High power microwave transmission is ultimately limited by window flashover at the vacuum-air dielectric boundary. While surface flashover in the presence of a vacuum has been studied in some detail, the mechanisms associated with flashover in an atmospheric environment need further investigation. For an aircraft based high power microwave system, atmospheric pressures ranging from 760 torr (sea level) to 90 torr (50,000 ft.) are of principal concern. The experimental setup uses a 2.85 GHz, 3 μs microwave pulse with a 10 to 90% rise time of approximately 600 ns from a magnetron capable of producing 5 MW. The slow rise time of the microwave pulse is sharply reduced by using a waveguide spark gap switch used for fast microwave reflection and a high power four port circulator . This reflected pulse has a reduced rise time on the order of 50 ns. The shorter rise time produces a more ideal step waveform that can be more easily compared with theoretical perfect square pulse excitation. Past investigations showed that the delay time for breakdown in air increases with pressure as is expected from the right hand side of the Paschen curve as long as the electron collision frequency is much larger than the microwave frequency. Surface flashover experiments have produced similar results. At a pressure of 155 torr, for instance, the breakdown electric field strength is 6 kV/cm (power density 0.08 MW/cm2) and the overall delay time from HPM pulse application to reaching critical breakdown plasma density is 600 ns. An overview of the experimental setup is given along with a discussion of breakdown delay times as a function of pressure as well as an investigation of surface flashover in the presence of external UV (ultraviolet) illumination.