The performance behavior of optical wireless links installed at Graz and Nice is discussed and analyzed for moderate continental fog and dense maritime fog conditions. The knowledge of the free space optical (FSO) wireless links behaviour and the corresponding measured attenuations analysis for the two most prevalent fog types is mandatory as fog severely affects the availability and reliability of the FSO links installed and results in complete link outages. A better understanding of the attenuations analysis could be useful in evaluating the availability and quality of service (QoS) requirements under increased predictability. For our FSO links analysis, we consider five winter months attenuation measurement data of moderate continental fog conditions at Graz and eight days measurement data for dense maritime fog conditions at Nice. We evaluate the diurnal variations patterns for Graz and Nice. The diurnal variations are presented for four time intervals i.e., 00:00-06:00, 06:00-12:00, 12:00-18:00, and 18:00-24:00.