This analysis of AOSERP sigma data is an attempt to synthesize the available plume dispersion data from the AOSERP study region to derive a useful procedure for predicting the plume sigma values. The report includes a critical review of many of the more widely known sigma specification schemes and an analysis of the characteristics of the sensor systems used to collect the data. With such a background, the discrepancies which existed between the measured plume sigma values and the predicted values were more understandable. The plume dispersion process was treated as a two-stage process with a source-dominated phase and an environmentally dominated phase. In the source-dominated phase, the plume rise and plume geometry are intimately related. It was found that the more recent formulations suggested by Briggs (1975) worked adequately for averaged values. The transition point from the source-dominated to environmentally dominated phases of dispersion (the sigma transition) could be clearly specified theoretically but would be difficult to do reliably in an operational or climatological mode.