Urban-related warming and drying trends are present in the data, but their effect is minimal. Moistening has occurred more » throughout the central United States while other regions have experienced drying. For the nation, trends in relative humidity show little change for the period 1947–2010, during which these data are more homogeneous. This moistening is especially pronounced during the summer in the Midwest. Trends since 1947 indicate that the warming of temperatures has coincided with increases in dewpoints and a moistening of specific humidity. Therefore, long-term decreases in relative humidity, which are strongest in winter, need to be viewed with caution. There is evidence of inhomogeneity in the relative humidity record that primarily affects data from prior to 1950. Average long-term trends (1930–2010) suggest that temperature has warmed but that little change has occurred in dewpoint and specific humidity. The effects were the same whether the adjustments were based on statistically detected or fixed-year breakpoints. Temperature trends were tempered by adjusting the data, whereas dewpoints were unaffected.
Dewpoints contained higher breakpoint detection rates associated with instrumentation changes than did temperatures. Due to numerous systematic instrument changes that have occurred, a homogeneity assessment was performed on temperatures and dewpoints. hourly surface observations are investigated at 145 stations to identify annual and seasonal changes in temperature, dewpoint, relative humidity, and specific humidity since 1930.
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