An appreciable number of nonurban stations in the United States and Canada have been identified with statistically significant (at the 90 % level) decreasing trends in the monthly mean diurnal temperature range between 1941 - 80. (skepticalscience.com)
These days of low diurnal temperature range occur naturally from time to time at individual locations. (bom.gov.au)
It is worth noting that the Bureau carried out extreme temperature analysis, namely the frequency of record - setting daily temperatures, while excluding all negative diurnal temperature range results. (bom.gov.au)