Figure 3

Image stacking was applied to identify and measure the trajectories of clasts. (a) The upper frame gives an impression of the number of particles ejected during a pulse (20 frames were stacked; this is essentially the picture that would have been created by a camera with an exposure time of 2/3 s). As this way of visualization only works at night and high contrast to a dark background, another standardized method was applied to track the trajectories: (b) 60 frames (i.e., 2 s of video footage) were stacked; the difference to each frame was calculated pixel by pixel and colored according to the respective frame. The image shows bent and straight trajectories, but only the latter were used for analysis. The visibility decreases on the right (downwind) side and is particularly low in the first quadrant (0° to 90°), as this sector is covered by ash clouds.