July lidar data indicated the continuing presence of stratospheric aerosols from El Chichón. A small layer in the upper troposphere or lower stratosphere over Mauna Loa, Hawaii in June was still present in July, although somewhat weaker. Layer altitudes, and values of peak and integrated backscatter at Hampton, Virginia in early July and early August were similar to late June figures. Collection of lidar data resumed at Fukuoka, Japan in mid-July. A single layer was reported in contrast to the pair of layers observed in May.

Brightness and duration of July twilights at Boulder, Colorado were the strongest since January, but Richard Keen noted that it was difficult to determine whether the cause was less cloudiness west of Boulder or the presence of thicker and/or higher aerosols. Bright salmon-pink topinkish-lavender twilights, peaking in color intensity at an SDA of 4-5°, were observed 20 March, 14-15 April, 7 May, 1-5 and 16-18 July, and 2 August. Rather dim reddish secondary twilight colors persisted to an SDA of 9° on 14-15 April. Brighter red secondary twilights remained visible to an SDA of 9° on 4 July, 11° on the 5th and 16th, 12° on the 17th and 13° on the 18th. Keen related the general lack of twilight effects March-June to frequent high-level coloudiness over the western United States in the spring. Dates in July without twilight effects corresponded to dates of thunderstorm activity over the Great Basin. Late May - early June dawns observed by Edward Brooks from Jeddah, Saudi Arabia were later than those of the previous year, indicating a significant reduction in stratospheric aerosols. Brooks noted that observations during the second week in June showed no firm evidence of volcanic aerosols in the stratosphere. Fred Schaaf noted that twilight glows were sometimes visible from Millville, New Jersey in June and July, after having been nearly absent March-May. Timing and position of the glow suggested maximum aerosol altitudes of little more than 8 km.

Information Contacts: R. Keen, Univ. of Colorado; E. Brooks, Saudi Arabia; F. Schaaf, Millville, NJ; M. Fujiwara and M. Hirono, Kyushu Univ., Japan; T. DeFoor, MLO; W. Fuller, NASA.