Abstract: A comprehensive fog experiment was carried out in the north suburb of Nanjing, China with many instruments, e.g., tethered balloon system, fog droplet spectrometer, visibility meter. Using the data of boundary layer profiles, fog droplet spectra, visibility, and NCEP reanalysis, a fog case on 14 December 2006 was chosen to study the jump features (sharp strengthening and weakening) of the fog top and the ground fogs density, and their causes are also discussed in detail. Result shows that the explosive development of the fog top is due to upward turbulent transport of moisture and its accumulation under uppe-layer inversion as well as substantial temperature decrease; the sharp strengthening of the ground fog is mainly caused by lower saturation vapor pressure in the near-surface layer under the influence of cold advection, enhancement of inversion owing to upper-layer systematic sinking motion, and moisture accumulation under inversion; during the fog top decline, turbulence happens near the fog top and upper-layer sinking motion causes the occurrence of temperature increase, fog double layer structure, and low-level jet; the sharp weakening of the ground fog is the result of solar radiation and downward transport of momentum; sinking motion has dual roles in the fog event; the fog double layer structure occurs during the sharp decline of the fog top and accelerates the fog top decline, which is extremely different from the previous results that the double layer structure causes the fog top development.