Evolution of AR 10953 over five consecutive days. This sunspot was observed by Hinode every day from 30 April 2007 to 4 May 2007. The white contours indicate a continuum intensity of 50% of the quiet Sun level, the yellow contour shows a B_z = 1714 G evaluated at log τ = −0.9 (the threshold for the Jurčák criterion for our data, see Sect. 4.2. For comparison, we also indicate in red a value of B_z = 1867 G (the original value for the threshold for the Jurčák criterion). We are interested in the UP boundary of the main umbral core of this spot, which is highlighted by the label M in all panels.