A small-scale microwave postdischarge at atmospheric pressure is studied. Its shape evolves from a straight beam of light to a diverging cone. By using the method of the synthetic spectrum, the mean temperature is shown to vary nonlinearly between 1000 and 2000K. Evaluation of the Reynolds number shows a linear dependence on flow rate and suggests a transition from laminar to turbulent flow. Simultaneously, the gas velocity increases and comes to exceed sound velocity. As the flow rate increases, three flow regimes are distinguished: laminar and subsonic, turbulent and transonic, and turbulent and supersonic.