EARTH'S NEW DUSTDECK ------------------ John Pazmino NYSkies Astronomy Inc www.nyskies.org nyskies!nyskies.org 1992 June 1
[This report on the Pinatubo Dustdeck was issued in two parts in 1992 May 1 and June 1. They are combined here under the later date. Minor cleanup done, otherwise the text is original.]
The dust expelled-from Pinatubo last summer  is still in the atmosphere and may be a longterm impediment to astronomy. Usually volcanic dust settles out after a few months, leaving the air clear and free for celestial observing. But Pinatubo's dust still circulates in the stratosphere like a veritable new dustdeck over the Earth. In New York this dustdeck by day tempers the blueness of the sky, smothers the low Sun, reddens the twilights, and shows faux nuage effects. At night the stars are dimmed and the Moon is muted. Pinatubo's dust hit New York on Friday 28 June 1991 in the late afternoon. Arline Caldwell and I saw the front creeping across the sky like a shroud over her Amagansett LI beachhouse. The daysky turned from normal blue to a dull grayish blue and the Sun set into a sandy haze. We were expecting the arrival any day from warnings posted by the press. However, the dust came in from the ocean and drifted toward the City -- it traversed Asia, Europe, and the Atlantic Ocean!! While governments are chewing their knuckles against a potential global Hawks Nest syndrome, we here treat only of the astronomy consequences of the Pinatubo Dustdeck. There was general fear that the dust would reach Mexico and Hawaii and impede views of the [July 1991] solar eclipse, then two weeks away. It in fact did cover these sites and hamper observations of that eclipse. Caldwell and I viewed from Waikeloa HI, but general overcast blocked the eclipse and masked any effects from Pinatubo. Astronomers believed that, as with EI Chicho'n and St Helens, the dust would quickly fall out of the atmosphere to allow normal astronomy activity again. But Pinatubo is different. Pinatubo is about the most violent volcano eruption of the 20th century. It heaved its stuff 30Km into the air, directly into the stratosphere, in stupendous amounts. Being that the stratosphere is a layered zone of the air, immune from weather and other convection forces, the dust is trapped and bound there. It circulates around the Earth for a long time, much like the CFCs. As a result, Earth is now tunicked by a new atmospheric layer, the Pinatubo Dustdeck. It sits 30Km to 40Km up and is of rather irregular texture. This texture can be assessed from the ground and many readers are actually keeping records, if only casually, of it. From New York the dust was not seriously noticed until the advent of the autumn clear sky season and the migration of sunset into the workhours. Then, in late October reports started coming into the AAA of a blue deficiency in the daysky and of some gorgeous sunsets. By November comments came in from layfolk -- not just astronomers -- about 'something wrong with the sky'. The public's perception of the Pinatubo Dustdeck is of four grand categories: blue deficiency in the daysky, attenuation of the low Sun, reddening of twilights, revelation of the Moon's surface markings. Simply put, since the eruption New York has not seen a truly blue sky. When the ground conditions and overall weather would normally generate dazzling brilliant blue skies, the best achieved is a limp blue. The horizon zone is always white or tan and the entire quadrant centered on the Sun is gray or tan. Strangely, this aspect is more noticeable to cityfolk. They are shaded from direct Sun by the towers around them and can see pieces of the sky in comfort between them. The sky is just plain not really blue anymore. When a thicker field of the dustdeck passes over, the sky is positively sand tinted with almost no blue at all in it. At noon there is a patch in the mid north sky of pastel blue and the Sun is ringed by a brown aureola. Occasionally the sky resembles a carpet, with 'creases' or 'wrinkles"'. Since January  this effect is much rarer. The attentuation of the low Sun is mentioned mainly by motorists. They contend with the glaring Sun shining in their eyes during the evening and morning commutes in normal times. But now the Sun is not blinding anymore; it is merely annoyingly bright. The tinting in the windshield or ordinary sunglasses cut the Sun's brightness enough for comfortable driving. On almost any day the Sun shows a true disc, not the usual blazing painful amorphous light. Virtually everyone mentions the beautiful vivid sunsets (and sunrises) over the City. As the Sun dips lower the sky in his quarter shifts to a red hue, mixed with what blue there is. The color ranges from a benign purple/violet on the clearest bluest days and the twilight resembles the ordinary ones. On the dust-laden days there is so little initial blue that the sky flares out in lavender, magenta, flamingo, or coral. So striking is this that ordinary people stop at street corners and marvel at the spectacle! The common person is noticing the face in the Moon. Normally the Moon in town is too bright to casually discern her markings. Seen thru the Pinatubo Dustdeck she is tempered enough to reveal her texture to the public. Some people think they 'discovered' the markings and are amazed they can be seen from the ground. At night the longer lightpath thru the dustdeck pretty much k11ls horizontal astronomy. The dust blocked easy viewing of Mercury at the March  elongation and Nova Cygni under the pole in February . Both events would be readily visible under normal New York skies. The Pinatube Dustdeck interdicts star observation. On the whole the best of the clear nights suffers a loss of half a magnitude in transparency. The stars seem dimmer, altho the background is still dark. The dust is far too high to catch the ground illuminations from the City and augment the light veiling of the stars. So in an otherwise normal looking sky there are fewer stars! We discovered an amazing anthropocentric and psychological phaenomenon. Under a dark sky with fewer stars some astronomers feel 'frightened' or 'scared'. The missing stars, despite the otherwise normal appearance of the sky, makes them feel terribly isolated from the rest of the universe. On the other hand, we learned that photometry, by eye and electric, is unperturbed by the dustdeck. Such photometry is done differentially, not absolutely. The target star is gauged against circumstant stars and all are, within a small area, uniformily dimmed by the dust. AAVSO, in particular, applies no 'correction' to its received assessment reports to compensate for the dustdeck. What does the near -- and far -- future hold for astronomy under the Pinatubo Dustdeck? At best it is tougher to wage. At worst it may be subject to suspension from time to time.