Observations of the stratospheric plume from the May 18, 1980, Mount St. Helens eruption suggest that it spread in the crosswind direction as an intrusive gravity current, as it was transported downwind. Grain size analyses of the plinian tephra are consistent with this model, suggesting that to distances of many hundreds of kilometers, turbulent atmospheric diffusion played a secondary role in plume spreading and tephra dispersal. ¿ American Geophysical Union 1992