Rayleigh sky model

The Rayleigh sky model describes the observed polarization pattern of the daytime sky. Within the atmosphere Rayleigh scattering of light from air molecules, water, dust, and aerosols causes the sky's light to have a defined polarization pattern. The same elastic scattering processes cause the sky to be blue. The polarization is characterized at each wavelength by its degree of polarization, and orientation (the e-vector angle, or scattering angle). The Rayleigh sky model describes the observed polarization pattern of the daytime sky. Within the atmosphere Rayleigh scattering of light from air molecules, water, dust, and aerosols causes the sky's light to have a defined polarization pattern. The same elastic scattering processes cause the sky to be blue. The polarization is characterized at each wavelength by its degree of polarization, and orientation (the e-vector angle, or scattering angle). The polarization pattern of the sky is dependent on the celestial position of the sun. While all scattered light is polarized to some extent, light is highly polarized at a scattering angle of 90° from the light source. In most cases the light source is the sun, but the moon creates the same pattern as well. The degree of polarization first increases with increasing distance from the sun, and then decreases away from the sun. Thus, the maximum degree of polarization occurs in a circular band 90° from the sun. In this band, degrees of polarization near 80% are typically reached. When the sun is located at the zenith, the band of maximal polarization wraps around the horizon. Light from the sky is polarized horizontally along the horizon. During twilight at either the Vernal or Autumnal equinox, the band of maximal polarization is defined by the North-Zenith-South plane, or meridian. In particular, the polarization is vertical at the horizon in the North and South, where the meridian meets the horizon. The polarization at twilight at an equinox is represented by the figure to the right. The red band represents the circle in the North-Zenith-South plane where the sky is highly polarized. The cardinal directions N, E, S, W are shown at 12-o'clock, 9 o'clock, 6 o'clock and 3 o'clock (counter-clockwise around the celestial sphere since the observer is looking up at the sky).