MONTHLY WEATHER REVIEW. NOVEMBER, 1916 624
in the reversed position of its luminous portion, decreas- .
%ne need but to observe the sky a little before the rising of the sun, or a few minutes after it has set, to recognize the henomenon or meteor with which we are concerned. 8 is very plain, and probably as old as the earth itself; and there IS every reason for astonishment that it does not receive more attention in both the modern and ancient works on-physics or astronomy. I know of but one work where it is expressly mentioned, and that is a work entitled On the Colors of the Sky., * M. Cramer-who had indeed well observed the anti- crepuscule and had even piade some optical studies on it-wss as surprised as I at the silence of writers in this respect. A number of years ago he wrote to me on the subject, and I conmiunicated t,o him what, I knew thereof together with the note on the work by Funccius. No doubt other occu ations prevented him from carrying
Happy would I be might I still consult on this, and all other subjects, a friend at once so faithful, so wise, so
enlightened and whose loss I shall et,ernally re ret.. * * * On the evening of R fine day, t,hen, one wit observe nt sunset or a few minutes aft.er, in t.hat ,ortion of the sky
o osite the sun and ininiediat.ely on t. h c horizon, a kind o!%and or d u d 8egm.en.6 of bluish and purple color sur- mount4 by a 11nwi.nmc~ and colored alreh which is whit,ish, orange, and finalIy 011 its upper bordcr of n rose color sonietinies verging on lire color. Bor t:hcse coloiu, or rather these shades of t i l e colors are never well, clearly, or sharply defined here. Also, it is only under niore or
less favorable circumstances, according as the air is niore or less free from vapors, exhalations, and clouds, that the anticrepuscule of one day or of one clininte differs from that of another. Otherwise taliere is nothing more uniformly constant t h n this phenomenon, which is a
purely optical one; therein quite different from the aurora borealis which belongs to physics hut! is variable and accidental. * * * Now as the sun sinks below the horizonl the t..\\:iliglit, or
crepuscule sinks, ailti t,he [anti-twilight or] ant.icrepusculc rises by n corresponding amount; t.hose solar ra~ys that had been striking tlie vault at, or near the zenith no longer reach so far, they are reflected from point,s nearer the sun, and the tmt,i-twilight continues to rise; it.s luminous, colored arch de taches itself from the hluish, purple seg- ment which soon shows nothing but a gray or ash shade, it [the c,oloretl arch ?] mounts steadily and finally conics
to the zenith where it is still visible when the air is there clear; for after havin nt,t,ainerl n certain elevation it grows weaker and we&er, until a t last it t,otauy tlisa - pears. I have observed the anti-t,wili lit innunierab 7 c
times in t,he more southern portions of brance, a t Pnris. and in its environs. The bluish and lmrple hand H t. t.lie horizon becoines gray and ash-colored when the anti-twilight or unticre- puscular arch det.aches itself therc*froni. because the rtrl
rays from the sun and from the inorc brilliant portion of the twilight (cl$puscule) no longer are reflected so far downward. (The author nlso remarks that while the secondary rainbow is frequently observed, he is not aware t,hnt n secondary anti-twilight arch has ever been seen.-c. A., jr.]
the old.- T L ,.
in brightness as the horizon is approached.
his researches fart R er, as well as from publishing them.
___--~-
4 I . e., An phenomenon or appesrana in the atm0spbere, tho first slgnincance ol
It lrlnkrwtlng to note here that MaIran urn the nnme %ntl-tWlight ar0hnot 1- tb.n 4 tlIW In thlr kcl8lnAsaement.- !hnul.
EXPLANATIONS OF TEE WESTERN PUBPLE LIQET AND THE EASTERN ARTEBGLO W (NACHGLu HEN! .I
By ALBERT HEIY.
[Translated lor the MONTHLY WEATEEB REVIEW by C. -4bbe, Jr.1
All observations of the phenoinenon ioint to the con- clusion t.httt t,he western urple light. ( d est urpur) is the condition necessary for t F, e afteGlow INac!&luhen). If t,he western purple light does not appear, t,hen the after- glow in the east is always absent; and if the western purple light is very weak, t$hen one sees hardly a weak sus icioii of the afterglow.
