The paper summarises data and preliminary observations arising out of engineering and physical studies of airgap breakdown at a moderately high altitude (1425 m), outdoor high-voltage research laboratory in South Africa. It is shown that in the case of positive switching impulses, the application of the present standard IEC atmospheric correction procedures can yield results which lie above the expected ‘sea level’ trends. The relative influences of humidity (and air density sensitivity) upon airgap breakdown characteristics are also examined, both in engineering breakdown tests, as well as in studies of physical parameters of the discharge.