Satellite-based quantum key distribution (QKD) offers a pathway to secure global communications, yet its performance is fundamentally constrained by atmospheric turbulence. This paper presents a detailed analysis of the BBM92 entanglement-based protocol under realistic satellite-to-ground conditions using the Hufnagel–Valley turbulence model. We examine the role of key atmospheric factors, including wind speed, ground station altitude, and diurnal variations, on quantum performance metrics across a mean photon number (MPN) range of 0.001–0.3. Results reveal that turbulence strength, captured via the refractive index structure constant, governs channel transmittance through Rytov variance and scintillation index, directly impacting quantum bit error rate (QBER) and secret key rate (SKR). Simulations show that elevated ground stations improve SKR by 15–25%, nighttime operation yields 40–80% gains compared with daytime, and wind speeds of 30 m/s can reduce SKR by up to 60% relative to 10 m/s conditions. Optimal operation lies in the MPN range of 0.1–0.2, beyond which multiphoton noise degrades security as QBER approaches the 9% threshold.