A land and lake breeze is very important to understand local climate and the pattern of air pollution in the Lake Biwa basin. Detailed features of the land and lake breeze in the basin, however, have not been elucidated, due to the small water area, complicated topography and other wind systems. This paper intends to make clear occurrence frequency and geographycal distribubution of the land and lake breeze, and to discuss effects of the above mentioned factors on them, by analyzing the hourly wind data at s stations in this area (Fig. 1) during the period from January 1 to December 31, 1979.
The method of selecting the occurrence days of the land and lake breeze is as follows : In the first place, the day when an on-shore wind blew in the daytime, and an off-shore wind in the nighttime at Imazu and Hikone located in plains near the shore of the Lake Biwa is noticed. In the second place, frequency distribution of the start and the end time of the on-shore wind at Imazu and Hikone was counted (Fig. 2). In the third place, criterion for judging the land and lake breeze was determined (Table 1), and the day when the on-shore wind at Imazu and Hikone started and ended within the time ranges shown in Table 1 was determined as the occurrence day of the land and lake breeze on the shore of the Lake Biwa.
The results obtained are summarized as follows:
1. The occurrence days of the land and lake breeze in the Lake Biwa basin amounted to 60 days in warm season and 45 days in cold season in 1979 (Table 2). They were classified into two main types, type I and II (Table 4), on the basis of the general wind direction and the existance of the mountain and valley wind at Tsuchiyama in the eastern mountainous part of the Lake Biwa basin (Table 3).
2. The type I prevails when the general wind is westerly to north-westerly (Fig. 3), In this type, the land and lake breeze on the shore penetrates inland and is combined with the mountain and valley wind which has the same direction. Consequently, the combined wind diverges from the lake to the boundary of the Lake Biwa basin in the daytime and converges in the opposite direction in the nighttime (Figs. 4 and 6).
3. On the other hand, the type II is connected with the general wind of which direction is south-east to south (Fig. 3). In this type, the mountain and valley wind is not seen in the eastern part of the Lake Biwa basin. Moreover, the penetration of the land and lake breeze toward inland is limited, and effects of the general wind on the land and lake breeze is intensified in the eastern part of the Lake Biwa (Figs. 5 and 7).
4. A feature common to two types is that the lake breeze changes its direction clockwise on the eastern shore, but counter clockwise on the western shore of the Lake Biwa.