A form of spark discharge preceded by a train of streamers of increasingly large current which occurs in air at atmospheric pressure at voltages as low as 5 kV per cm of gap length is investigated. A mechanism involving the development of a heated channel of air in the gap is proposed to explain the increase of streamer size in the discharge. This mechanism is verified by means of a scanning interferometer which measures the phase shift caused by the altered index of refraction of the heated air. The dimensions and temperature of the heated channel are such that the peak phase shift produced is about one‐tenth of one wavelength. Analysis of the phase shift data permits the growth of the heated channel to be observed as a function of time and space. Further experimentation involving time‐resolved photography of the streamers is used to demonstrate a correlation of the streamers paths with the development of the heated channel. An appendix is included describing a numerical method for solving the Abel integral equation which was used to analyze the inteferometer data.