The performances of free space optical (FSO) communication systems are highly susceptible to changes in atmospheric weather conditions. In places where dense fog is rare, atmospheric turbulence is the primary factor responsible for power losses in the received optical signal level. In this paper, the impact of irradiance fluctuations on FSO systems was estimated using the modified Rytov theory for major cities across South Africa. The extent to which the refractive index structure parameter, propagation distance and link margin affect the optical signal power at the receiver is discussed and the different methods used in evaluating the atmospheric turbulence effect are investigated. The results presented are based on the horizontal-path transmission of spherical and plane waves.