In the paper the long arc in free air is investigated. First, the arc is initiated under laboratory conditions in the high power test laboratory FGH-Mannheim (Germany). Based on the laboratory obtained data records, the main features of the arc are derived. By this arc voltage and current are processed. An important feature of the arc, the variation of arc length, is analyzed. Based on the analysis of arc voltage and current wave forms, the new dynamic arc model, taking into account the arc length variation, is derived. The arc is modeled as a current dependent voltage source with the characteristic distorted rectangular waveform and arc length dependent amplitude. The arc is considered as a source of higher harmonics distorting by this other signals in the network. An example of arc computer simulation using the new dynamic model is given. The main features of the simulated arc are also presented. Features of both the real and the simulated arc are given in the time and the spectral domains.