It is well known that DTR has seasonal maximums and minimums for a given year. These include spring and fall peaks and summer and winter dips. DTR peaks in the early spring because of increasing solar radiation and relatively long and clear nights but relatively low atmospheric moisture content. This condition allows sufficient rise in daily temperature and notable radiative cooling at night that leads to spring DTR peak. In the summer DTR dips because of higher moisture content in the boundary layer of the atmosphere, long day-time heating, and shorter nights; in the fall it peaks again much for same reason as spring and in the winter it dips again because long-nights and significant cloud cover which suppresses large DTR.

This study is using data from 95 USHCN stations in the United States east of the 100-degree West meridian to determine shifts in the seasonal peaks and dips in the 20th century. We suggest that due to recent warming of the atmosphere and resultant expansion of length of growing season would also modify the timing of these peaks and dips. As expected this study finds that shifts in the timing of DTR peaks and dips and the magnitude of these changes varies by the season. For example, in Kentucky analysis suggests that compared to pre-1950 there were -13, +2, -1, and -2 days shifts in spring DTR peak, summer DTR dip, Fall DTR peak, and winter DTR dip, respectively, for the post-1950 period. Hence, except for summer and compared to pre-1950 period, DTR peaks and dips occurred earlier during post-1950 years.