Visibility and the Position of the Sun

[摘要] There is some reason that the visbility of a terrestrial object at any direction can never be independent of the position of the sun in the sky, because the degree of the molecular scattering in the sight line is different for each position of the sun. Moreover, this scattering is some function of wave length, and also the reflective power of the earth's surface and the penetrative power of the atmosphere is the function of the wave length, therefore it is supposed that the visibility of the distant ob_??_ect depends not only on the position of the sun, but also on the spectral distribution of the light which reaches the observer's eye. To examine the primitive relations which must exist between these elements, some observations were made during the summer month of the last year. For the sake of avoiding the complexity which may arise from the other causes above mentioned, and also to make the conditions of the visibility marks the same as possible, the topography of the observing station was carefully reconnoitreed, and Mt. -Haruna, a high plateau of about 1200m above the sea level, where the air is almost clear and the various dusts are very scantly, was selected as the most suitable station for this observation.Two kinds of visibility marks were used. One of them is a set of flags, each of which consisted with rcd, white and black colour of about 50cm, Three sets of such flags were made, and each of which was installed at three directions east, south and west at about 400m from the observer. At the background of these flags, the green grass of the same tint had been grewn.As the conditions for northern direction, especially with respect to the background, are somewhat different from those of other directions, so that the observation for this direction was not executed from the beginning. As the second kind of marks, the natural mountain peaks, which were properly distributed around the observer, were used.The observations were made with the Wigand's visibility meter with the glass colour filter of several kinds. From the plentiful materials of observation, which were made each 30 minutes from 8 o'clock to 16 o'clock during one month, it was remarkably perceived that the visibility for the eastern marks, flags or mountain peaks, is better in A. N than that of A. M, and for the western marks, in the contary, the visibility is better in A. M than that of A. N. It secms, as if the visibility is better when the sun situates as the back of the observer. This variation of visibility of more remarkable when the colour filters were used, especially the red filter was most effective. Even when the flags can not be perceived by the dense fog covering the field, it was clearly perceived by using the colour filter. For the southern marks, it is expected that the variation of visibility with time becomes symmetry with respect to the true noon. It reality, the results of our observations meet this expectation. But, opposite to the above general rule that the visibility is smaller in the direction facing the sun, it is the largest at true noon. The reason of which can not be easily explained, and perhaps it may be attributed to the fact that the sun's altitude at its upper culmination exceeds more than 70° for the latitude and season of our observation, so that such cases can be considered as the same as the case when the sun is behind us.

[学科分类] 大气科学