Since a deep and long-term fading in optical intensity results in considerable burst errors in the data, a terrestrial free-space optical (FSO) system has to be designed with consideration of a frequency characteristic of optical scintillation to achieve high quality wireless services over the link. In designing a terrestrial FSO link, we had better design the system considering variations caused by some slow time-varying parameters. This paper proposes a Butterworth-type spectral model of optical scintillation to design a terrestrial FSO link, which enables us to estimate the power spectral density of optical scintillation in a current optical wireless channel when time zone and weather parameters, such as temperature and rainfall intensity, are given. The spectral parameters of optical scintillation, cut-off frequency, and spectral slope are estimated from the data obtained in the experiment, and then their dependencies on time zone, temperature, and rainfall intensity are examined. (C) 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3557487]