The leader propagation is one of the most important stages in long air gap discharge. The mechanism behind leader re-illumination remains unclear. In high humidity conditions (20.0-30.1 g/m(3)), we have conducted experiments of long sparks in a 10 m ultra-high voltage (UHV) transmission line gap under switching impulse voltages. The positive leaders predominantly propagate discontinuously, with almost no significantly continuous propagation occurring. The leader channels are intensely luminous and each elongation segment is straight, with streamers resembling the "branch type" which differs fr...
