宇宙成因的天然氚和热核试验所引入的人工氚均以HTO形式参与自然界的水分循环。热核试验以前降水中的氚含量极微,平均值为5—10 T.U.;1954年以后,历次的核试验产生的大量人工氚对降水氚含量有明显影响。1963年降水中氚含量达到历史最高水平,北美局部地区高达10000 T.U., 到了七十年代,北半球降水中氚浓度明显下降,一般为几十到几百 Artificial tritium introduced by natural tritium and thermonuclear tests of cosmic origin are all involved in the natural circulation of water in the form of HTO. Before the thermonuclear test, the tritium content in precipitation was minimal with an average value of 5-10 T.U. After 1954, the large amount of artificial tritium produced by previous nuclear tests had a significant effect on the tritium content of precipitation. In 1963, the tritium content in precipitation reached its highest level in history, with some areas in North America reaching as high as 10,000 T.U. By the seventies, the tritium concentration in precipitation in the northern hemisphere decreased significantly, generally from tens to hundreds