Leo James Rainwater was born on December 9, 1917, in Council, Idaho, the son of a former civil Engineer who ran the local general store, Leo Jaspar Rainwater and his wife Edna Eliza née Teague. He never used his first name and was always referred to as James or Jim. His father died in the great influenza epidemic of 1918 and Rainwater and his mother moved to Hanford, California, where she married George Fowler, a widower with two sons, Freeman and John. In time he also acquired a half-brother, George Fowler, Jr., who became naval officer. At high school he excelled in mathematics, chemistry and physics and was admitted to the California Institute of Technology on the strength of a chemistry competition. He received his Bachelor of Science degree as a physics major in 1939.