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 [1991] 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 [1992] 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 [1992] elongation and Nova Cygni under the pole in February
[1992]. 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.