Temporal and spatial variabilities of stack plumes are analyzed by means of field data analysis and computer simulations. In this investigation, photographs from field programs of the study Visibility Impairment due to Sulfur Transport and Transformation in the Atmosphere were analyzed via sensitometry, and PLUVUE II model simulations were conducted for a case study. Analysis of the temporal variability of plume visibility led to documentation of the fact that the visual effects of plumes vary with time because of (1) turbulent fluctuations, (2) changes in atmospheric stability, and (3) changes in the observer-plume-sun scattering angle. Likewise, the analysis of the spatial variability of plume visibility led to the fact that the visual effects of plumes vary with downwind distance from the stacks because of (1) turbulent fluctuations, (2) dilution of the plume and/or a change in the observer-plume distance, and (3) changes in the observer-plume-sun scattering angle.