Satellite, lidar, and balloon data continue to indicate that little latitudinal movement of the stratospheric cloud has occurred in recent months. Mean monthly sea surface temperatures determined from the NOAA 7 satellite have been as much as 3°C lower in some regions than the actual ocean temperatures measured at the same time and place by ships, apparently because of interference from the El Chichón cloud. Previous work had shown that the satellite temperatures are normally quite accurate, varying from ship measurements by a maximum of about 0.5°C. Significant discrepancies between actual temperatures and satellite temperatures have remained between 10°S and 30°N, with maximum variations from 15-20°N. No substantial northward movement has been detected through September. Variations peaked in June and July, declining somewhat in August and September.

September lidar data showed gradual vertical expansion of the cloud but little evidence of large-scale northward movement. Layers were detected below the tropopause in Hawaii, but this material may be from Galunggung or some other source, rather than from El Chichón. Balloon soundings from Wyoming (41°N) continued to show a very broad layer around 18 km altitude and some layers as high as 25 km during September, but there were large variations between soundings. The upper layers were not as broad as they had been over Texas (about 27.5°N) during previous measurements. When samples collected from these layers by the Wyoming sondes were heated to 150°C, 98% of the material volatilized, a result consistent with an H₂SO₄-H₂S composition. The composition of the particulate matter, the 2% that did not volatilize, has not been determined.

Brilliant sunsets continued to be seen by H.H. Lamb from Norwich, England through mid-September. A probable Bishop's Ring was seen after sunset 7 September and long, smooth streaks at 20-25 km altitude were illuminated the next evening, but low-level haze prevented any useful observations 9-10 September. No anomalous features were present at sunset or during twilight on the 11th, but a brightly-colored layer could be seen around sunset 12-14 September. The timing of the end of illumination of this layer indicated that it was at about 21 km altitude. When dark, it showed a structure of long streaks.

In late October, government and university scientists will begin a satellite and field experiment designed to determine the key radiative, dynamical, and chemical properties of the El Chichón stratospheric eruption cloud. A NASA Electra aircraft, equipped with a number of remote sensors (including 2-wavelength lidar, 13-channel sun photometer, 4-channel direct diffuse photometer, and Brewer spectrometer) will collect data over the US, the Caribbean, and Central and South America between 19 October and 5 November. The flight plan will be coordinated with a number of concurrrent in-situ and satellite (SME, NOAA/TIROS N, NIMBUS 7, and GOES) measurements. Coordinated rendezvous are planned with high-flying aircraft and with balloon experiments in Texas, New Mexico, and Wyoming. Comprehensive data sets from this research will be made readily available to the scientific community. For more information, contact Pat McCormick or George Maddrea at NASA Langley Research Center.

Information Contacts: W. Fuller, P. McCormick, and G. Maddrea, NASA; T. DeFoor, MLO; M. Hirono, Kyushu Univ., Japan; A. Strong and M. Matson, NOAA/NESS; D. Hofmann, Univ. of Wyoming; B. Mendonça, NOAA; H. Lamb, Univ. of East Anglia, England.