Two-dimensional PDFs of the gradient norms with respect to the values of the fields of every run for each field and different smoothings (R_f ∈ {1, 2, 6}, see each column). The first row corresponds to the 21cmFAST reionisation times, and the second row to their corresponding GRFs. The grey-scale lines are the isocontours of the histograms. Here, ν represents the value of the normalised reionisation times.