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318 Two Halo Displays Over Eastern U. S. in December 1948

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318 Two Halo Displays Over Eastern U. S. in December 1948 318 BULLETIN AMERICAN METEOROLOGICAL SOCIETY Two Halo Displays over Eastern U. S. in December 1948 GENE C. STEVENS AND SIGMUND FRITZ U. S. Weather Bureau, Washington AND ROBERT J. LIST * Smithsonian Institution HE halo around the sun is a frequently most due west at an elevation of about 24° there observed optical phenomenon. Only oc- was a sundog (P) or mock sun (paranthelion, also Tcasionally, however, is the halo accom- termed parhelion of 120°) which appeared as a panied by less familiar phenomena such as mock bright white spot on the clouds. Also toward the suns, white bands, and/or colored arcs. A galaxy northeast at the same elevation was another mock of such elements is called a halo display. At sun (P')> but not as bright as the former. Since Washington, D. C., two interesting halo displays this latter sundog was the same angular distance were observed within a period of only eleven days from the real sun, it also was a paranthelion. At —the first on December 10 and the other on the same time a halo of 22° radius (H) encircled December 21, 1948. The upper-air conditions the sun. Tangent to the western edge of this halo accompanying and producing such displays have was another mock sun (A) (parhelion of 22°) ; a been studied and several interesting relations seem complementary parhelion (A') was similarly tan- to be substantiated in these cases. gent to the eastern edge. The brightness of these two parhelia produced a glare at each point which made their shapes nebulous. Connecting the four sundogs and the real sun was a faint white line (CC) forming a part of a circle (parhelic circle) which almost girded the sky. Only in the north- west quadrant was this parhelic circle not visible. By about 10 :20 a.m. the appearance of the phe- nomenon had changed slightly due to movement of both the clouds and the sun. The western par- anthelion had dimmed to a faint spot while the northeastern one had taken on new prominence. In addition, an arc of a second halo, of 46° radius (FF), had appeared as a faint white line above the sun. Tangent to this halo, but with opposite curvature, was a brilliantly colored rainbow-like arc (GG). The center of this arc was the point directly overhead (zenith) and the arc is hence termed a circumzenithal arc. The entire display reached its climax at ap- proximately 10 :25 a.m. A narrow vertical band FIG. 1. Schematic diagram of the halo display of De- of color (B,B') appeared in about the center of cember 10, 1948. The outer circle represents the horizon each parhelion of 22°. These two bands of color and the center (Z) represents the zenith. The elements displayed are (S) the sun; (A,A') parhelia of 22°; (possibly segments of Lowitz' arcs) were small (B,B') bands of color which were possibly segments of but brilliant. The colors in the 22° halo were Lowitz' arcs; (H) halo of 22°; (CC) parhelic circle; quite evident at this time, even though they were (P,P') paranthelia; (FF) arc of halo of 46°; and (GG) diffuse. The crowning glory to the phenomenon circumzenithal arc. was the vivid coloring of the circumzenithal arc which attained a brilliance that was dazzling. DECEMBER 10, 1948 The distribution of color in each of these four spectra was red on the side nearest the sun to The display of December 10 (FIG. 1) was first blue on the opposite side. noticed at about 10:05 a.m. (EST) when the sky Within the next five minutes the display began was overcast with thin cirrostratus clouds. At al- to fade, the circumzenithal arc being the first to * Now with U. S. Weather Bureau. disappear. The last element of the phenomenon Unauthenticated | Downloaded 10/01/21 04:27 AM UTC VOL. 31, No. 9, NOVEMBER, 1950 319 to retire was the eastern parhelion of 22° which between the sun and parhelion. It was then that faded about 10:40 a.m. the parhelic circle (CC) became quite evident. Except for the short space between the western DECEMBER 21, 1948 parhelion and the sun, the parhelic circle was On December 21 a similar display (FIG. 2) visible from southeast through south and west to was observed when six- to seven-tenths of cirrus northwest where it terminated at the paranthelion. and cirrostratus clouds were present. The display Every effort was made to discern a halo of 46° began in the form of the circumzenithal arc (GG) radius but without success. at 1:28 p.m. (EST). At that time the arc was As the cirrostratus clouds dissipated, the display colored and faint but was rapidly becoming bril- gradually faded; the last visible phenomenon was liant in color. Immediately an effort was made to the eastern parhelion which disappeared at 1 :45 