When the relative humidity reaches over than 95% and the visibility under the 1000 meter fog has been occurred. In this paper we survey the fog phenomena frequency and the conditions of it in Isfahan city during 1992-2011. The calculations showed that in Isfahan city in Feb, mar, Jan& Dec the fog phenomena more occurred. Significant synoptic pattern in foggy days shows that the dominant pattern is warm fronts. Usually in warm section (the area between warm and cold front) fog phenomena is seen.

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... Receiving humidity from Persian gulf and Oman sea cause intensifying warm front ,and cold air which penetrate from Siberian area to central part of Iran cause fog formation over Isfahan city.