Ash clouds from a volcanic eruption-invisible to radar and nearly indistinguishable from weather clouds-pose a serious hazard to aviation safety. This article describes a system developed by the Alaska Volcano Observatory and the Arctic Region Supercomputing Center for predicting and visualizing the movement of ash clouds. Using meteorological and geophysical data from volcanic eruptions, a supercomputer model provides predictions of ash cloud movements for up to 72 hours. The AVS (Ash-cloud Visualization System) controls the execution of the ash cloud model and displays the model output in 3D form, showing the location of the ash cloud over a digital terrain model. >