&OH, who wns t,he fiivt t.0 study the alpenglow with exactness, reached t.lie conclusion-which is confinled Mnurcr-tha.t t,he evening glow of t,he western sk ,
wcstmn purple, was reflect,ecl a t n s arp nng e froin the m i y r i n i undersurfaces of high air layers. Mirror re- flectioii oes not essentially change the c.olor of the light falling upon the mirror, hut the wi(lt1i of the effective surfa.r.c and the breadth of the bundlt- oE rays prevents the production of sharp shadows. rhcrefore, a portion of the evening red (Ahendrot,) is reflected downward once
more into t-hc blue ea.rth sharlow. If t.liis is the nianner in which the aft,er low originates, t,lien the iianie reflex
If the uft,erglow (Nachgluhen) is a reflcs glow, t,hen we niay conipute tlie altitude of the niii-roring air layers. The result,s give dtit.utles of only 20 t,o 35 kin. for the aft,erglow a t n solar depression of 4i0 to 6O, and of 70 t.0
SO krn. for a depression of go. The different phases of the afterglow correspond to niirroring layers at different 1ieight.s. It seems to me tllat the following esplnnation of the afterglow is also a possible one:
The upper air layers reoeive almost solely yellow-red,
they are directly illuminated by the yellow-red. The air reflects diffusely not simply blue done, but dl colors,
as is shown by the whitish n earsnce alon the horizon; however! it reflects mom of \!i ue than of t a e other com- ponent,s of whitre light. If it receives yellow-red almost esclusively, then i t must also reflect diffusely the yellow- red, of course weakened. Thus there originates tlirough- out the wholo zone of sir st,anclinu in the eart,hs shadow n. difkx? fvening red (Ahendrot) bcRw th.e (iire.dly i.Zbusmined
evening red. This in at one and the same timc the ~u le light in t.ho west and the afterglow in the cast. 1 n 3 e urpIe light, we see not,hing. other than the higher air Lycrs illunlined by the evcning red from the sun. Yel- low-red illurninatmion of t.he air appears as pu And the afterglow in the east is the reflection^%$^ dwin, Refles, Abglmz) of the western pur le. Finally, there is su port for tho probebiity that dif-
higher and still direotly ilfumined air layers a t the time of the purple b h t and the after low. Kiesling, Penter,
fraotion was the cause of the western
x w b\ iich we see projcrt.ed int,o t,he hi ti air In rem as e
f
glow (Spiegelglu fl en) is ~ust~ifietl.
fraotion affects the lig f t rays shining down to us from the
and Riggenbaceh, particularly a ave assumed that dif-
artic P es. It seems to refiectibn and C f iffuse reflection send the light down to us
According to Riggenbaahs tion of hrakatoa it was very dear resulted from the widening of Bishops Rin certainly owed its ca per-red-brown to the tion bv dust
.
~ -
I Heim, Albert L~ft-Farben Holer & Co. .4.-G. Ziirioh, 1912. 8 cm. pp. 70-74.
[Notable Illust&tlons in color!]
NOVEMBER, 1916. MONTHLY WEATHER REVIEW. 6%
from the hi her illuminated air masses and that diffrac-
The color and the time of occurrence of the western purple light and of the afterglow are in agreement with all three classes of explanpt,ions. The relahe rarhy of the afterglow and its varying intensity with the osit.ion
of the sun speak in favor of the-iden of mirror re ection. It seems to me, however, that if the .oause were mirror reflection from the sunset zone tslie l!ght woul? appear somewhat less scattered and would gve more llght wid shade efkct in the mountains. As a matter of fact I have
often wondered, as I stood in the mountains in the after- glow itself, at the shadowless or ver weakly shadowing
a te low but fills the whole interveniii space wit.11 its a-pervading mysterious, gloomy ye1 ow-red. The breadth of the light-giving surface nnd of t.lie mirror together are after all an inadsquatme explanation, it seems to me, for this diffuse behavior of the after low. Just as
wit8h blue haze durin the dsv, so nom- the sky is yurplc,
va PI eys aro filled wit.li these colors. If reflectioii throws the western purple light. into our eyes and upon the east,ern mountains, then t,here must. he n much stronger pohbriza- tion of the purple. If the skys blue were a true fluorescence of bhe air, then the western purple light and the afterglow- would he blue instead of red. I conclude, from their colors, that the skys blue is mere1 a pseudo-fluorescence.
the western urple light often begins, has not yet been explained, w R ile the arched form of the western purple seems to be a matter of course.. Evidently we have not. vet the last word in esplanation of the western purple bght and of the afterglow. In the case of the afterglow
we may have to do mt,h the conibined effects of several factors. Nature is. indeed. always more coniplicated than we like to kqsurne in thought,. On! t.lling is quite
certain, thhe afterp.lOw on the ea&m m.oimtuana miid the cast-
e r n 8k.v iS not a direct evening red. but an. inc7abect PrerLing
red which k brought down into the ea.rths .Q?t.m70ii7 h y mir-
wriri or difuse rqflcrtion. rc.nQ (1.ifkctiin.n. Put niorc
sim &: The afterglow in t,he east is the reflcct,ed s >lendor
the afterglow a o as an indirect, dpen low. Here also is
repeat.ed R relation similar to t,hat. of t. ie fiist dpe~~glow: The west furnishes light and color, t.he east is illuniinat,cd thereby. Thus it. conies about t.hat light, nncl color are
stronger in the west than they are siniult.aneouslv in t,he
east. If one stands on t.he eastern mountitins and wholly
in the afterglow: there appcura no repet,ition of sunset in the west; one sees only a reat surface of purple light.