see whether any other elements were visible. p.m. It was thought that the display had termi- While looking for parhelia, one (A') seemed to nated. However, at 1 :50 p.m. the circumzenithal appear as if by magic on the eastern side of the arc reappeared almost as brilliantly colored as the sun. Close scrutiny revealed an arc of the 22° first time. Three minutes later, at 1 :53, the arc vanished. Between 2:10 and 2:13 p.m. the par- anthelion to the northwest wTas again visible. This time it covered a larger area than before although no part of the parhelic circle could be seen. Again at 2 :15 the circumzenithal arc made an appearance although it was quite faint. The recurrence of these elements was occasioned by the now patchy cirrostratus drifting into the correct location. When the arc faded at 2:17 p.m., the display had finally terminated. COMPARISON OF THE Two DISPLAYS A comparison between the two halo displays brings forth one major difference—the location of the parhelia with respect to the 22° halo. In the first instance they appeared tangent to the halo, while in the second display they appeared slightly outside the halo. A theoretical analysis [1] shows that for a solar altitude of 25° the parhelia should be removed from the halo by approximately 2°. FIG. 2. Schematic diagram of the halo display of December 21, 1948. The outer circle represents the Since on both dates the solar altitude was very horizon and the center (Z) represents the zenith. The close to 25° (24° on the 10th, and 23° on the elements displayed are (S) the sun; (A,A') parhelia of 21st), it follows that the parhelia should have 22°; (H,H') arcs of the halo of 22°; (C,C) parhelic been slightly displaced from the halo. Perhaps circle; (P) paranthelion; and (GG) circumzenithal arc. the fact that they appeared tangent in the first instance is explained by the glare surrounding halo (H') passing between the sun and the par- each parhelion—the brightness producing an illu- helion, very close to the latter. The complemen- sion as to their exact position. In the case of the tary parhelion was not visible, nor was the oppo- second display, the parhelia were less bright and site arc of the halo. A visual sweep of the entire their positions were easily discernible. Other dif- sky revealed a faint paranthelion (P) toward the ferences between the two displays were the absence northwest. The cirrostratus clouds which covered of certain elements on the second date (the 46° the sky from southeast through west to north halo, the second paranthelion, the coloring of the were moving from the west-southwest and rapidly parhelia), and the increased prominence of the dissipating on the forward edge. Since there were parhelic circle on the second occasion. no clouds to the east, no paranthelion was possible in that direction. When attention was again cen- METEOROLOGICAL CONDITIONS tered toward the sun, the parhelion (A) to the The individual elements of such halo displays west of the sun was present. Also the arc of the have been seen on many occasions and the theories 22° halo (H) was faintly visible, likewise running explaining their presence are widely discussed. Unauthenticated | Downloaded 10/01/21 04:27 AM UTC 320 BULLETIN AMERICAN METEOROLOGICAL SOCIETY stability. The dotted line (moist adiabat) in FIGURE 3 shows the lapse rate which would be associated with unstable saturated air. Thus the air was fairly stable between 17,000 and 34,000 feet, the latter being the highest point of observa- tion. The lapse rate of December 21 shows in- creased stability at least between 16,000 and 24,000 feet. Both of the areas coincide reasonably well with the approximate heights of cirrostratus clouds. Since, with other factors being equal, more stability indicates less turbulence, the theory is somewhat substantiated. FIG. 3. Lapse rates determined from radiosonde obser- vations at Washington, D. C., near the times of the halo displays. A moist adiabat is included for comparison. Arrows indicate the base of the layer of increased stability. During the occurrence of complex halo displays, cirrus or cirrostratus clouds generally consist of ice crystals with flat bases and with hexagonal pattern [2], Refraction and reflection of light by FIG. 4. Weather map for 1030 EST, December 10, the faces of these crystals produce the optical phe- 1948. Stations reporting snow (*) or rain (••) are nomena already described. The physical explana- indicated. Broad arrow indicates air flow at the cirrus tion of each individual element is presented in the level. comprehensive work by Humphreys [ 1 ]. A study of the weather situation at the time of The fact that the cirrostratus level was non- these halo displays (especially on December 10) turbulent * does not imply that the air was calm.
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