In this study, hourly data for the 1951-1990 period for 178 stations in the coterminous United States were used to establish temporal trends in dew point temperature. Although the data had been quality controlled previously (Robinson, 1998. Monthly variations of dew point temperatures in the coterminous United States. International Journal of Climatology 18: 1539-1556), comparisons of values between nearby stations suggested that instrumental changes, combined with locational changes, may have modified the results by as much as 1 degrees C during the 40-year period. Nevertheless, seasonally averaged results indicated an increase over much of the area, of slightly over 1 degrees C/100 years in spring and autumn, slightly less than this in summer. Winter displayed a drying of over 1 degrees C/100 years. When only the 1961-1990 period was considered, the patterns were similar and trends increased by approximately 1-2 degrees C/100 years, except in autumn, which displayed a slight drying. Analyses for specific stations indicated periods of both increasing and decreasing T-d, the change between them varying with observation hour. No single change point was common over a wide area, although January commonly indicated maximum values early in the period in the east and west, and much later in the north-central portion. Rates of increase were generally higher in daytime than at night, especially in summer. Investigation of the inter-decadal differences in dew point, as a function of wind conditions, indicated that changes during calm conditions were commonly similar in magnitude to that of the overall average changes, suggesting an important role for the local hydrologic cycle in driving changes. Other inter-decadal changes could be attributed to the changes in the frequency and moisture content of invading air-streams. This was particularly clear for the changes in north-south flow in the interior. Copyright (C) 2000 Royal Meteorological Society.
