We analyzed hourly coterminous U.S. airport data, beginning in 1948, for changes in daily temperature and dewpoint regimes. We found an ubiquitous phase shift in the daytime cycle of warming, towards later in the day. Overall day and night temperature changes were very consistent with the results of Karl et al. [1993> even though this is an entirely different source of data, and one that is not specifically deurbanized, although we attempted to use stations that were not from downtown airports. We divided our results into eastern and western subsets as a first approximation for high and low sulfate aerosol conditions, and found evidence that was consistent with a sulfate effect on overall temperature, but inconsistent with modeled estimates of the effect of sulfates on the intradiurnal regime. It is difficult to establish the causes of the observed intradiurnal phase shift of warming time although urbanization and anthropogenic emissions are likely to be involved. The nature of our finding is subtle, and is consistent with other results that show very modest (and nonobvious) responses to greenhouse changes. Further research will ultimately clarify its causes and effects. ¿ American Geophysical Union 1996