GRAND TETON NATURE NOTES

A SOLAR HALO

by Howard R. Stagner
Junior Park Naturalist

A beautiful and quite rare optical phenomenen was observed in the sky on the morning of January 20 near the Granite Canon Showshoe Cabin by Superintendent Whitcraft and the writer. A series of colored rings, concentric around the sun, tangent arcs, and bright spots appeared in the display.

The preceding day had brought heavy, low-hanging clouds and considerable snowfall on the higher slopes, but during the night the atmosphere cleared and the temperature dropped to 21 degrees below zero. The morning of the twentieth the sun rose in a cloudless sky and the thermometer hovered near zero throughout the day. A fine shower of tiny, light frost crystals was falling during the early forenoon.

The haloes were first seen shortly before ten o'clock in the morning. They maintained their brilliance for about fifteen minutes, then gradually faded to extinction by noon.

The display consisted of two rings or haloes around the sun, horizontal band of light passing through the sun, two upward bending arcs tangent to the halos, and seven bright spots on the rings. Of the halos, the inner one was the more intensely illuminated and colored. Both were colored like the rainbow except that in both rings red appeared on the inner side. The inner halo was complete, while the outer one was visible through an arc of some 200 degrees with the lower part passing below the horizon. The band of light passing through the sun appeared to be a part of a huge arc which was faintly visible only within the rings and for a short distance beyond. Tangent to the upper side of each ring were arcs, quite short and similar to the rings in coloration. Bright spots were formed at the intersections of both the light band and the two arcs with the two rings, the brightest being on the inner ring. At the lower edge of the inner halo a bright spot appeared, possibly marking the tangent point of another colored arc below this point.

According to Moore (Descriptive Meteorology, Appleton, N.Y., 1914) and Weld and Palmer (Textbook of Modern Physics, Blackiston, Philadelphia, 1925) solar haloes are interference phenomena produced by refraction and dispersion of light in passing through tiny frost crystals. The crystals, hexagonal prisms capped by pyramids, falling slowly through a quiet atmosphere, are believed to be in rapid rotation around the longer crytal axis which in falling assumes a horizontal position. To form the inner colored halo, light rays undergo refraction both in entering and in leaving the crystals, and are thus bent toward the observer. In addition, the light rays which produce the sensations of the various colors are separated or dispersed to form the colored bands. A similar refraction and dispersion forms the outer colored ring, but in addition, at this greater angle from the sun, internal reflection occurs within the crystal. The inner ring forms at about 22 degrees away from the sun, and the outer at 46 degrees. White light reflected from the surfaces of certain crystals without refraction or dispersion is responsible for the band of white light which passes through the sun.

While such haloes are considered quite rare outside the polar regions, several such displays of less development have been observed both in Grand Teton and in Yellowstone National Parks. In Yellowstone Nature Notes, Vol. XIV Nos. 1 and 2 for January-February, 1937, William E. Kearns, Assistant Naturalist, describes a partial solar halo, apparently almost identical with the one herein described, which was seen in Yellowstone Park on December 29, 1936.

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