Report on Atmospheric Effects (1980-1989) — December 1984
Scientific Event Alert Network Bulletin, vol. 9, no. 12 (December 1984)
Managing Editor: Lindsay McClelland.
Atmospheric Effects (1980-1989) Strong twilight colors resume over England
Please cite this report as:
Global Volcanism Program, 1984. Report on Atmospheric Effects (1980-1989) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 9:12. Smithsonian Institution.
Atmospheric Effects (1980-1989)
All times are local (unless otherwise noted)
H. H. Lamb reported a resumption of strong sunset colors in late November in the vicinity of Holt, England. The effects were stronger than they had been in more than 12 months. The timings and elevations of the optical phenomena seemed to suggest the illumination of an aerosol layer at about 22-25 km altitude. Lamb noted that for both the March-April 1982 eruption of El Chichón, and the August 1883 eruption of Krakatau, the first strong optical effects over England were at about the same time of year.
On 25 November, Lamb noted a strong purple patch that extended to more than 20° elevation and appeared about 30 minutes after sunset. At sunset on 28 November, the slightly greenish orange sun was surrounded by strong orange diffused light, and a purplish patch more prominent than on the 25th was present 50-60 minutes after sunset. However, no anomalous colors or other optical effects were seen at sunset in clear weather on 30 November.
During clear weather in early December, optical phenomena similar to those of late 1982 and early 1983 yet stronger than those of late 1983 were consistently observed by Lamb and his colleague Michael Kelly. The 6 December effects were very similar to the most impressive twilight seen in November 1982. As the sun set on 6 December, it was surrounded by orange light to about 3 solar diameters. From 10 to 20 minutes after sunset, a horizontal pinkish purple band appeared, evidently from the illumination of a layer above the surface haze. This band gradually climbed to 10° elevation, becoming broader and more diffuse. At 1600, 20 minutes after sunset, the entire western sky was a brilliant yellow to 35° elevation, edged by a brown layer along the horizon. By 1610, a purple patch had developed from 10-30° elevation above a shield-shaped area of bright white sky. At 1616, the maximum elevation of the purple glow was 20° and the sky from the horizon to 3° was a fiery deep brownish red. The next morning, the rising sun was pale yellow and surrounded by orange diffused light to 4 solar diameters. Twilight observations that evening were similar to those the previous day, but the purple patch at 1615 was asymmetrical, roughly triangular, with a vertical northern edge. It then narrowed to a broad column of light, which reached only 10-12° elevation by 1622 and faded fast. Sunrise on 8 December was obscured by clouds. That evening, purple light development was less pronounced than on the 6th and 7th, and gradually changed to a dirty gray. A fiery red band was present along the horizon at 1630. After sunset on 10 December, the purple patch was especially beautifully colored and in the form of crepuscular rays, reaching 25° elevation at 1615 and fading soon after 1620, when the maximum elevation was about 18°.
Richard Keen reported that brightness and duration of twilights at Boulder, Colorado showed a noticeable and steady decrease from late August through early December, indicative of a continued thinning and/or lowering of the aerosol layer. Enhanced salmon-pink to lavender twilights, peaking in color intensity at an SDA of 4°, were observed on either the mornings or evenings of 26, 27, 29, and 30 August; 2, 3, 5, and 15 September, 7 October, and 4-6 December. Extended lavender to purplish twilights were visible to an SDA of 11° in either the morning or evening sky on all these dates except 26 and 29 August and 5 December. Keen related the occurrence of extended twilights on individual dates to the absence of cirrus clouds for 1000-2000 km in the direction of the sun.
The shapes of lidar profiles and total aerosol backscattering at Mauna Loa, Hawaii varied considerably in December. Data on 11 and 19 December were similar to those of October and early November, with higher integrated backscattering than in late November and early December. Breaks in slope in recorded profiles on those dates suggested that aerosols were present to 34 and 39 km altitudes. Aerosol concentrations and maximum layer altitudes decreased again at the end of December.
Information Contacts: H. Lamb, Univ. of East Anglia, England; R. Keen, Univ. of Colorado; T. DeFoor, MLO.