A Classification Study On Foggy Days Over China: Climatic Features, Formation Mechanism And Relationship With Climate Change

The climatic features of foggy days over China were analyzed in this paper including its temporal and spatial distributions, effect factors and changes by climate variations. Results show that the spatial distribution area of annual fog is quantitatively more in southeast than in northwest, and decreases along with the reduced visibility. Over most areas there exists an indistinctive decreasing trend in foggy days on general. Differences were presented in classification of foggy days, showing great regional features. The severe fog along the Yangtze River and eastern coastal line, however, shows a significant increase since 1970s. The foggy days also shows great seasonal variations, the number of foggy days in autumn and winter is larger than in spring and summer over most fog districts except for east coastal line.The starting time in foggy days mostly occurs at 06～08 am local time in inland areas and at 20～21 pm local time in coastal areas. The frequency of foggy days experiences a low-high-low inter-decadal variation. The starting time postpones in a few regions. In general, the duration time of fog shorter than three hours reached a high frequency before 1990s, while the fog with duration time over twelve hours locates mostly along the coast, the northeast China and eastern Gansu-western Shanxi. The maximum frequency of this category happens in 1990s along the coast, the Sichuan basin and Yunnan-Guizhou Plateau.The quantity of foggy days shows certain relationships with the intensity of winter and summer monsoon in East Asia by using composite analysis. The contrary results were obtained in different seasons: strong winter monsoon induces less foggy days over China. The week winter monsoon brings on more foggy days; while the strong summer monsoon conduces to more foggy days over northeast China, the week summer monsoon results in less foggy days in same area. Companied by the climate variations, the foggy days change through the effect of certain key factor such as the relative humidity and lowest temperature. Conclusion shows good positive correlation between foggy days and relative humidity. The quantity of foggy days can be reduced range from one day to seven days by relative humidity reducing one percent. The lowest temperature, however, shows different correlation with foggy days in summer and winter. The correlation presents good negative correlation between foggy days and lowest temperature over summer, while, the same correlation was not evident in winter, even the north coastal line shows obvious positive correlation.