Based on the observed and NCEP reanalysis data from 1985 to 2006, the climate background and synoptic situation of fog at Great Wall Station were analyzed.It is shown that the seasonal variation of fog is controlled by the change of general circulation and local pressure field.Three favorable typical synoptic situations for fog development arc found,the Front-of-A-Depression type,the Saddle-Shaped-Field type and the Passing-Weak-Cyclone type.The first one is the most important situation.Advection cooling fog is dominant at Great Wall Station,but there are other kinds of fog as well.As a result,some helpful principles for local fog forecasting are given.
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