Decreased visibility due to fog causes disruption in air and ground transportation systems, so recognizing the factors affecting the formation of this phenomenon has a significant role in reducing human and financial losses. The purpose of this study is to investigate the factors affecting the formation of fog and identify its types in the central strip of the country. To this aim, data related to the time of fog occurrence and in the period of 48 hours before it was collected from synoptic stations of Kermanshah, Isfahan, Mehrabad, Yazd, Kerman and Zahedan. In addition, the re-analysis maps of geopotential height at 500 hPa level and sea level pressure for foggy days have been investigated. The results show that the maximum frequency of occurrence is in winter and coincides with the minimum temperature. Further study also shows the correlation between fog occurrence and precipitation report in the last 48 hours. Finally, the synoptic patterns indicate the frequent occurrence of frontal and post-frontal fog in the central strip of the country.