Abstract : A full-sky imaging polarimeter has been developed to measure the four Stokes parameters in each pixel of a full-sky image. The Stokes image set can be acquired in typically less than 1 sec., making possible polarimetric measurements in a partly cloudy sky without creating artifacts from moving clouds. This instrument was used to study the variation of atmospheric polarization with cloudiness, finding that the degree of polarization is consistently less in the clear portions of a partly cloudy sky than in a fully clear sky. The instrument also was applied in a validation study of the polarized MODTRAN radiative transfer code. This validation showed that the code generally performs well in conditions of low aerosol optical depth, but the single-scattering model breaks down with higher optical depths. The study shows the need for improved aerosol characterization in future polarization studies, as aerosols have a very large influence on the observed polarization state of visible skylight.