1978 Volume 27 Issue 12 Pages 709-714
Since Japan belongs to the temperate monsoon climate, thee seasonal difference of the origin of air mass is very clear. Thus the tritium concentration in precipitationn changes seasonally ccording to the origin of air masses.
The author measured the tritium concentration in precipitation of Tokyo for every ten-day period from August 1972 to May 1974. Judging from the daily synoptic weather chart, the rainfall-inducing air masses in Japan were classified into five types; polar maritime air mass (Pm), polar continental air mass (Pc), tropical maritime air mass (Tm), tropical continental air mass (Tc), and equatorial maritime air mass (Em) (Fig. 3) . And the precipitation for every ten-day period sampled for tritium measurement were classified into these five types (Table 2) .
Based on this classification, it is confirmed that there exist clear difference in the tritium concentration between the rainfall from the continental air mass and ones from the maritime air mass (Table 3) . It is characteristic that the tritium concentration in rainfall induced by.equatonal maritime air mass such as typhoon in summer and early fall season is very low whereas the tritium concentration in rainfall and snowfall induced directly by the polar continental air mass in late winter season is very high. The regional difference of the tritium concentration in intermonthly precipitation could considerably be explained by this synoptic meteological classification of rainfall-inducing air mass.
In spite of these regional differences rences of tritium concentration in precipitation, use of the tritium concentration of Tokyo as a representative value of Japan may be allowed because of the similarities of the changing pattern and annual mean tritium concentration.
The time series variations of tritium concentration in precipitation of Tokyo from August 1972 to December 1977, Tsukuba from December 1976 to Apri1 1978, and Nagaoka from April 1977 to March 1978 are listed in Fig. 2.