Nanosecond repetitively pulsed discharges can be classified into three types: the corona, the glow and the spark. The experimental parameters triggering the transition between these regimes are well understood. Corona and glow are non-equilibrium plasmas. The spark plasma can be in non-equilibrium or in equilibrium, the equilibrium regime being called hereafter the arc regime. The transition from the non-equilibrium spark to the arc regime for nanosecond discharges is not entirely understood. This is accompanied by a sharp rise of the gas temperature and electron number density that cannot be explained by conventional mechanisms. In this paper, spatially and temporally resolved emission was performed before, during and after the arc transition, which was found on a time scale below 1 ns. We compare the measurements of the electron number density and the electronic temperature before and after the transition. According to the present results, the electron number density rises from typical values of 10 15 to 10 19 cm-3 and the gas temperature increases from 1,000 K to 40,000 K. The presented measurements are intended to help explain the transition mechanism.