The initial dispersal of the major stratospheric cloud from the March-April, 1982 eruption of El Chichón is described in the El Chichón [volcano reports]. Its persistent atmospheric effects are reported below.

Lidar data collected in June at Mauna Loa, Hawaii showed backscattering that typically increased from near the base of the stratosphere to a peak at 26-27 km altitude, with significantly enhanced values to 33-34 km. Data were less variable from night to night and layering within the cloud was less distinct than in May. The cloud above Hawaii has apparently affected incoming solar radiation, measured for about 50 years by the Hawaiian Sugar Planters Association. Although mean daily solar radiation would normally have been about 110% of the long-term average (because precipitation in May at the primary station was only 27% of normal), the measured value for the month was only 92% of average.

Lidar at Fukuoka, Japan showed decreased backscattering from the 21-29 km layer in late June, but backscattering increased again in early July. The less dense layer at 18.5 km remained stable through this period. In early July, backscattering detected by lidar at Hampton, Virginia increased sharply for the (highest) layer centered at about 25 km altitude, approaching those values measured at lower latitudes for the first time.

To assess latitudinal variation in the stratospheric cloud, a lidar-equipped NASA aircraft flew from Wallops Island, Virginia to Puerto Rico during the night of 8-9 July, to about 12°N (near the coast of Venezuela) 10 July, and from Puerto Rico to the vicinity of Albany, New York (about 42°N) 11 July. From 25-30°N to the southern limit of the flight, preliminary data show greatly enhanced backscattering from a dense layer between 21 and 33 km altitude. Some material was present below 21 km, but it was much less dense. Strong local variation in the cloud was observed. Although the cloud diminished in density N of 25-30°N, significantly enhanced stratospheric backscattering was detected to the N limit of the flight.

Brilliant sunrises and sunsets - Saudi Arabia. Weather satellite images first showed the front of the 4 April stratospheric cloud (visible over water during the day) over the Red Sea 21 April. Edward Brooks, who has made frequent sunrise and sunset observations from Jeddah, Saudi Arabia (21.5°N, 39.16°E) saw WNW-ESE-trending bands of haze in the WNW sky after sunset 20 April and similar bands before dawn the next morning. The twilight of 24 April was a brilliant pink from bands and patches of WSW-ENE-trending aerosol. During the next several weeks, volcanic cloud effects could be seen in the sky around sunrise and sunset most days, often as bands of material oriented within 45° of E-W. Brooks observed a layer 10° above the horizon at twilight 18 May and calculated its altitude at roughly 20-25 km. Multiple layers began to be visible in June. At sunrise on 5 June, criss-cross bands of aerosol trended SW-NE and SSE-NNW. Brilliant sunrises and sunsets were common in mid-June. Beginning on 19 June, many sunrises illuminated two distinct layers, at about 30-minute intervals. This effect weakened later in the month and by 30 June the higher layer (illuminated earlier in the morning) had disappeared. Brooks and others also noted that on 6 July roughly the upper half of the eclipsed moon was considerably darker than the lower half, which Brooks interprets as indicating the presence of volcanic aerosols in the atmosphere of the earth's northern (but not southern) hemisphere.

Further References. Galindo, I., Hofmann, D.J., and McCormick, M.P., eds., 1984, Atmospheric Effects of the Volcanic Eruption of El Chichón; Geof¡sica Internacional, v. 23, nos. 2-3, p. 113-448 (22 papers).

Pollack, J.B., Toon, O.B., Danielsen, E.F., Hofmann, D., et al., eds., 1983, The El Chichón Volcanic Cloud: An Introduction; Geophysical Research Letters, v. 10, no. 11, p. 989-1128 (18 papers).

Information Contacts: W. Fuller and P. McCormick, NASA; T. DeFoor, MLO; M. Hirono, Kyushu Univ., Japan; E. Brooks, Saudi Arabia; K. How, Hawaiian Sugar Planters Assoc.; M. Matson, NOAA/NESS.