This paper presents a user-friendly graphically based numerical model of one-dimensional steady state homogeneous volcanic plumes that calculates and plots profiles of upward velocity, plume density, radius, temperature, and other parameters as a function of height. The model considers effects of water condensation and ice formation on plume dynamics as well as the effect of water added to the plume at the vent. Atmospheric conditions may be specified through input parameters of constant lapse rates and relative humidity, or by loading profiles of actual atmospheric soundings. To illustrate the utility of the model, we compare calculations with field-based estimates of plume height (~9 km) and eruption rate (>~4 W 105 kg/s) during a brief tephra eruption at Mount St. Helens on 8 March 2005. Results show that the atmospheric conditions on that day boosted plume height by 1--3 km over that in a standard dry atmosphere. Although the eruption temperature was unknown, model calculations most closely match the observations for a temperature that is below magmatic but above 1000C.