Formation hieght of the radiation at (a) SB3.1 (τ_3.2 mm = 1.0), (b) SB3.4 (τ_{2.8 mm} = 1.0), (c) difference in height between SB3.1 and SB3.4, (d) SB6.1 (τ_{1.3 mm} = 1.0), (e) SB6.4 (τ_{1.2 mm} = 1.0) and (f) difference in height between SB3.1 and SB3.4. The selected locations A and B for the time-series analysis (Figs. 3–5) are marked for reference. The colour scale of the sub-band differences (c) and (f) are positive at all times (they are not absolute) and are capped to 1.0 Mm from 1.42 Mm at band 3 (c) and 1.17 Mm at band 6 (f) in order to reveal the small-scale structures.