The warm season dip in diurnal temperature range over the eastern United States

In light of numerous studies documenting a decline in the diurnal temperatu re range (DTR) over much of the globe, some authors have in recent years ex amined the annual march of the DTR in an effort to understand better the fa ctors that influence the DTR's seasonal variations. These papers show that, over the southern two-thirds of the eastern United States, the DTR's clima tology features peaks in spring and autumn and minima in winter and mid-to- late summer. However, the factors responsible for these characteristics rem ain uncertain. In this study, the DTR climatology of the eastern United Sta tes is analyzed in detail using daily surface and 850-mb data, with emphasi s on possible relationships to seasonal changes in vegetation. It is shown that the warm season dip in the DTR deepens and widens from north to south across the study area, in accordance with a lengthening of the growing seas on. Furthermore, the dip is particularly prominent in the annual march of t he DTR on mostly sunny days, indicating that seasonal variations in cloudin ess are not responsible for this feature. The climatologies of daily maximu m and daily minimum temperatures are found to be very different from each o ther: the former flattens out after the springtime peak in the DTR whereas the latter exhibits a pronounced mid-to-late summer maximum. These findings suggest that, by inhibiting daytime surface heating, evapotranspiration fr om vegetation contributes significantly to the dip in the DTR during the wa rm season in the eastern United States.