Two-month periods as highlighted with blue horizontal bars in Figure 2: (a, d) October and November 2011; (b, e) May and June 2012; (c, f) February and March, 2014. (Upper row, a–c) As for the upper plot of Figure 2 with PREMOS-B groups 7–12 also plotted (see Table 1). (Lower row, d–f) As for the lower plot of Figure 2, with PREMOS-B groups 7–12 (see Table 1 also plotted; errors on 7 (red) have been propagated to groups 9–12 in (f). Error bars in (a)–(c) represent 68% confidence intervals, and 95% confidence intervals in (f). Grey dots with error bars represent dose-based estimates of exposure time based on Lyman-α.