The pressure fields for the heavy fog events occurred at Huludao weather station around the Northern Bohai Bay for nearly 30 years are analyzed by using the Fuzzy CMeans (FCM) Clustering Algorithm, and the optimal cluster number is decided based on four scalar validity measures, and then the pressure pattern and monthly frequency for each cluster, wind direction and magnitude, relative humidity, diurnal variation of visibility at the weather station are also studiedThe results show that the pressure patterns for heavy fog weather at the Huludao station can be divided into four types: ahead of a cold front, ahead of a high pressure system, rear of a high pressure system, and col pressure fieldThe pattern ahead of a cold front occurs mainly in spring and summer, while patterns ahead of a high pressure system and col pressure field happen mostly in winter and spring, and that in the rear of a high pressure system can arise all the year, within which the annual frequency of heavy fogs for only col pressure pattern passed the 95% significant test For patterns ahead of a cold front and rear of a high pressure system, the prevailing wind during a heavy fog is southwesterly, because of the effect of land and sea breeze in the rear of high pressure, the percentage of northerly is higher than that of southerly, but the percentage of southwest wind is higher in the col pressure field