Observed changes in the diurnal dewpoint cycles across North America

We analyzed the record of hourly dewpoint data from 92 climatological stations in North America. The data were centered with respect to sunrise and sunset and divided into four geographic regions of approximately equal area in an attempt to uncover regional trends. Few statistically significant regional trends in mean dewpoint temperatures were found. However, the within‐day variations were very well behaved and consistent across regions. They showed a relative decline in dewpoints in the late afternoon at most stations, and small declines just after sunrise. The results are consistent with urbanization signals described by Oke [1987]. An observed rise in midday dewpoint, followed by a fall in late afternoon, indicates some regional enhancement of convection, which may in part be from the urban effect, but other causes are possible. Our dewpoint changes are somewhat consistent with precipitation changes that have described by other researchers.