Based on the high temporalspatial resolution visibility data from 2011 to 2016, the spatial distribution characteristics of visibility and its variation in the fog formation process are analyzed. Results show: the ratio of the normal distribution or lognormal distribution in visibility spatial distribution was more than 95%. The ratio decreased significantly while there were fogs; the coefficient of variation formed a peak structure in fog formation process; the highdensity visibility data was used to make nowcasting or early warning of heavy fogs and the time was 1.9 hours ahead of the warning time averagely.