As diode pumped alkali lasers (DPAL) are scaled to powers exceeding 1 kW, the effects of atmospheric transmission,
including thermal blooming, is explored. The cesium and rubidium lasers operate near 894 and 795 nm, respectively, in
the vicinity of atmospheric water vapor absorption lines. The potassium laser line lies in the high rotational limit of the
O2 X-b (0,0) band near 770 nm. We assess the effects of atmospheric transmission on high power propagation using the High Energy Laser End-to End Operational Simulation. HELEEOS uses the scaling laws of the Scaling the High energy laser And Relay Engagements (SHaRE) toolbox which is anchored to the wave optics code WaveTrain and all significant degradation effects, including thermal blooming due to molecular and aerosol absorption, scattering extinction, and optical turbulence, are represented in the model. The HELEEOS model enables the evaluation of uncertainty in low-altitude high energy laser engagements due to all major low altitude atmospheric effects to include physically-based representations of water clouds, fog, light rain, and aerosols. Worldwide seasonal, diurnal, and geographical spatial-temporal variability in key climatological parameters is organized into probability density function databases in HELEEOS using a variety of recently available resources to include the Extreme and Percentile Environmental Reference Tables (ExPERT) for 408 sites worldwide, the Surface Marine Gridded Climatology (SMGC) database which provides coverage over all ocean areas, the Master Database for Optical Turbulence Research in support of the Airborne Laser, and the Global Aerosol Data Set (GADS) used to provide worldwide aerosol densities. The spectral transmission model is anchored to field data from an open-path Tunable Diode Laser Absorption (TDLAS) system composed of narrow band (~300 kHz) diode laser fiber coupled to a 12" Ritchey-Chrétien transmit telescope. The ruggedized system has been field deployed and tested for propagation distances of greater than 1 km. The TDLAS approach achieves a minimum observable absorbance of 0.2%, whereas an FTIR instrument is almost 20 times less sensitive.