  The polarization of the atmosphere has been a long-standing concern for
ground-based experiments targeting cosmic microwave background (CMB)
polarization. Ice crystals in upper tropospheric clouds scatter thermal
radiation from the ground and produce a horizontally-polarized signal. We
report the detailed analysis of the cloud signal using a ground-based CMB
experiment, POLARBEAR, located at the Atacama desert in Chile and observing at
150 GHz. We observe horizontally-polarized temporal increases of low-frequency
fluctuations ("polarized bursts," hereafter) of $\lesssim$0.1 K when clouds
appear in a webcam monitoring the telescope and the sky. The hypothesis of no
correlation between polarized bursts and clouds is rejected with $>$24$\sigma$
statistical significance using three years of data. We consider many other
possibilities including instrumental and environmental effects, and find no
other reasons other than clouds that can explain the data better. We also
discuss the impact of the cloud polarization on future ground-based CMB
polarization experiments.