Here and there a sheaf of divergent rays [crepuscultlr
raw] from a mountain or a cloud in the west.ern sky, reitches high up into the recently enlpurpled heavens.
(See Heinis fi . 18.) The shadow rays [or crepusaular
conclu a e that the air tmhatm appears to he already in the western purple, as we stand down below it, still receives direct rays from the sun at high levels above us. The
western purple light therefore arises first from direct il- lumination of the u per air lapis. Diffuse reflection and
down to our eyes in t.he eart,hs shadow or across to the mom t aim. This does not enable one to decide. however, whether the purple light originates in the west behind the cloud or
tion can in B uence only the color of the light. .
character of this eculiar flesh-red lig i t: and how i t often a pears not mere P y as the western purple and tho eastern
P
the sk is blue and tho mountains and va f leys are fillrd
eo per-oolornd, or re d dish yetlow and the mountains and
B
The peculiar form o B a  salmon-colored spot  in which
of t e western urple light. We may t,hcrrfore cescrihe 1 R 5
spread over almost the who f; e westmein sky.
rays] a pear b f ue-green in t.lie purple. Hence one niaq
diffraction are but t K e processes t.hat conduct the splendor
only on our side of the latter. If the former, than the clouds intercept the purple-colored solar rays; if the latter, then the purple doe8 not develop where the clouds shadow falls. The former appears to me much the more probable case. The western purple light, then, seems to be the evening glow of the hi her air directly illumined by the setting
The eastern afterglow shows us mountains and air in- directly illuminated by the reflection of this lofty evening
sun, reflected 8; own into the earths shadow to our eyes.
glow. 7, .,.
TWILIGHT PHENOMENA IN ABIZONA, SEPTEYBEB TO DECEMBEB, 1916.
By Prof. ANDREW ELLICOTT DOIJ~LASS.
[Dated: Deparlniciit of Physim 3nd Astrouomy, University of -4Iizm8, Tucson, Dec. 2, 1916.1
[In his letter transmitting this gper Profemor Douglaee correcta his statement, quoted by nie in the EVIEW for August, 1916, 44:434, to t.he effect that red sunseta were not observed by him after the middle of September, 1916. His accompanying description of twilight rolora is closely in accord with the claeaical description b von Besold, which, t,ogether with Exners classification of the twilidt phenomenon will be found translated on pages 620 to 6 3 of this number of the REVIEW. Therefore. with the consent of Professor Douglaes, I have inserted in brackets in his text the designation given bv Exner to the hase of the t.wilight. deecribed. Thus, [(C) Fimt twilight arch] ie to %e inter- greted aa nieaning that the phase of twili8ht under consideration will e found under (c ) of Exners classification to have the deeignation here given.--If. H . hhball.]
Prom September 16, 1916, to tihe present time the writer has wat,ched ever. evening aft,er sunset for the occurrence of bright. t,wi l ght and afterglow colors, and
shndow phenomena. The clearness of our atmosphere permits all t,hese henomena to be seen with greatest
about. one degree in elevation in all directions. From west-northwest to wst,-southwest the average height is about I S o , while in the northwest the apparent horizon descends a very slight amount below the true horizon. In the enst t.hc niounta.ins r i p s from 1 to 3 above the true horizon. On account3 of t .1 ~ clearness of the air t,he sun is never under any circumstances faint enough t.o be looked at directly hy t,he naked eye. Clouds are rare and two levels of clouds still more rare, so that it heconies possible to estimate cloiid heights lq- t,he t.inie when direct. sunlight c-enses to illuniinat,c thein. I t thus becomes possible here
to usc nieniis of inrestigat.ion that, would seem incredible to an observer nccust,omed only to A4t,ln~it.ic coast. con- d i t,ions. Out, of 80 niglit.s, cloucls have interferred to tl serious extent on only li nights. Plight. cloudiness has prevailed on over 51) per cent of t.he nights. In fact, it is a con- dit,ion of n fcw thin, scat,t.ered ciriiis clouds that is most favorable t.o t,he heautiful ray phenomena so often noticcd. [See Ahlsos translations of Heims and Bezolds descriptions of t,liesc crepuscular r a p : this REVIEW, p. 622
and p. 625.1
As soon as the disk of the sun is behind the western mountain, a heavy bronze area [(b ) Twilight low] is apparent. estending to a dist,a.nce of 4 or 5 in a 5 direc- tions from the sui1 itself. At, first this bronze is of a
&her yellowish color, which soon changes to a reddish tone, lasting some 20 minutes after the disappearance of t,he sun. -4fter the sun has really set the clear western sky in that. general vicinity shows a structure which seem8
likely t,o be due to a high layer of haze in the atmos- phere. This structure usually ap ars as a faint, soft etching of large numbers of paralel p lines. In general
ease down to our x orizon of mountains, which averages