An all-sky polarization spectral imager developed under prior support was deployed along with an atmospheric lidar, a sun-tracking multi-channel solar radiometer, an infrared cloud imager, and a variety of ground-based aerosol sensors to study the effect of variable clouds and aerosols on skylight polarization in the 450 780 nm spectral region. Near the end the performance period the imaging polarization imager was re-engineered to operate in an autonomous manner outdoors in a weather-proof housing to allow long-term collection of data in a wide range of conditions. This study has shown that clouds always significantly alter the degree of polarization of skylight (either below the cloud or in a cloud-free portion of the sky), but that they often do not alter the angle of polarization beneath the clouds. Under certain illumination conditions, the angle of polarization beneath a cloud is rotated by 90-degrees relative to the clear-sky polarization. A relationship also was developed for an initial model of how increasing surface albedo reduces the overhead skylight polarization.
