Due to recent advances in the peak output power densities and pulse widths of high-power microwave (HPM) devices, the ability to radiate this power into the atmosphere is limited by surface plasma formation at the vacuum-air interface. Very little is known about this window flashover under HPM excitation at "air" side pressures from atmospheric down to approximately 90 Torr, and this paper reports one such study at 2.85 GHz and MW cm2 pulsed power densities. Due to the high (∼600 GHz at standard temperature and pressure) elastic collision frequencies of the electrons with the neutral gas molecules and added energy-loss channels through molecule excitation, proven concepts of vacuum flashover, such as multipactoring electrons, have to be abandoned. The observed flashover field is roughly a factor 3 higher in SF6 compared to air, which is consistent with unipolar volume breakdown data. Quantitative comparisons of HPM flashover data with results from a recently developed computer code are given.