Between 1946 and 1963, the United States Atomic Energy Commission (AEC) conducted over two-hundred atmospheric nuclear weapons tests at the Nevada and Pacific test sites. Most historical studies of the fallout controversy during this period have centered on the scientific debate over the hazards associated with low-level radiation exposure. While such studies have contributed to our understanding of one of the most salient issues driving the fallout controversy, they have also tended to underplay equally important scientific debates about fallout, particularly questions concerning the environmental mechanisms through which fallout from the tests might reach human bodies. This paper will trace the evolution of AEC sponsored meteorological research of the atmosphere culminating in the controversy between U.S. Weather Bureau scientist Lester Machta and AEC Commissioner Willard Libby in the late 1950s over the meteorological conditions mediating the deposition of fallout. In particular, the paper will focus on two claims made by Libby that comprised the controversy: the first, Libby's assertion that H-bomb fallout deposited uniformly around the globe and, second his claim that the stratosphere acted as a kind of natural storage barrier for fallout which allowed long-lived fallout radioisotopes such as strontium 90 to decay significantly. The paper will conclude by exploring how Machta's opposing and prevailing views helped to usher in new understandings of the atmosphere as a dynamic and vertically integrated space and how, ironically, fallout as an experimental tool utilized by meteorologists to trace the atmosphere emerged as a critical practice through which the controversy came to a close.