When dealing with the propagation of High Power Microwaves (HPM), special precautions must be used to prevent the onset of plasma generation. In this paper, we investigate the plasma located on the high pressure side of the dielectric boundary separating the vacuum environment of the microwave source from the high pressure environment of the transmitting medium, e.g., atmosphere. Because the collisional ionization rates are a monotonously increasing function of E eff/p in the range of interest, the effective electric field normalized with pressure, implementation of HPM in high altitude (low pressure) environments are subject to dielectric breakdown due to this generated plasma, more than at sea-level altitudes. Dielectric breakdown causes the interruption in transmission of electromagnetic radiation due to the reflection and absorption properties of the plasma generated on the dielectric surface. In this paper, transmission, reflection, and absorption data is presented for plasma generated under various pressures ranging from 5 to 155 torr in N 2 and air environments. In addition, seed electrons from UV illumination of the dielectric surface and physical vapor deposited metallic points are implemented and their implications to the overall transmission properties are discussed.