The B-mode polarization, a parity-odd component of polarization anisotropies on the cosmic microwave background (CMB), is a unique probe of primordial gravitational waves from the cosmic inflation. One of the challenges for the measurement is mitigation of the low-frequency noise, also called the 1/f noise, which exponentially degrades the measurement accuracy for large angular scales. The POLARBEAR experiment is a ground-based CMB polarization measurement at the Atacama Desert in Chile with 1,274 transition-edge-sensor (TES) bolometers sensitive to the 150 GHz band. To control the 1/f noise, POLARBEAR installed a continuously rotating half-wave plate at prime focus in 2014. In the test observation, we have found that the 1/f noise mainly comes from the instrumental polarization leakage, which we can remove analytically (Takakura et al. 2017). In science observations, however, we occasionally have spurious polarized bursts, especially in cloudy days. It is likely that ice crystals in clouds scatter the thermal radiation from the ground and create horizontal polarization (Pietranera et al. 2007). I will present the model of cloud polarization and discuss prescriptions in future experiments.

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