This study estimates the amplitude and phase of the climatological diurnal cycle of temperature, from the surface to 10 hPa. The analysis is based on four-times-daily radiosonde data from 53 stations in four regions in the Northern Hemisphere, equatorial soundings from the Tropical Ocean Global Atmosphere/Coupled Ocean Atmosphere Response Experiment, and more recent eight-times-daily radiosonde data from the Atmospheric Radiation Measurement program's Central Facility in Oklahoma. Our results are in general qualitative agreement with earlier studies, with some quantitative differences, but provide more detail about vertical, seasonal, and geographic variations. The amplitude of the diurnal cycle (half the diurnal temperature range) is largest (1 to 4 K) at the surface. At 850 hPa and above, the regional-average amplitudes are <1 K throughout the troposphere and stratosphere. The amplitude of the diurnal cycle in the boundary layer is larger over land than over ocean, and generally larger in summer than winter (except for monsoon regions, where it is larger in the dry season). In the upper-troposphere and stratosphere, land-sea and seasonal differences are not prominent. The diurnal cycle peaks a few hours after local noon at the surface, a few hours later at 850 hPa, and somewhat earlier in the upper troposphere. The timing of the diurnal cycle peak in the stratosphere is more uncertain. Radiosonde data are also used to simulate deep-layer mean temperatures that would be observed by the satellite-borne microwave sounding unit, and the amplitude and phase of their diurnal cycles are estimated. An evaluation is made of the uncertainty in these results due to the temporal resolution of the sounding data, which is only barely adequate for resolving the first harmonic of the diurnal cycle, the precision of radiosonde temperature data, and potential biases in daytime stratospheric temperature observations.