This paper deals with statistical analyses to clarify the frequency of fog occurrence in some cities in Japan (Fig.l). In this work, the data of Japan Meteorological Agency were used to analyze at about 80 points in Japan (Tab.l) between 1951 to 2000. The main results of this study were summarized as follows. 1. It is tendency in fog occurrence to present the regional and seasonal characteristics during recent 50 years (Fig.2, 3 & 4). 2. Judging from the regional and seasonal characteristics in occurrence frequency of fog in Japan, the main causes of fog are advection type as well as radiation one (Fig.4). 3. The secular change of annual number days with fog at each station tends to decrease gradually in many points, especially in inland or basin stations (Fig.5). 4. In spite of above mention, the annual number days with fog have slightly increased at the stations along the coast of Pacific in Northeastern Japan. 5. It will be able to point out that the factors of decreasing of number days with fog in secular change are equal to lower the value of relative humidity or to rise the extreme value of daily minimum temperature (Fig.6)
