気温の平年値について

抄録

There has not been a difinite conclusion to determine the normal priod of mean monthy temperature, which is an important process in climatology and meteorology, though in 1935 International Meteorological Organization proposed thirty years from 1901 to 1930 as a nomal period. From the statistical point of view it is doubtful to adopt the same period strictly for each station. Because the confidence interval of population mean temperature differs generally from each other, even for the same period. Consequently it is more reasonable to use the mean temperature with the same confidence interval rather than for the same period.
In the first place the relationship between the confidence interval and the number of years is explained, and then the minimum period necessary to obtain the mean temperature with the definite confidence interval 0.5°C is calculated. Finally the characteristic distribution of the minimum period is discussed from the standpoint of dynamic climato-logy.
The variations of accumulative mean temperature and the correspond-ing confidence interval are shown in Fig. 1 to 3 and numerical expressions of confidence limit are given in Tab. 1 to 3. In general the mean monthly temperature ranges about 1°C with the increasing number of years. The range depends upon both season and latitude, greater in winter than summer as well as in higher latitudes than lower, and the seasonal difference is more outstandinding in lower latitudes. The decrease of confidence interval with the increasing period is considerably steep for ten or so years at first, while it runs more gradually thereafter. As the difference of interval, for instance, between thirty and sixty years amounts to only 0.1°C, it is meaningless in practical sense to adopt an extremely long period.
The minimum normal period increases with latitude in both seasons, especially increases abruptly in the boundary region between Central and Northeastern Japan. The geographical distribution of nomal period is shown in Fig. 4 and 5. According to such a discontinuous pattern, Japan may be devided into two regions, Northeastern and Southwestern, with respect to the temperature variability.
The meridional gradient of normal period is resemble to the one of temperature, being larger in Northeastern Japan. As the region with large temperature variability is expected to be found.in the region with large gradient of temperature, it is inferred that the characteristic cir-culation type controlling the temperature pattern over the whole region is the primary factor leading to the regional difference of normal period. In summer the boundary region corresponds to the southern margin of frequent cool summer damage, a serious climatic disaster in Northeastern Japan, which is indentified by the average position of daily front or the climatic front. In wintet it seems the pattern of nomal period is deter-mined by the temperature regime in the Siberian air mass itself.