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Specular reflection from horizontally oriented plates (HOPs) has significant effects on lidar backscatter. The intensity of specular reflection from HOPs is high in warm mixed-phase clouds and low in cold ice clouds. The theoretical simulations of lidar backscatter and depolarization ratio are consistent with spaceborne measurements for optically thick mixed-phase and ice clouds if an equivalent percentage of HOPs of 0.08%-0.3% is assumed. Based on the joint probability density function of the attenuated backscatter and depolarization ratio observed by the Cloud-Aerosol Lidar with Orthogonal Polarization aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations platform, it is estimated that HOPs exist in approximately 60% of optically thick (τ > 3) ice and mixed-phase cloud layers. The cloud-layer temperature is the primary factor affecting the distribution of HOPs. Specifically, HOPs exist in approximately 88% of optically thick ice and mixed-phase cloud layers warmer than -30°C, in approximately 84% of ice and mixed-phase cloud layers between -30°C and -45°C, and in approximately 29% of cold ice cloud layers below -45°C.