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There is an increasing interest in free space optics (FSO) communication systems among data service providers. The cost of installing a new fiber network in an urban environment is often very expensive, and a less expensive method is needed. A high-speed data link, for example, can be designed using either optical or millimeter-wave (MMW) point-to-point communication systems. The optical system is often the preferred method because of its simplicity and security. The data rate for short distance FSO systems in an ideal condition can exceed one Gbit/sec. However, both MMW and optical links are susceptible to adverse weather conditions. The traditional method to estimate the effects of an atmospheric channel is to calculate the attenuation characteristics based on the size distribution and concentration of particles along the signal path length. From the signal attenuation rate, the bit-error rate is usually estimated. This approach, however, does not consider the propagation characteristics of modulated signals. We give a numerical method to calculate the waveform of an intensity modulated optical signal through a layer of fog and cloud. Our approach takes into account the multiple scattering effects and reveals a substantial amount of waveform distortion.