Monthly mean maximum and minimum temperatures for over 50% (10%) of the Northern (Southern) Hemisphere landmass, accounting for 37% of the global landmass, indicate that the rise of the minimum temperature has occurred at a rate three times that of the maximum temperature during the period 1951–90 (0.84°C versus 0.28°C). The decrease of the diurnal temperature range is approximately equal to the increase of mean temperature. The asymmetry is detectable in all seasons and in most of the regions studied.

The decrease in the daily temperature range is partially related to increases in cloud cover. Furthermore, a large number of atmospheric and surface boundary conditions are shown to differentially affect the maximum and minimum temperature. Linkages of the observed changes in the diurnal temperature range to large-scale climate forcings, such as anthropogenic increases in sulfate aerosols, greenhouse gases, or biomass burning (smoke), remain tentative. Nonetheless, the observed decrease of the diurnal temperature range is clearly important, both scientifically and practically.

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*National Climatic Data Center, Federal Building, Asheville, North Carolina.

+Climatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, Norfolk, England

@Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York.

**Bureau of Meteorology, Melbourne, Australia.

++Research Institute of Hydrometeorological Information, Obninsk, Kalugu, Russia.

@@Climate Research Group, University of Wirswatersrand, South Africa.

***University of Washington, Department of Atmospheric Sciences and Institute for Environmental Studies, Seattle, Washington.