624 MONTHLY WEATHER REVIEW. NOVEMBER,1916 in. the reversed position of its luminous portion, decreas- EXPLANATIONS OF TEE WESTERN PUBPLE LIQET AND THE in brightness as the horizon is approached. EASTERN AR’TEBGLOW (NACHGLuHEN! .I %ne need but to observe the sky a little before the By ALBERTHEIY. rising of the sun, or a few minutes after it has set, to recognize the henomenon or meteor with which we are [Translated lor the MONTHLY WEATEEB REVIEWby C. -4bbe, Jr.1 concerned. 8 is very plain, and probably as old as the All observations of the phenoinenon ioint to the con- earth itself; and there IS every reason for astonishment clusion t.httt t,he western urple light. (d est urpur) is the that it does not receive more attention in both the condition necessary for tF, e afteGlow INac!&luhen). If modern and ancient works on-physics or astronomy. I t,he western purple light does not appear, t,hen the after- know of but one work where it is expressly mentioned, glow in the east is always absent; and if the western and that is a work entitled “On the Colors of the Sky.,’* purple light is very weak, t$hen one sees hardly a weak M. Cramer-who had indeed well observed the anti- sus icioii of the afterglow. crepuscule and had even piade some optical studies on &OH, who wns t,he fiivt t.0 study the alpenglow with it-wss as surprised as I at the silence of writers in this exactness, reached t.lie conclusion-which is confinled respect. A number of years ago he wrote to me on the Mnurcr-tha.t t,he evening glow of t,he western sk , subject, and I conmiunicated t,o him what, I knew thereof wb\ iich we see projcrt.ed int,o t,he hi ti air Inrem as e together with the note on the work by Funccius. No wcstmn purple, was reflect,ecl at n sf arp nng e froin thex doubt other occu ations prevented him from carrying miyrini undersurfaces of high air layers. Mirror re- his researches fartR er, as well as from publishing them. flectioii oes not essentially change the c.olor of the light Happy would I be might I still consult on this, and all falling upon the mirror, hut the wi(lt1i of the effective other subjects, a friend at once so faithful, so wise, so surfa.r.c and the breadth of the bundlt- oE rays prevents enlightened and whose loss I shall et,ernally re ret.. * * * the production of sharp shadows. ‘rhcrefore, a portion On the evening of R fine day, t,hen, one wit observe nt of the evening red (Ahendrot,) is reflected downward once sunset or a few minutes aft.er, in t.hat ,ortion of the sky more into t-hcblue ea.rth sharlow. If t.liis is the nianner o osite the sun and ininiediat.ely on t.h c horizon, a kind in which the aft,er low originates, t,lien the iianie “reflex o!%and or dud 8egm.en.6 of bluish and purple color sur- glow” (Spiegelglufl en) is ~ust~ifietl. mount4 by a 11nwi.nmc~and colored alreh which is whit,ish, If the uft,erglow (Nachgluhen) is a reflcs glow, t,hen we orange, and finalIy 011 its upper bordcr of n rose color niay conipute tlie altitude of the niii-roring air layers. sonietinies verging on lire color. Bor t:hcse coloiu, or The result,s give dtit.utles of only 20 t,o 35 kin. for the rather these shades of tile colors are never well, clearly, aft,erglowat n solar depression of 4i0 to 6O, and of 70 t.0 or sharply defined here. Also, it is only under niore or SO krn. for a depression of go. The different phases of less favorable circumstances, according as the air is niore the afterglow correspond to niirroring layers at different or less free from vapors, exhalations, and clouds, that the 1ieight.s. anticrepuscule of one day or of one clininte differs from It seems to me tllat the following esplnnation of the that of another. Otherwise taliere is nothing more afterglow is also a possible one: uniformly constant thn this phenomenon, which is a The upper air layers reoeive almost solely yellow-red, purely optical one; therein quite different from the they are directly illuminated by the yellow-red. The aurora borealis which belongs to physics hut! is variable air reflects diffusely not simply blue done, but dl colors, and accidental. * * * as is shown by the whitish n earsnce alon the horizon; Now as the sun sinks below the horizonl the t..