pagiven@drutx.UUCP (GivenP) (11/05/84)
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Thanks to all who replied to "Sunrise Phenomenon."
Briefly, netters were asked if they could explain the "rosy fingers
of dawn:" on a cloudless morning, shafts of alternating deep blue
and orange light radiating from the horizon just before sunrise.
Can it happen at sunset, too?
Summary of responses:
1. Yes, it has been observed to happen at sunset by at least one
respondent.
2. The orange light is caused by atmospheric refraction, as we
observe at the usual sunrise or sunset.
3. The light appears in shafts because of either cloudmass or
terrain irregularities that lie beyond the horizon and block
some of the refracted red light.
4. Although the sky appears cloudless, it really has a thin haze
of cirrus clouds that act as a "movie screen" upon which is
projected shafts of light caused by sunrise over either
irregular terrain or cumulus clouds. Perspective gives the
illusion of radiating bars.
5. The blue light is there because the strongly refracted red
light is blocked thereby letting the observer see the weakly
refracted blue light.
6. The horizon must be far enough away from the observer to
allow refracted light to reach, thus the phenomenon is never
observed at sunset from Denver: the mountainous western hor-
izon is too close. Nor is it observed if the cloudmass is
above the horizon.
The common thread in these explanations seems to be the requirement
of irregular terrain or cloudmass just beyond the horizon. I am
reminded of the solar eclipse phenomenon where sunlight peers
through lunar surface asperity, is it called "Bailey's Beads?"
However, none of the explanations deal with the *striking* regular-
ity and even spacing of those bars of light! Random chance? or
slipshod observing? Nor do the proffered explanations address the
phenomenon's rarity. If unchanging irregular terrain and a clear
sky are all that is required, why do we not see it more often? (at
least for those of us foolish enough to be up at such a ridiculous
hour :-))
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Paul Given {ihnp4, houxe, stcvax!ihnp4}!drutx!pagiven
AT&T Information Systems Laboratories
11900 N. Pecos, Rm 1B04, Denver 80234 (303)-538-4058
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