The diurnal temperature range (DTR) is the difference between the daily maximum and minimum temperature. Changes in DTR have multiple possible causes (cloud cover, urban heat, land use change, aerosols, water vapor and greenhouse gases). Different regions are affected by different factors. Some researchers say decrease of DTR is evidence of “climate change”. Others say the decrease in DTR leveled off in the 1990s.

http://appinsys.com/globalwarming/DTR.htm