Universal time-Year variations in am averaged over all F for: (a) and (d) 1960–1979; (b) and (e) 1980–1999; and (c) and (f) 2000–2019. The top panels show the UT variation in the 90% quantile of the distribution of am values (q(0.9), cyan lines) and the 99.99% quantile(q(0.9999), blue lines). The dashed lines show the variation for the mean am. Bottom panels show the variations for mean observed am (black lines) and modelled am, am_m (mauve lines).