The concentration of 238U in rain and snow collected at Fayetteville [Arkansas, USA] (36.degree.N, 94.degree.W), showed a marked increase during the summer months of 1980 and also during spring 1981, while Mount St. Helens remained active. This observed increase of 238U was due to the fallout of natural U from the eruption of Mount St. Helens. Based on the results from the analysis of rain and snow samples for the period March 1980-Oct. 1982, the amount of 238U brought down by the rain is estimated to be 6.55 .times. 103 Ci.