The formation of thermal sparks generated by nanosecond pulses is studied experimentally and numerically. The increase of the electron number density, up to 10 19 cm-3 , is measured with sub-nanosecond resolution using Stark broadening of N + , Stark broadening of Hα, continuum emission of the electron, and the intensity of N + lines. Our experimental results are then compared to 0-D kinetic simulations including the electron-impact ionization of excited electronic states of N and O. The electron temperature, in equilibrium with the gas temperature, reaches 42,000K, which agrees with our experimental findings and the literature on the thermal spark.