\\:iliglit,or however! it reflects mom of \!iue than of ta e other com- crepuscule sinks, ailti t,he [anti-twilight or] ant.icrepusculc ponent,s of whitre light. If it receives yellow-red almost rises by n corresponding amount; t.hose solar ra~ysthat esclusively, then it must also reflect diffusely the yellow- had been striking tlie vault at, or near the zenith no longer red, of course weakened. Thus there originates tlirough- reach so far, they are reflected from point,s nearer the sun, out the wholo zone of sir st,anclinu in the eart,h’s shadow and the tmt,i-twilight continues to rise; it.s luminous, n. difkx?fvening red (Ahendrot) bcRw th.e (iire.dlyi.Zbusmined colored arch detaches itself from the hluish, purple seg- evening red. This in at one and the same timc the ~u le ment which soon shows nothing but a gray or ash shade, light in t.ho west and the afterglow in the cast. 1n3e it [the c,oloretl arch ?] mounts steadily and finally conics urpIe light, we see not,hing. other than the higher air to the zenith where it is still visible when the air is there Lycrs illunlined by the evcning red from the sun. Yel- clear; for after havin nt,t,ainerl n certain elevation it low-red illurninatmionof t.he air appears as pu grows weaker and we&er, until at last it t,otauy tlisa - And the afterglow in the east is the reflection^%$^ pears. I have observed the anti-t,wili lit innunierab7 c dwin, Refles, Abglmz) of the western pur le. times in t,he more southern portions of brance, at Pnris. Finally, there is su port for tho probebiity that dif- and in its environs. fraotion affects the ligf t rays shining down to us from the The bluish and lmrple hand H t. t.lie horizon becoines higher and still direotly ilfumined air layers at the time gray and ash-colored when the anti-twilight or unticre- of the purple bht and the after low. Kiesling, Penter, puscular arch det.aches itself therc*froni.because the rtrl and Riggenbaceh, particularly aave assumed that dif- rays from the sun and from the inorc brilliant portion of fraotion was the cause of the western the twilight (cl$puscule) no longer are reflected so far According to Riggenbaah’s downward. tion of hrakatoa it was very dear (The author nlso remarks that while the secondary resulted from the widening of Bishop’s Rin rainbow is frequently observed, he is not aware t,hnt n certainly owed its ca per-red-brown to the secondary anti-twilight arch has ever been seen.-c. A., jr.] tion bv dust articP es. It seems to ___--~- refiectibn and Cfiffuse reflection send the light down to us 4 I. e., “An phenomenon or appesrana in the atm0spbere,” tho first slgnincance ol . ~ - the “old.- TL,. It lrlnkrwtlng to note here that MaIran urn the nnme %ntl-tWlight ar0h”not 1- I Heim, Albert L~ft-Farben Holer & Co. .4.-G. Ziirioh, 1912. 8 cm. pp. 70-74. tb.n 4 tlIW In thlr kcl8lnAsaement.- !hnul. [Notable Illust&tlons in color!]’

Unauthenticated | Downloaded 09/30/21 07:32 PM UTC NOVEMBER,1916. MONTHLY WEATHER REVIEW. 6% from the hi her illuminated air masses and that diffrac- only on our side of the latter. If the former, than the tion can in Buence only the color of the light. . clouds intercept the purple-colored solar rays; if the The color and the time of occurrence of the western latter, then the purple doe8 not develop where the cloud’s purple light and of the afterglow are in agreement with shadow falls. The former appears to me much the more all three classes of explanpt,ions. The relahe rarhy of probable case. the afterglow and its varying intensity with the osit.ion The western purple light, then, seems to be the evening of the sun speak in favor of the-iden of mirror re ection. glow of the hi her air directly illumined by the setting It seems to me, however, that if the .oause were mirror sun, reflected 8;own into the earth’s shadow to our eyes. reflection from the sunset zone tslie l!ght woul? appear The eastern afterglow shows us mountains and air in- somewhat less scattered and would gve more llght wid directly illuminated by the reflection of this lofty evening shade efkct in the mountains. As a matter of fact I have glow. .,. often wondered, as I stood in the mountains in the after- 7, at glow itself, the shadowless or ver weakly shadowing TWILIGHT PHENOMENA IN ABIZONA, SEPTEYBEB TO character of this eculiar flesh-red ligi t: and how it often DECEMBEB, 1916. a pears not mereP y as the western purple and tho eastern aP te low but fills the whole interveniii space wit.11 By Prof. ANDREWELLICOTT DOIJ~LASS. its a-pervading mysterious, gloomy ye1B ow-red. The [Dated: Deparlniciit of Physim 3nd Astrouomy, University of -4Iizm8, Tucson, breadth of the light-giving surface nnd of t.lie mirror Dec. 2, 1916.1 together are after all an inadsquatmeexplanation, it seems [In his letter transmitting this gper Profemor Douglaee correcta his to me, for this diffuse behavior of the after low. Just as 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 the sk is blue and tho mountains and vaf leys are fillrd of September, 1916. His accompanying description of twilight rolora wit8hblue haze durin the dsv, so nom- the sky is yurplc, is closely in accord with the claeaical description b von Besold, which, eo per-oolornd, or re dish yetlow and the mountains and t,ogether with Exner’s classification of the twilidt phenomenon will d be found translated on pages 620 to 63of this number of the REVIEW. vaPI eys aro filled wit.li these colors. If reflectioii throws Therefore. with the consent of Professor Douglaes, I have inserted in the western purple light. into our eyes and upon the east,ern brackets in his text the designation given bv Exner to the hase of mountains, t’hen t,here must. he n much stronger pohbriza- the t.wilight.deecribed. Thus, [(C) Fimt twilight arch] ie to %e inter- tion of the purple. greted aa nieaning that the phase of twili8ht under consideration will e found under (c) of Exner’s classification to have the deeignation If the sky’s blue were a true fluorescence of bhe air, given.