Mexico
United States
Gulf of
Mexico
Atlantic Ocean
52 ASTRONOMY • APRIL 2019
A flecked
Hawaiian sky
On March 28, 1982, El
Chichón, a dormant volcano
in Chiapas, Mexico, awoke
from 600 years of slumber,
erupting violently three times
in a week. One of the most
important volcanic events of
the 20th century, the unex-
pected blast released 7.5 million
metric tons of sulfur dioxide
into the stratosphere, warm-
ing it by 7.2 degrees Fahrenheit
(4 degrees Celsius), and cooling
the Northern Hemisphere by
0.72 F (0.4 C). The resultant
cloud encircled the globe in
20 days and altered Earth’s
climate for years afterward.The stratospheric aerosol
cloud initially moved westward
from southern Mexico toward
Hawaii, where I was living at
the time. In a 1983 Applied
Optics paper, Kinsell L.
Coulson notes that “a consider-
able enhancement of intensity”
occurred throughout the main
part of the day, causing a “dif-
fuse type of aureole” over a
large portion of the sky. Mauna
Loa Observatory lidar mea-
surements over Hawaii in 1982
revealed a sixfold increase in
scattering due to aerosols, and
a 25 percent decrease in direct
incident radiation.
In my studies of the El
Chichón-inf luenced daytimesky, I noticed it had a “nervous”
quality, caused by the interplay
of minute f lecks of complemen-
tary colors. This is why I refer
to it in my Hawaiian diaries as
an Impressionist’s sky. To a
casual viewer, the El Chichón
aerosols had buffed away the
normally crystal blue sky and
replaced it with a frost-glass
glare of Pointillist light — light
predominantly infused with
flecks of blue and orange, with
dabs of yellow and white, that
scintillated with subtle pris-
matic effects like tossed con-
fetti. This description is
reminiscent of one recorded
one month after the Krakatau
paroxysm by Captain Parson of
the Earnock, who noticed the
eastern sky before sunrise
appeared “silver grey, changing
to light blue, f lecked with
numerous small cirrus trim-
ming, pink and rosy.”
Some of the color associated
with the aerosol umbrella I
witnessed was linked to the
Bishop’s ring atmospheric phe-
nomenon. This enormous dif-
fraction corona (in this case
created by the scattering effects
of volcanic aerosols) covered
half of the visible sky and
displayed the color-contrastStephen James O’Meara is
a columnist and contributing
editor of Astronomy.The Total Ozone Mapping Spectrometer, carried by the Nimbus 7
meteorological satellite, created this plot that shows the largely
westward spread of the volcanic aerosol veil from El Chichón on
April 8, 1982, only four days after the largest blast. NASAThis Pointillist image
shows colors of
complementary light
scattered by pollen
grains in a diminutive
atmospheric corona.
The bright glow is an
edge effect from a
roof used to block the
Sun, around which the
colorful corona appeared.
STEPHEN JAMES O’MEARA