Visibility and Some Elements in Meteorological Optics

Abstract

At the same time and place with the previous report (Jour. Met. Soc. Japan: Vol. 24 No.1_??_12), another party has executed some observations to examine the relation between the visibility and some other optical elements in meteorology which were not touched in the previous paper.
In the first place, the relation between the visibility _??_nd the intensity of the. ultra-violet rays was examined. The former was observed with the Wigand's visibility-meter, and the latter with Dr. Miyake's dosimeter. The results of observations show that as shown in Fig. 2, the seeing is better as the intensity of ultra-violet rays becomes stronger, just as was expected from the beginning. This better seeing seems to be more remarkable for white objects than for red objects.
In the second place, they have examined the efficiency of the red glass colour filter for improving the visibility for white and red marks, and found that as shown in Tab. 4, in which S₀ and S_R, are the visibility without and with the red filter respectively, the red filter rather diminishes the seeing of white objects, but it increases the seeing for red objects very remarkably. Especially, we have frequently experienced the fact that, when the fog covered the _??_ield and the objects can no more be perceived these re-appeared using the red filter, though the quantitative relation could not be de termined, because we had no method of classifying the mean density of the fog in the sight line as a whole. The difference of seeig without and with filter seems to _??_e large when the weather is bad compared with those of clear weather. Perhaps it is due to the change of contrast bet- ween the object and its background.
The daily variation of the difference between the eastern and western visibility S_E-S_W is very clear (Fig. 5). i. e. S_E-S_W is very small in the early morning, and very large in the evening. Both visibility S_E, S_W coincide at about noon when the position of the sun became symmetry with respect to these eastern and western marks. Further, examining the Fig. 2 in detail, we can find the seeing for southern objects are always smaller than those of the other directions for the same condition of the weather. The reason of which can not be easily explained, but it seems to be perhaps due to, one part at least, the instrumental causes, because the visibility in the case when the ground glass was shaded with hand n_??_ are remarkably large compared with those when the direct sunshine is incident upon the ground glass of the Wigand's meter n_??_ Tab. 5). It seems that this phenomena are the similar ones as the flre for the lenses. The ratio n-/n₀ is very large in the early morning and approaches to unity at about . 10_??_30^m when the sun's altitude is about 45°. Thus the Wigand's meter is only useful when the incident angle of the solar beam upon the ground glass exceeds 45°, for the small incident angle it must be used shading the ground glass by any means. It was a remarkable fact also that the seeing is better when the direct sunshine illuminates the field between the observer and the marks than when sunshine is shaded with cloud (Tab. 8).
As to the effect of contrast, numerious observations of visibility for mountain peaks were made, the background of these mountain peaks being always the free sky. For the classification of the blueness of the sky standard tint classes in 20 steps extending the Linke's 14 steps were made. The observation shows that when the weather is bad and the sky is appearing in grey colour, the visibility is sometimes very large and sometimes is very small, and it seems to be at random. Then, as the blueness of the sky increases the visibility increas s proportionally, but when the blueness becomes very thick, the rate of incr_??_ses becomes small and approaches asymptotically to some finit_??_ value.