An all-sky polarization spectral imager developed under prior support was deployed along with an atmospheric lidar, a sun-tracking multi-channel solar radiometer, and a variety of ground-based aerosol sensors to study the effect of variable aerosols, clouds, and surface reflectance on skylight polarization in the 450 780 nm spectral region. The entire sensor suite operated continuously from 2009-2012 except for brief periods of maintenance. This study produced a computer model for predicting clear-sky polarization as a function of aerosol properties and surface reflectance, which was validated with measurements and then used to predict clear-sky polarization spectra for various locations around the world. A surface reflectance with a strong wavelength dependence results in lower skylight degree of polarization at wavelengths with higher surface reflectance. Similarly, changing aerosol types and concentrations lead to spatially and temporally variable skylight polarization.