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Based the classification of fogs in Chongqing, the circulation patterns, streamline field features and the vertical circulation structures during several typical radiation and water fog events were analyzed by using of a composite method. The formation causes of the two types of fogs were studied. Radiation fogs may form under the background of longitudinal circulation with the main influencing system being the high-pressure ridge over Middle Asia. In the longitudinal vertical circulation cross-section, there existed a well-developed and deasil vertical longitudinal circulation in the middle troposphere on the east side of the Tibet Plateau. The descending currents in the front forced an anticlockwises vertical circulation over the Sichuan basin along the topographical sloping fields, which strengthened the low-level downdraft around Chongqing. The nighttime radiation cooling was obvious and there occurred temperature inversion around the ground with the air stratification being stable. Water fogs may form under the condition of relatively weak longitudinal circulation over the Asia region. The Tibetan Plateau low-pressure trough and the surface cold front were the main influencing systems. The vertical circulation cross-section shows that the eastward moving upper low-pressure trough led to an anticlockwises vertical circulation. The updraft on the east side of the circulation was coupled with the updraft of the deasil longitudinal vertical circulation, which was produced by the weak surface cold air over the Sichuan basin. Thus the updraft around Chongqing was strengthened; surface warm and moist air was relatively active; and the obvious humidity increasing process played an important role in water fog forming.