 The temperature inside the lightning channel, measured by spectral analysis, varies during its 50 μs existence, rising sharply from an initial temperature of about 20,000 K to about 30,000 K, then dropping away gradually to about 10,000 K. The average is about .This heating causes a rapid outward expansion, impacting the surrounding cooler air at a speed faster than sound would otherwise travel.The resultant outward-moving pulse is a shock wave,
similar in principle to the shock wave formed by an explosion, or at the front of a supersonic aircraft.