We analyze the link reliability of satellite-to-ground free-space optical (FSO) communication systems. Using ten-year meteorological data of five major cities in South Korea, we theoretically estimate the link reliability limited by the power losses arising from cloud cover, scintillation, and aerosol scattering. For conservative estimation of the link reliability determined by cloud cover, we propose a loss model based on the maximum value of cloud droplet concentration. The results show that the link reliability for 3-hours continuous operation ranges from 45% to 75% when a single ground station is placed at a major city with 20-dB power budget allocated for atmospheric loss. Also, we find out that the link reliability is not improved by using the site diversity.