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Sun's Image on Airborne Ice Crystals?
D. Deirmendjian
The Rand Corporation, 1700 Main Street, Santa Monica,
California 90406.
Received 31 December 1967.
On 15 October 1967 at about 1345 h local time, en route from
Chicago to Denver by jet airliner, I observed the following inter-
esting optical phenomenon.
A relatively bright spot was seen near the ground, traveling
with the aircraft at what appeared the position for specular reflec-
tion of the sun's image. That this was indeed precisely the posi-
tion was corroborated repeatedly by observing that the spot be-
came dazzlingly bright whenever it passed over a small body of
water (ponds, rivers, etc.). The sun's zenith distance was esti-
mated at about 55°. There were some broken clouds be-
low, possibly of the altocumulus type; the spot disappeared
entirely when passing over these. The glory phenomenon was
not seen around the shadow point of the aircraft when passing
over these clouds. The spot appeared only
between clouds
where
there might have been some tenuous haze, but disappeared again
556 APPLIED OPTICS / Vol. 7, No. 3 / March 1968
in larger cloudless areas over bare terrain and vegetation. There
was no snow or ice cover apparent on the ground.
I have no ready explanation for the phenomenon nor have I
come across references to it by others in print or otherwise.
Perhaps the haze (which, if it existed, did not prevent a clear
visibility of the terrain below) consisted of a thin layer of other-
wise invisible ice crystals in the form of hexagonal platelets1 such
as are thought to produce the well-known light pillars below and
above the sun. If so, these crystals must have been quite stable
(no wobble) with their flat tops absolutely parallel to the hori-
zontal plane, for the bright spot I observed was circular and of an
angular diameter comparable to that of the sun's disk.
One wonders whether the phenomenon is related to a quite
similar specular spot appearing in a sequence of time lapse pic-
tures of the sunlit earth, transmitted by the ATS–1 NASA syn-
chronous satellite,2 a film projection of which I had seen recently.
I was also reminded of a reference to a paper by Barabashov3 in
which he reported a quasi-specular brightness maximum observed
on the disk of Venus.
References
1.
M. Minnaert, Light and
Color
in
the Open
Sky (Dover Publica-
tions,
New York, 1954), pp. 196 and 202.
2.
R. H. McQuain, Bull. Am. Meteorol. Soc. 48, 74 (1967).
3.
N. P. Barabashov, Vestnik Kharkov Univ., No. 4, Astron.
Series, No. 1, pp. 13-21 (1965).
