Response functions of the intensity profile of the Ca II 8542 Å line during the development of a UF. Panel a: response function of Stokes I to the temperature at the core of the line (dashed line) and to the velocity at the wavelength where the response is maximum (8542.2 Å, solid line) at t = 1405 s (see profile in Fig. 2k). The lower axis corresponds to the optical depth and the upper axis to the geometrical height. Panel b: optical depth of the maximum of the response function to temperature. Panel c: geometrical height of the maximum of the response function to temperature. The color-shaded regions in panels b and c have the same meaning as in Fig. 2.