 1985; Bebbington et al., 1987; Santalla et al., 1999). The results provide a simple way of understanding the depolarization processes and of seeing the comparative advantages of using different polarizations for meteorological measurements.