Abstract. This paper examines trend uncertainties in layer-average free atmosphere temperatures arising from the use of different trend estimation methods. It also considers statistical issues that arise in assessing the significance of individual trends and of trend differences between data sets: Possible causes of these trends are not addressed. We use data from satellite and radiosonde measurements and from two reanalysis projects. To facilitate intercomparison, we compute from reanalyses and radiosonde data temperatures equivalent to those from the satellite-based Microwave Sounding Unit (MSU). We compare linear trends based on minimization of absolute deviations (LA) and minimization of squared deviations (LS). Differences are generally less than 0.05øC/decade over 1959-1996. Over 1979-1993, they exceed 0.10øC/decade for lower tropospheric time series and 0.15øC/decade for the lower stratosphere. Trend fitting by the LA method can degrade the lower-tropospheric trend agreement of 0.03øC/decade (over 1979-1996) previously reported for the MSU and radiosonde data. In assessing trend significance we employ two methods to account for temporal autocorrelation effects. With our preferred method, virtually none of the individual 1979-1993 trends in deep-layer temperatures are significantly different from zero. To examine trend differences between data sets we compute 95% confidence intervals for individual trends and show that these overlap for almost all data sets considered. Confidence intervals for lower-tropospheric trends encompass both zero and the model-projected trends due to anthropogenic effects. We also test the significance of a trend in d(t), the time series of differences between a pair of data sets. Use of d(t) removes variability common to both time series and facilitates identification of small trend differences. This more discerning test reveals that roughly 30% of the data set comparisons have significant differences in lower-tropospheric trends, primarily related to differences in measurement system. Our study gives empirical estimates of statistical uncertainties in recent atmospheric temperature trends. These estimates and the simple significance testing framework used here facilitate the interpretation of previous temperature trend comparisons involving satellite, radiosonde, and reanalysis data sets.

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This paper considers the use of upper-air data from radiosondes in long-term climate studies. The accuracy and precision of radiosonde humidity measurements, including temperature and pressure measurements used in calculating them, and their effects on the precision of reported and derived variables are estimated. Focusing on the U.S. radiosonde system, we outline the history of changes in instruments and reporting practices and attempt to assess the implications of such changes for studies of temporal variations in lower-tropospheric water vapor. Changes in biases in the data are highlighted, as these can lead to misinterpretation of climate change. We conclude that the upper-air data record for the United States is not homogeneous, especially before 1973. Because of problems with the humidity data in cold, dry conditions, the water vapor climatology in the upper troposphere, nominally above the 500-mb level, is not well known.

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.