A thick zone of probable volcanic material was observed in the upper troposphere and lower stratosphere during the night of 8-9 June. NASA's lidar at Hampton, Virginia (37.1°N, 76.3°W), operating at the ruby wavelength of 0.6943 µm, recorded several layers with scattering ratios greater than the normal background reading of 1.1 (scattering ratio = 1 + aerosol scatter/molecular scatter). Thin layers were centered at altitudes of 17.4 km (about 1 km thick, scattering ratio 1.5) and 16.5 km (about 1/2 km thick, scattering ratio 1.3). A much broader layer extended downward from 16 km through the tropopause at 13.3 km to 12 km altitude. Within this broad layer, clearly-defined scattering peaks were located at 14.9 km and 14.1 km, both with scattering ratios of 2. Residual material from St. Helens remains in the stratosphere, raising the background scattering ratio to 1.2.

Weather balloons launched from Laramie, Wyoming (41.33°N, 105.63°W) began to detect volcanic material 16 May. Since then, a thick zone showing some variation in structure has remained between 12 and 18 km altitude. During the night of 8-9 June, an intense new volcanic layer between 11 and 14 km altitude joined the post-26 May material over Laramie and nearby Boulder, Colorado. The previous material, concentrated between 16 and 18 km, was truncated by a reversal in wind direction at 18 km altitude.

The two most likely source volcanoes are Alaid and Pagan, which both injected eruption columns into the stratosphere, at the end of April and on 15 May respectively. No other major explosive eruptions have been reported since then.

Information Contacts: P. McCormick, NASA Langley Research Center, VA; D. Hofmann and J. Rosen, Univ. of Wyoming.