Delay times of high power microwave surface flashover are affected by radiation illuminating the dielectric. A controlled environment of pure Argon at a range of low pressures as compared to normal atmospheric pressure was used with 2 mW/cm2 UV-radiation illuminating the test window. Argon was chosen due to its relatively small number of processes involved such as inelastic electron collisions and due to the well-known cross-sections for these processes. Delay times in the presence of UV are significantly shorter than without UV illumination. The initial electron density contribution due the UV source is very roughly estimated to be -106 cm-3. A small admixture of radioactive Krypton-85 showed only marginal changes in the observed delay times, likely due to an insufficient concentration of Kr-85 producing ionization events only every few microseconds and the high energy distribution associated with the emitted electrons. A detailed discussion of experimental breakdown delay data, along with theoretical expectations and discussion of the statistically dependent mechanisms and analysis, will be given. The ultimate goal is to develop a model for HPM window breakdown in a UV environment, to describe the role of discharge initiating electrons, and to quantify breakdown at high altitudes.