• anticorrelation with the solar activity cycle as measured by sunspot numbers; and
• The 22-year cycle in the amplitude of the diurnal variation.

His pioneering work provided the empirical foundations and inspiration for an immense amount of subsequent research. For the most part, he made only tentative forays into the detailed theory of the relationships he established. Nonetheless he was thoroughly aware of the speculative interpretations of others and embraced them enthusiastically insofar as they were consistent with his perception of the facts. This characteristic of his work is well illustrated by his classical review article ''Time-Variations of Cosmic Rays" in the Handbuch der Physik (1966).

Forbush was born in 1904 on a farm near Hudson, Ohio. His boyhood was typical of the rural Midwest during that epoch, walking back and forth to a country school 2 miles from his home and taking over a progressively increasing share of the farm chores. His mother, a teacher, nurtured his curiosity and keen interest in learning and enrolled him in the nearby Western Reserve Academy, from which he graduated second in his class in 1920. A year later he entered the Case School of Applied Science in Cleveland and graduated in June 1925 with a major in physics. He then tried graduate study in physics at Ohio State University for a brief period, but he visualized observational geophysics as much more appealing than pure physics and sought employment in that field. He resumed formal graduate work later but with a fresh appreciation of its direct applicability to his research.