Climatology and Trends of U.S. Surface Humidity and Temperature

Abstract

Climatological annual and seasonal dewpoint, specific humidity, and relative humidity maps for the United States are presented using hourly data from 188 first-order weather stations for the period 1961-90. Separate climatologies were calculated for daytime (three observations per day between 0800 and 1600 LST), nighttime (three observations per day between 2000 and 0400 LST), and the full day (eight observations per day, every 3 h). With extended datasets for the period 1961-95, trends in these same variables and temperature are calculated for each of 170 stations and for eight regions of the country. The data show increases in specific humidity of several percent per decade, and increases in dewpoint of several tenths of a degree per decade, over most of the country in winter, spring, and summer. Nighttime humidity trends are larger than daytime trends. The specific humidity increases are consistent with upward temperature trends. The upward temperature and humidity trends are also consistent with upward trends in apparent temperature, a measure of human comfort based on temperature and humidity. Relative humidity trends are weaker than the specific humidity trends, but they do show evidence of increases, especially in winter and spring. The possibility that the detected trends may be artifacts of changes in instrumentation was examined, but several lines of reasoning suggest that they are not. Anthropogenic water vapor produced from fossil fuel consumption, both locally and globally, is too small a source to explain the observed trends.

A critical overview of pan evaporation trends over the last 50 years

“…But it is "attributed to a range of changes, from changes in the frequency of the various 'air mass' types influencing a station, to changes in the intensity of one or more these air masses, and to changes in the local vertical exchanges of energy and moisture" (Robinson 2000). The increase of dew point could be the result of higher humidity, which is partly caused by changes in evaporation rate (Robinson 2000), because dew point temperature is a function of specific humidity and atmospheric pressure (Gaffen and Ross 1999). As these changes are different from station to station, the dew points can be increasing at one station and decreasing at another, or increasing in a season and decrease in another (Robinson 2000).…”

“…Moreover, Gaffen and Ross (1999) indicate that night-time dew point temperature increases generally exceed the daytime increases whereas Robinson (2000) indicates that the rates of dew point temperature increases are generally higher in the daytime than the night. Therefore, it is hard to conclude that the dew point temperature must also increase faster than average temperature based on the observed increase in minimum temperature.…”

Evidence of urban-induced precipitation variability in arid climate regimes

“…Moisture advection into the preferred convergence region by southerly flow regimes could enhance convection. Figure 9 is taken from Gaffen and Ross (1999) and illustrates the sign and statistical significance level of station trends in dewpoint temperatures Td for each season based on full-day data. The trends are generally positive and statistically significant, particularly at Phoenix station.…”

“…The trends are generally positive and statistically significant, particularly at Phoenix station. Gaffen and Ross (1999) note that locally, anthropogenic modification of the hydrological cycle is important to consider. Within the conterminous United States, the U.S. Geological Survey has estimated that consumptive use of water in agricultural irrigation contributes 100 billion gallons of water per day to the atmosphere, compared with 2800 billion gallons per day from evaporation and transpiration from surface water bodies, land surface, and vcgetation.…”

Relating changes in agricultural practices to increasing dew points in extreme Chicago heat waves

“…Dew-point measurement techniques at firstorder NWS stations such as Chicago Midway have changed over time. Elliott (1995), Gaffen & Ross (1999), Robinson (2000b), andSparks et al (2002) documented that through the early 1960s sling psychrometers were primarily used. Then, from the early 1960s through the mid 1980s lithium chloride hygrothermometers were used, and from the mid 1980s through the mid 1990s the hygrothermometer model HO-83 was used.…”