Complex sentence: Downstream (i.e. external to the nozzle) the flow is free to expand to supersonic velocities; however Mach 1 can be a very high speed for a hot gas because the speed of sound varies as the square root of absolute temperature.
Split into simpler sentences: Downstream (i.e. external to the nozzle) the flow is free to expand to supersonic velocities. Note that the Mach 1 can be a very high speed for a hot gas; since the speed of sound varies as the square root of absolute temperature.