Volume density of hydrogen plotted against v_LSR for our HVC absorber sample. The blue data points show the barycentre absorbers, the red data points indicate the anti-barycentre absorbers. The densities are taken from our Cloudy modelling. Upper panel: data points for the 49 HVCs with reliable n_H measurements are shown. Lower limits are indicated by light blue symbols and indicated by the arrow. HVCs with log n_H ≤ −3.54 are shown in dark blue. Lower panel: same as above, but data are binned in 50 km s⁻¹-wide velocity bins. For this plot, we regard the individual densities as fixed values (i.e. in this plot we ignore that densities may represent lower limits).