Measurements of Tropospheric Ice Clouds with a Ground-based CMB Polarization Experiment, POLARBEAR

The polarizationof the atmosphere has been a long-standing concern for ground-based experiments targeting cosmic microwave background(CMB) polarization. Ice crystalsin upper tropospheric clouds scatter thermal radiationfrom the ground and produce a horizontally- polarizedsignal. 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- polarizedtemporal increases of low-frequency fluctuations (" polarizedbursts," hereafter) of $\lesssim$0.1 K when clouds appear in a webcam monitoring the telescope and the sky. The hypothesis of no correlation between polarizedbursts 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 polarizationon future ground-based CMB polarizationexperiments.