10 TI. A. Sr” in Chicago Rains Summary of Chicago Rain Data, 1953 through 1955 . Table 1 shows the estimated monthly and yearly totals of sr90 deposited per square foot in the Chisago area, together with totals for rains actually measurede The 1953 rain samples were roof run-off samples collected by the Chicago Tritium Research Group. For these, it is assumed that the result in disintegra- tions per minute was constant for each rain, and the Sr70 deposited per square foot was computed from the total precipitation in inches reported by the Weather Bureau for the University of Chicago station, The 1954 samples were individual rains collected in a galvanized wash tub on the roof of the Jones Chemistry Laboratory building, For these, the total sample activity divided by the collection area was taken as the Sr90 deposited per square foot for individual rains, For both 1953 and 195rains, the total monthly deposit was estimated by multiplying the deposit in disintegrations per minute per square foot per inch of rain for the rains masured by the total monthly precipitation in inches. Table 2 shows the total monthly deposits for 1955 rains, At all three locations the 1955 collections were made in galvanized tubs, and the total monthly precipitation was taken, For these, the deposit per square foot was computed from total sample activity divided by collection area. The 1955 collections were made with the tubs exposed continuously and thus any dry material falling out between rains was included in the sample.