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Results of spectroscopic investigation of an arc formed over an ice surface are presented. Direct-current positive voltage is applied to a flat ice surface with a 6-cm air gap to initiate electric discharge. Time-resolved spectra are taken during the propagation of arc along the surface. From the emission spectra, the profile of hydrogen-alpha from Blamer series is used to measure the electron density of the arc column. A method based on broadening of the Hα line considering instrumentation broadening is employed. The electron densities obtained by optical emission spectroscopy show an increase from 1 to 2 × 1017 cm-3 corresponding to current increase from 200 to 700 mA. The electron density at the flashover instant is calculated to be about 2.5 × 1017 cm-3.