Based on the observations and theNCEP reanalysis data, a heavy fog eventoccurred inHebeiPlain from November7 to 13,2007 is investigated from such aspects as theweatherbackground, temperature and humidity condition, the atmospheric stratification,dynamic and thermal features. The results show that the background situation was stablewith zonal circulation at themiddle and high levels and no strong cold airmoving during the heavy fog. The groundwind speed atnight ranged from 0 to2m /s. Plenty vaporand radiation cooling process atground layerwere in favor of the formation andmaintenance of the heavy fog. The atmospheric stratification
was stablewith an inversion layernear the ground, and theweak convergence and the persistentwarm advection near the groundmade formaintaining of the inversion layer, which played a very important role in the long-timemaintenance of the fog event. The heavy fogeventwent through three phases including advection-radiation fog, radiation fog and advection fog, and the intensity ofground invertion andmoist thicknesswere different in differentdeveloping phases of the fog.