Diurnal cycle of PWV for in situ data (black markers) and ERA5 data (coloured markers). The ERA5 integrated PWV is shown as well as the ERA5 total column water output (‘tew’, blue). The grey and coloured shadings represent the standard deviation of the in situ and ERA5 data based on hourly averaged data, respectively. Time is in universal time (UT). In situ data are missing for Cerro Tololo, La Palma, Siding Spring, Sutherland and SPM. Data for La Silla is interpolated from 3-hourly data. The integration limit for the integrated PWV is 600 hPa for Mauna Kea (a, dark-yellow line), 750 hPa for Paranal (b, bright blue line) and 775 hPa for La Silla (c, green line).