The NCAR/PSU MM5v3 model is used to simulate a fog event in the Wuqing area from 30 Nov to 1 Dec 2009. This severe fog was distributed mainly in Tianjin, Hebei, and Shandong; Wuqing was at the edge of the fog area. This fog event can be seen as having three phases: After the formation phase from 1700 LST 30 Nov to 0000 LST 1 Dec, fog began to form at 0000 LST 1 Dec. The development phase lasted from 0000 LST to 0900 LST; after 0900 LST the fog began to disperse. With a cooldown caused by long-wave radiation cooling, an inversion appeared; in addition, sufficient water vapor was transferred by a warm wet flow. Weak convergent upward motion occurred in the surface layer. Under these conditions, fog formed. The maintenance of the inversion and sustained long-wave radiation cooling is conducive to continuous fog development. In the third phase, fog began to dissipate as a result of vapor divergence. Turbulence transfers water vapor, but if the turbulence is too strong, fog will dissipate quickly.