The diurnal variation of the planetary boundary layer height (PBLH) over the contiguous United States (CONUS) is examined using Aircraft Meteorological Data Reports (AMDAR) at 54 major airports and the ERA5 reanalysis product. The bulk Richardson number method is used to derive the PBLH at hourly scales from the AMDAR profiles. It is found that the estimated PBLH is sensitive to the value of the critical bulk Richardson number and to the consideration of the surface friction effect, especially under stable and near‐neutral conditions. Parameters constrained by field campaign observations are finally selected to estimate the PBLH. The results reveal that the diurnal climatology of PBLH exhibits seasonal and spatial variations, with greater diurnal variations in spring and summer and in the western regions. It is demonstrated that the traditional twice‐daily radiosonde data may roughly capture the diurnal amplitude of PBLH in the western but not the eastern CONUS. Compared to the AMDAR data, the ERA5 reanalysis product overestimates the daytime PBLH throughout the CONUS by 18–41% and shows an earlier peak time by 1–2 hr in the western regions and a later peak time by 1 hr in the eastern regions. Key Points The estimated PBLH is sensitive to the critical Rib and surface friction effect in the computation with the Rib method The diurnal climatology of PBLH revealed by AMDAR profiles exhibits expected seasonal and spatial variations The ERA5 reanalysis product overestimates the daytime PBLH and fails to capture its peak time