Abstract
Calms or breezes are very important to understand air pollution, especially from the viewpoint of the potentiality of spontaneous clarification of atmosphere itself. Little has been studied on calms or breezes, nevertheless many climatological researches have been done on-gales, storms and damages caused by them. This paper intends to make clear (i) the rela-tionships between calms (less than 0.5 m/sec) and atmospheric stabilities in the Ichihara area, Chiba Prefecture, (ii) geographical distribution of calm-ratios (percentages of calm-frequency to total observations of wind), and (iii) explanation of some causes of calm.
The results obtained are summarized as follows:
i) The diurnal variations of calm frequencies and atmospheric stabilities are shown in Figs. 1 (a) and (b) for winter and in Figs. 1 (c) and (d) for summer. In general, calms occur very frequently from late evening to early morning when the atmospheric layer is stable ; namely, in the case of temperature inversions or smaller lapse rates. This tendency is remarkable in winter. Although Ichihara faces Tôkyô Bay, the dawn calms are not so indistinguishable because there is no obvious exchange of sea and land breeze.
ii) The annual fluctuation of calm-ratio was compared with that of inversion-ratio in Fig. 2e The latters mean percentages of inversion frequencies to whole measurements of ver-tical air temperature. The correlation coefficient between them is fairly high (0.749) and the regression curve is expressed as
I=0.80C-0.86
where I and C are inversion-and calm-ratio respectively.
iii) Tables 1_??_4 show the relation of calm-ratio to inversion-ratio mainly using the data S for Sendai, Miyagi Prefecture. It can be seen that both calm and inversion decrease with increasing altitude as the annual mean conditions, but calm-ratio at the 1000m-level in sum-mer is comparatively larger while inversion ratio at the 500m-level in spring and at 1000m in summer are rather greater than at the surface-level. With respect to the daily variation, calms appear more often at the 1000m-level than at the 500 m-level and the surface-level. In short, the atmosphere over the urban area very frequently shows a higher calm-ratio than the surface-level, which is also confirmed by the vertial distribution of SO2-concentration, etc.
iv) Local phenomena of geographical distribution of calm-ratio are discussed in order to verify the influence of surface friction to the wind. Twenty-four maps in Fig. 3 present the horizontal distribution of calm-ratio in the Ichihara area. Generally speaking, calm-ratio (C) decrease as the distance from seashore increases. Thus the following equations are obtained:
For winter (January, 1967 and 1968),
C=4.5D+14.1 and
For summer (July, 1966 and 1967),
C=2.7D+20.1
where D is the distance (km). In Fig. 5, the relations of winter half-year are shown. The initial conditions at the coast (D=0) are larger in summer than in winter while the regional differences are smaller in summer than in winter.
v) Geographical distribution of calm-ratio in Japan is given in Fig. 6. It is evident that the calm-ratio is smaller in coastal regions and larger in inland areas particularly in basins. As exceptional cases, the regions facing the inland seas or bays have large calm-frequencies.
