The diurnal cycle is useful as a testbed for model evaluation because it is related to many model parameterizations, and at the same time it is also a widely observed climate parameter. The daily average radiation budget and the availability of water for evaporation determine the daily mean temperature and relative humidity (in the absence of strong thermal advection). The diurnal temperature range, while related to many model processes, is most strongly coupled to the surface net long-wave flux [Betts, J. Geophys. Res., 2006]. In turn the net long-wave flux is a function of cloud cover and relative humidity, which is closely related to lifting condensation level. Model and flux tower data will be used to illustrate these relationships over land, and suggest strategies for model evaluation.