The formation and decay of the thermal spark generated by a single nanosecond high-voltage pulse between pin electrodes are characterized in this study. The influence of air pressure in the range 50-1000 mbar is investigated at 300 K. By performing short-gate imaging and optical emission spectroscopy (OES), we find that the thermal sparks exhibit an intense emission from excited electronic states of N+, in contrast with non-thermal sparks for which the emission is dominated by electronic transitions of N2. Spark thermalization consists of the following steps: (i) partial ionization of the plasma channel accompanied by N2 emission, (ii) creation of a fully ionized filament at the cathode characterized by N+ emission, (iii) formation of a fully ionized filament at the anode, (iv) propagation of these filaments toward the middle of the interelectrode gap, and (v) merging of the filaments. The formation of the filaments, steps (ii) and (iii), occurs at sub-nanosecond timescales. The propagation speed of the filaments is on the order of 104 m s−1 during step (iv). For the 1 bar condition, the electron number densities are measured from the Stark broadening of N+ and Hα lines, with spatial and temporal resolution. The electron temperature is also determined, from the relative emission intensity of N+ excited states, attaining a peak value of 48 000 K. In the post-discharge, the electron number density decays from 4 × 1019 to 2 × 1018 cm−3 in 100 ns. We show that this decay curve can be interpreted as the isentropic expansion of a plasma in chemical equilibrium. Comparisons with previous experiments from the literature support this conclusion. Expressions for the Van der Waals and resonant broadenings of Hα, Hβ, and several lines of O, O+, N and, N+ are derived in the appendix.