Evening star., 1913-06-22
THE ALPENGL0W.
Splendid Spectacle of Color Among
the Snow-Capped Mountains.
From the Scientific American.


Everybody who has sojourned among
the Alps. or other snow-capped noun
tainS. must have admired the gorgeous
coloring of the peaks at sunrise and sun
set. Watching the splendid spectacle
from day to day the attentive observer
soon discovers that it is complex phe-
nomenon, varying greatly with tho
weather. and sometimes exhibiting GER
tain peculiarities that to the uninitiated
are decidedly mysterious.
In fine weather the following sequence
of events may be noted: When the sun
has sunk nearly to the observers horizon.
the peaks to the eastward begin to have
a reddish or golden hue. This fades grad
ually. but in few minutes, when the
sun is little below the observers hori-
zon. but the peaks still bathed in
direct sunlight, an intense red glow be-
gins down the slopes and moves upward
to the SUBMITS. This is Identical with
the rosy twilight arch that in clear
weather rises from the eastern horizon
as the sun sinks below the western: AMI
it is bordered below by the blue shadow
of the earth Now, for Il few minutes.
the peaks are i'll the earth-shadow; their
rocks and SHOWS assume a livid appear
ance. aptly described by the inhabitants
of the vale of Chamounix, whence the
phenomena in question are well seen
'teinte cddaYereuSe.
Presently occurs the remarkable phe-
nomenon known .'recoloration' or ''af-
terglow. I'm Chamounix it is called the
resurrection of Mont Blanc" The peaks,
from which all color had faded, again as
sumo rosy tint: but this time gradually.
and without any sharp line of demarca-
ton between the glow and the shadow
beneath. The recolorat ton is by no
means a daily Occurrence-in fact. it is
tl,<
in appearance and duration. Sometimes
il lasts until an hour after sunset. and II
passes away from below upward. Very
rarely "A faint second recoloration may be
seen. All these phenomena may be seen,
in reverse order. at sunrise. though they
are less common then than at sunset.
They are known, collectively. the
"Alpenglow.'
The recolcration of the Alps has been
varously explained but there seems to
be little doubt that it is due, at least
principally, to the reflection from the
peaks of the rosy glow which forms in
the western sky after sunset. known as
the purple light, and which is some
times intense enough to constitute sec
ondary luminous source. The diffuse ap-
pearance of the recOloration, as compared
with the well defined zone OF rosy liGh,
that rests upon the peaks while the
latter are still in direct sunlight is eX-
planned by the broad expanse of the
purple light as compared with the
small disk of the sun. Sometimes the
sky itself ii. e.. the fine dust i'm the
upper atmosphere reflects the purple
light. giving rise to a second purple
light for an observer situated at a suit
able angle of vision. This in turp may
be reflected by the mountains giving the
rare phenomenon of second recolora-
ton.
\ rival explanation of recoloration, pro-
posed by Amsler. according to which the
phenomenon is due to a rapid change In
the refractive index of the lower air
owing To cooling after sunset, is no,
now accepted, chiefly for two reasons
viz, il fails to account for the CORE
spending sequence of phenomena before
sunrise and il assumes stronger con-
trasts of temperature i'll the vertical than
have generally been found to occur. Ii
is not impossible, however, that the pro-
cess described by Amsler may sometimes
cooperate in producing the Alpenglow
at sunset.