Gradient spectra of T_eff derived from the Kurucz model at different effective temperatures, as marked in the figure. We assume log g = 4.5 and [Fe/H] = 0. The A0 dwarf (T_eff = 9500 K) has the minimal gradients in hydrogen lines (close to 0), and they form an inflection point in the gradient of the hydrogen lines as a function of T_eff, which leads to the artificial A0 star desert of the stellar distribution in the T_eff-log g diagram as seen in Fig. 9.