Abstract: Based on the historical meteorological data from the meteorological information comprehensive analysis processing system (MICAPS), the main influencing factors in the typical regional heavy fog weather cases from 2008 to 2010 were analyzed by the physical mechanism of heavy fog formation. Seven factors that were favorable to the formation of heavy fog were selected. The results indicate that there are some certain features for sever factors, namely, the inversion layer thickness is 20-200 hPa; the ground temperature dew point difference is mostly less than 1 ℃, and it is mostly less than 3 ℃ on the previous day; the surface wet air thickness is 0.05-0.70 km; the inversion layer high point 0 ℃ line is the feature line of rime occurrence; the wind speed of southerly wind near  the ground is 2 to 6 m? s-1; air temperature is from -15 ℃ to 20 ℃ and most of temperature are between – 5.0 ℃ and 5.0 ℃, and 0 ℃ line is the feature line of fog occurrence; high pressure degenerates or weakens. These characters could be found 12 to 24 h earlier. Therefore, a heavy fog falling area conceptual model is established, so is a corresponding forecast system according to the model. Test shows that regional fog forecast can be advanced by 12 to 24 h, and the accuracy rate of 24 h regional fog forecast can reach 87.5 %.

Key words: Fog falling area, Concept model, Operational system