The NCEP reanalysis grid data, conventional observations, numerical forecast products, as well as satellite imagery, are used to diagnose and analyze a dense fog event occurred in the central Zhejiang Province and the surrounding areas on 6 April 2009. The results show that the circulation pattern of “the warm ridge behind a trough” is conducive to the surface cooling resulted from longwave radiation and the midlevel warming, which is advantageous to the formation of temperature inversion, so to provide favorable conditions for fog generation. The formation and maintenance of the temperature inversion or isothermal layer in the boundary layer played an important role in the development and sustentation of the fog event. The main reasons for the higher visibility in mountain areas than that in urban areas include more condensation nuclei and stronger turbulent mixing. The crossover temperature can be used to indicate the humidity state of the potential fog layer, and the difference between temperature and crossover temperature can be used as an indicator of the generation, development, and dissipation processes of fogs. The model products can provide the environmental conditions for fog formation, and the satellite images are helpful in predicting the generation, development and dissipation of fogs.