Some statistics have been made to see the general tendency of horizontal visibility by the data of recent two years observed at the Tôkyô Air Port of Haneda.
The author has obtained monthly mean values of visibilities at 6, 10, 14, 18, 22h as is shown in Table 1 and those for four directions in Table 2. They are expressed by visible mean distance in km. and there is the following relation international scale and that the author used.
The main results obtained from the present investigation are as follows:-
(1) Diurnal variation: Visibility becomes the worst in the morning, best in the daytime and rather good in the night compared with the early morning. When the monsoon develops in winter, it becomes sometimes very good on the west even in the early morning.
(2) Annular variation: Good in winter and summer, bad in spring and baiu or rainy season of summer and rather bad in the early winter.
(3) Variation due to the direction: The worst on the north (direction for the Tôkyô city), the best on the south (for the Bay of Tôkyô), rather good in winter on the west and good in summer on the east.
