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In the design of high-altitude high-voltage transmission systems and in the optimization of the reliability of power sources in aeronautical environments, it is important to have reliable data concerning the effect of air density and humidity on reducing air insulation performance under local conditions. This is helpful also in the modeling of discharges and lightning in mountainous regions. This paper describes a systematic laboratory investigation of the combined effects of humidity and gas density on the breakdown strength of a 0.2 m rod/plane air gap that is subjected to positive-polarity lightning impulses, using a test chamber to reproduce the conditions commonly found in such regions. The results show that there are appreciable deviations between the measured humidity and air density correction factors and those of the 1973 and 1989 IEC Standards when extended to low air density. The paper proposes the adoption of a radically different correction procedure, based upon the influence of density and humidity on streamer propagation field, and including the effect of the high gradient in the anode region.