A mathematical model is proposed which allows the prediction of the contour, for a given concentration, of a plume caused by a release into the atmosphere of substances either heavier or lighter than air. The application of the model to different cases has given results close to those obtained from other models. Its introduction into the computer code of any dispersion model provides a tool useful in risk analysis and environmental assessment. (C) 2002 Elsevier Science Ltd. All rights reserved.