Modelling studies are presented of positive streamers developing at various pressures in air in a point-to-plane discharge geometry. The modelling results emphasize that the quenching of the singlet excited states of molecular nitrogen emitting photoionizing radiation is responsible for non-similar behaviour of streamers at pressures higher than approximately 30 Torr. Our modelling results are consistent with recent experimental work showing that streamers have more and thinner channels and

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... ch more frequently at higher (i.e. near atmospheric) pressures than at lower pressures. The results also demonstrate the importance of accounting for effects associated with electrode geometry for the interpretation of experimental studies on similarity properties of streamers at various pressures.