Sensitivity studies were performed with a complex nonhydrostatic volcano plume model that explicitly treats turbulence and microphysics. The impact of environmental conditions such as wind, temperature and humidity profiles was studied for standard observational data. To investigate the wind effects, a two-dimensional Cartesian formulation of the model was used, while for the temperature and humidity effects a cylindrical coordinate system had to be applied, since this treats the entrainment process more realistically. It was found that horizontal wind generally reduces the height of the ash plume. The gaseous part of the plume sometimes may rise higher than without wind owing to the more effective separation between gas and solid material. Besides reduced static stability, the absolute temperature and humidity also increase the plume height. All environmental impacts strongly depend on the strength of entrainment and thus on the quality of the prognostic turbulence. ¿ 1999 American Geophysical Union