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A statistical analysis is made of the spatialtemporal distribution, weather circulation background and formation causes of radiation fogs in Yunnan Province by means of the radiosonde data in recent five years and the conventional meteorological data from 1976 to 2005 of 126 weather stations over Yunnan. The results show that the number of foggy days exhibited a yearbyyear undulatory variation trend and an overall dropping tendency, but the diurnal variation of fogs is relatively stable; the synoptic patterns favorable to fog formation include the westerly flow and the northerly flow patterns, with the northerly pattern accompanying heavier and longerduration fog process. The analysis of formation mechanisms of fogs indicates that the formation conditions for fogs include such weather situations as the good water vapor condition around the surface layer, calm or weak wind, clear sky or slight cloudiness, uniform pressure field behind the ground transformed cold anticyclone, weak ascending movement and warm advection at the lower levels, strong descending movement and cold advection at middle and upper levels, and an inversion layer in the boundary layer.