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Results from experiments on long positive streamers in atmospheric air under lightning impulse voltage are presented. The length of the rod-plane gap is 57 cm. The peak voltages applied to the gap are+ 210–290 kV. The voltage and current are measured through a synchronized measurement system. The streamer diameters are obtained by the analysis of photographs taken by an intensified CCD (ICCD) camera. The four continuous photographs of single streamer discharge allow us to investigate the variation of the streamer diameters with time. For the hemispherical electrode 2 cm in diameter the diameters vary in the range 1.6–6.3 mm when the streamer length is 2–16 cm. The streamers are thicker when the electrode diameter is doubled from 2 to 4 cm. There is a downward trend in the streamer diameter with the propagation of the streamer heads. Their diameters for higher voltage decline at a lower rate. A …