--If. H. h’hball.] then the western purple light and the afterglow- would he here blue instead of red. I conclude, from their colors, that Prom September 16, 1916, to tihe present time the the sky’s blue is mere1 a pseudo-fluorescence. writer has wat,ched ever. evening aft,er sunset for the The peculiar form oB a “ salmon-colored spot ” in which occurrence of bright. t,wil ‘ght and afterglow colors, and the western urple light often begins, has not yet been shndow phenomena. The clearness of our atmosphere explained, wR ile the arched form of the western purple permits all t,hese henomena to be seen with greatest seems to be a matter of course.. Evidently we have not. ease down to our x orizon of mountains, which averages vet the last word in esplanation of the western purple about. one degree in elevation in all directions. From bght and of the afterglow. In the case of the afterglow west-northwest to wst,-southwest the average height is we may have to do mt,h the conibined effects of several about ISo, while in the northwest the apparent horizon factors. Nature is. indeed. always more coniplicated descends a very slight amount below the true horizon. than we like to kqsurne in thought,. On! t.lling is quite In the enst t.hc niounta.ins rips from 1’ to 3’ above the certain, thhe afterp.lOw on the ea&m m.oimtuana miid the cast- true horizon. ern 8k.v iS not a direct evening red. but an.inc7abect PrerLing On account3of t.1~clearness of the air t,he sun is never red which k brought down into the ea.rth’s .Q?t.m70ii7 hy mir- under any circumstances faint enough t.o be looked at wriri or difuse rqflcrtion. rc.nQ (1.ifkctiin.n. Put niorc directly hy t,he naked eye. Clouds are rare and two levels sim &: The afterglow in t,he east is the reflcct,eds >lendor of clouds still more rare, so that it heconies possible to of t e western urple light. We may t,hcrrfore cescrihe1 estimate cloiid heights lq- t,he t.inie when direct. sunlight the Rafterglow a o as an indirect, dpen low. Here also is c-enses to illuniinat,c thein. It thus becomes possible here repeat.ed R relation similar to t,hat. of t.5 ie fiist dpe~~glow:to usc nieniis of inrestigat.ion that, would seem incredible The west furnishes light and color, t.he east is illuniinat,cd to an observer nccust,omed only to A4t,ln~it.iccoast. con- thereby. Thus it. conies about t.hat light, nncl color are d i t,ions. stronger in the west than they are siniult.aneouslv in t,he Out, of 80 niglit.s, cloucls have interferred to tl serious east. If one stands on t.he eastern mountitins and wholly extent on only li nights. Plight. cloudiness has prevailed in the afterglow: there appcura no repet,ition of sunset in on over 51) per cent of t.he nights. In fact, it is a con- the west; one sees only a reat surface of purple light. dit,ion of n fcw thin, scat,t.eredciriiis clouds that is most spread over almost the whof; e westmeinsky. favorable t.o t,he heautiful ray phenomena so often Here and there a sheaf of divergent rays [crepuscultlr noticcd. [See Ahlso’s translations of Heim’s and Bezold’s raw] from a mountain or a cloud in the west.ern sky, descriptions of t,liesc crepuscular rap: this REVIEW,p. 622 reitches high up into the recently enlpurpled heavens. and p. 625.1 (See Heini’s fi . 18.) The shadow rays [or crepusaular As soon as the disk of t’he sun is behind the western rays] a pear bf ue-green in t.lie purple. Hence one niaq‘ mountain, a heavy bronze area [(b) Twilight low] is conclua e that the air tmhatmappears to he already in the apparent. estending to a dist,a.nceof 4’ or 5’ in a5 direc- western purple, as we stand down below it, still receives tions from the sui1 itself. At, first this bronze is of a direct rays from the sun at high levels above us. The &her yellowish color, which soon changes to a reddish western purple light therefore arises first from direct il- tone, lasting some 20 minutes after the disappearance lumination of the u per air lapis. Diffusereflection and of t,he sun. diffraction are but tK e processes t.hat conduct the splendor -4fter the sun has really set the clear western sky in down to our eyes in t.he eart,h’s shadow or across to the that. general vicinity shows a structure which seem8 momt aim. likely t,o be due to a high layer of haze in the atmos- This does not enable one to decide. however, whether phere. This structure usually ap ars as a faint, soft the purple light originates in the west behind the cloud or etching of large numbers of paralelp“ lines. In general

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