GRAND TETON NATURE NOTES

A DOUBLE LUNAR RAINBOW

by
Dr. F. M. Fryxell

Lunar rainbows, popularly called "white rainbows", receive only passing mention in most discussion of atmospheric optics, since the principles they involve are practically the same as for solar rainbows. Such references as one can discover convey the impression that lunar rainbows there at best faint and indistinct phenomena. That there may be occasional and impressive exceptions to the rule was established by the occurrence of a nocturnal rainbow of extraordinary brilliance in Jackson Hole, Wyoming, on July 9, 1930, witnessed by Mr. Karl Kent and the writer.

Early in the evening a thunderstorm invaded Jackson Hole from the southwest, crossing the Teton Range in the vicinity of Teton Pass and then, changing direction slightly, moved north-northeast up the length of the basin and crossed Jackson Lake — a course often followed by summer thunderstorms. As the storm formation moved overhead at Jenny Lake no showers resulted, but when we set out from the lake about 8:45, shortly after dark, it was apparently raining in the north end of the basin, for our attention was attracted to a lunar rainbow forming against the dark sky to the north. The great arch bridged the basin and Jackson Lake, one end resting on the Tetons, the other on the mountains east of the Hole. Gradually it deepened in intensity, its outlines and colors becoming strikingly distinct. Meanwhile a secondary, outer bow came into existence, its colors much paler than those of the primary bow, brightest near the horizon and fading out towards the crest.

At the time of the maximum intensity the primary bow compared favorably in brilliancy and distinctness of coloration with the magnificent solar rainbows we see so frequently in these mountains following afternoon thunderstorms. In its night setting the double lunar rainbow formed a scene of awesome splendor, impossible to describe satisfactorily, the most impressive nocturnal spectacle I have ever witnessed.

During this apparition to the north the moon was in the south-southeast, partially obscured by small clouds which alternately hid and disclosed it. The constantly changing light effects resulting from the illumination by the moon of these drifting clouds contrasted strangely with the steady radiance of the double rainbow silhouetted against the overcast sky to the north.

With the retreat of the storm the rainbow at last faded, and at the same time the scattered clouds of the south were dissipated. The lunar bow had been in sight from about nine o'clock to ten-fifteen.

Nothing in our past experience in the mountains had even remotely suggested to us the possibility of such a spectacle, nor have any accounts of similarly brilliant nocturnal rainbows come to our attention. Probably they are of exceptional occurrence, calling for a delicate adjustment rarely attained of those physical conditions requisite to the formation of rainbows.