Since the Clean Air Act's passing in 1963 as a landmark piece of legislation aimed at controlling air quality at the national level, research has indicated that "the mean wet deposition flux... has decreased in the U.S. over time" since its enactment. Even then, however, the same research indicated that measurements in the amounts of chemical pollutants contaminating rain, snow, and fog "follows an exponential probability density function at all sites".[1] Such a finding suggests that alleged variability in rainfall patterns is the likely driving factor for the study's seemingly promising results, as opposed to there being a clear significance stemming from the policy change.[34] It is within this context that while beneficial, the Clean Air Act alone cannot stand as the only firm rationale for climate policies in the United States moving forward.[59]