Hold the left mouse button down as you drag the slider above the image (or use the arrow buttons) to fade between the GOES-8 visible image and the corresponding GOES-8 IR difference product and 6.7 micron IR ("water vapor") images at 16:15 UTC on 07 October 1999. At this time (about 4 hours after the eruption), the plume is difficult to identify on the IR difference product; the thickest portion of the easternmost ash plume is apparent on the visible image, but the entire eastern portion of the plume is well defined on the water vapor image.