上野盆地における霧の局地気候学的研究

Fog is one of the remarkable features of the basin climate. In basins, generally, fog appears in early morning and disappears soon after sunrise. Autumn and early winter are the fog seasons in the inland of Japan. The observation and survey of the basin fog in the Ueno Basin, lie Prefecture, have been made by the authors since the summer of 1971. This is the first report on the fog observations in the Ueno Basin.<br> There are many studies on the basin fog, and these studies clarify the features of basin fog in each basin. But these are either based on the meteorological data recorded at a few stations situated in a basin, or description of limited cases of fog occurrence. Accordingly, the general features and the regionalities of the basin fog are somewhat obscured. The authors intend to clarify the regionalities and the mechanismus of for-mation of the basin fog.<br> The Ueno Basin is situated in the middle of Kinki District. Fog appears frequent-ly in the basin from autumn to early winter. Table 1 shows the number of days with fog in each month. The annual number of days with fog reaches to about 80 in the normal. The months from August to November are the fog season. The number of days with fog exceeds 10 days in these months. Accordingly, the fog observations were mainly performed in autumn. The main methods of observations are as follows ;<br> (1) The observations of fog appearances were continued through November of 1972, at 28 stations.<br> (2) The perspective observations of fog layer, from the slope of Takahata Mountain, were repeatedly made in the autumns and the winters of 1971, 1972 and 1973.<br> (3) Daily minimum air temperatures were observed at 25 stations in November, 1971.<br> (4) Air temperature observations were made by thermographs at 5 stations, from November to December, 1972. The elevations of each station are 135 m (at the basin bottom), 210 m, 330 m, 430 m and 580 m (on the slope of Takahata Mountain).<br> (5) The moving observations of air temperature by automobiles equipped with thermometers were performed in early mornings of November, 1971.<br> Fig. 1 shows the number of days with fog at each station, in November, 1972. Day with fog is defined as the day in which visibility becomes below 1 km by fog. Day with dense fog is defined as the day in which visibility becomes below 200 m by fog. The number of days with fog are larger in the lower areas of the basin, that is, the areas along the rivers. Especially, appearances of dense fog are concentrated on the lower areas. 4n the contrary, the areas higher than 300 m (shown by the broken line in Fig. 1) have a few days with fog. According to these facts, it is suggested that the formation of fog is influenced by the conditions of radiational cooling at night, accumulation of cold air in the basin bottom and water vapour supply from rivers to the air.<br> When fog appears in the basin, it covers firstly the northern and the western parts of the basin bottom along rivers. Then, it spreads to all of the basin bottom. When fog have appeared before the midnight, it tends to be a dense fog in the early morning. But when it appears immediately before the sunrise, it tends to be a thin fog in the morning. As soon as fog appears, the air temperature of the basin bottom rises a little. Perhaps, this rising may be resulted from the discharge of latent heat by the condensation of water vapour. Accordingly, the time of fog appearance is inferable by the records of thermographs. The temperature records on the bottom of the basin (135 m) frequently show a small rising at night. Table 2 shows the frequency distribution of the time when temperature rising occurs, that is, fog appears on the bottom of basin. The investigated period is from 10 November to 7 December, 1972. According to the Table 2, it is supposed that fog appears mostly between 22h and l h.<br>The depth of fog layer increases gradually, after fog appeared on the bottom of the basin at first.