<p>We present a unified framework for modeling free-space optical (FSO) communication channels under realistic atmospheric conditions, with a focus on both satellite-to-Earth downlinks and horizontal near-ground links. The model accounts for three key factors affecting transmittance: deterministic attenuation from atmospheric absorption and scattering, geometric diffraction losses due to transverse beam broadening, and stochastic intensity fluctuations induced by atmospheric turbulence. For downlink scenarios, we analyze how the ground station altitude influences channel performance, particularly in terms of turbulence-induced variability. Our findings indicate that higher-altitude stations offer enhanced stability of received signals, even when average transmittance remains largely unchanged. The framework is also extended to horizontal FSO links, relevant for inter-city communication. We show that atmospheric effects, including turbulence and attenuation, play a critical role even over moderate distances, emphasizing the importance of realistic modeling for both classical and quantum optical technologies. The results provide practical insight for designing high-fidelity FSO systems, especially in the context of quantum communication, where both loss and stability are essential performance metrics.</p>