Positive Leaders Propagate Slower at Higher Altitudes: Experimental Evidence and Theoretical Explanation

Abstract 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.