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We provide the clearest experimental evidence to date that positive leader velocity decreases with the decline of air density in the troposphere. The experiments are conducted at altitudes of 4,300 m in Tibet and 53 m in Beijing using the long air gap discharge technique. Rigorous comparative studies are performed using synchronized measurement of positive leader velocity and current under different air densities and similar temperature and humidity conditions. The effect of reduced air density on positive leader velocity is analyzed with a detailed thermal‐hydrodynamic model. It is found that the widely used similarity law setting initial leader radius inversely proportional to air density does not hold in estimating positive leader speeds in the troposphere. Plain Language Summary: The effect of reduced air density on leader discharge is a fundamental concern in several subjects such as lightning physics, atmospheric electricity, and electrical engineering. Increasing observation results have shown that lightning leaders seem to propagate slower at higher altitudes. To rigorously investigate the effect of reduced air density on positive leader velocity, electrical discharge experiments were performed at an altitude of 4,300 m in Tibet under the basis of excluding other influencing factors, such as discharge current and other atmospheric conditions. By comparing measurements near sea level and at high altitudes under similar temperature and humidity conditions, we provide reliable evidence to confirm that the positive leader velocity decreases with the decline of air density. Based on a thermal‐hydrodynamic model, it is found that the leader velocity calculated with the use of similarity law deviates significantly from the experimental measurements. A revised scaling formula for the initial conditions of leader stem at reduced air density is proposed. Our work can motivate scientists to revisit the use of similarity law in analyzing lightning discharges and upper‐atmospheric discharges such as gigantic jets. Key Points: We report a rigorous experimental study confirming that positive leaders propagate slower at higher altitudesBased on an improved thermal‐hydrodynamic model, the effect of air density on leader velocity is discussedOur study indicates that the similarity law for leader radius needs to be revised for electrical discharges in the troposphere [ABSTRACT FROM AUTHOR]