The words halo, nimbus, aureole, gloriole and glory all refer to the same thing--a cloud of light around the head of a figure of great spiritual importance, and often represented in art, often as a golden cloud radiating light; the golden crowns of kings represent this as an item of adornment, since an actual aureole has always been found lacking. These things have never been observed by anyone at any time, and on this evidence they are an utter fiction.
The natural phenomenon of the glory or anti-corona has, however, definitely been observed. It is seen as colored rings surrounding the shadow of the observer's head on a cloud in sunlight. Since one had to be on a high place with the sun rising and clouds below, it was very seldom seen. The top of the Brocken, in the Harz Mountains, was one of the places that it could be seen, and so is also called the Brocken Spectre. All of the References relate the early sightings of the glory, and the wonder that it engendered.
The glory, as seen in sunlight, usually has a bright centre with a red border, surrounded by one or more rings blue on the inside and red on the outside. The whole display may be surrounded by a white fogbow under certain conditions. It moves with the observer, as does the rainbow, and is centred at the shadow of the observer's eye, or at the lens of the camera that takes a photograph. If you hold the camera at waist level, the glory will be around your belly, as Tricker likes to point out. Your companions may cast shadows on the cloud as well, but they will not be adorned with glories.
I have seen the glory many times when flying over Wyoming during the afternoons in the 1950's in DC-3's that dodged the thunderheads. The shadow of the plane was often cast on a cloud, with excellent glories, all centred on my window. This is probably not as easy these days, since the planes on such local flights are gone, and what flights do exist fly too high and the Portuguese-tight-pack seating style rarely gets you a window.
The glory should not be confused with the heiligenschein, which is light returned by reflection in which the water drops act as lenses, and reflect light like the "cat's eyes" used in road signs. This light may show colored flecks, but is mainly white, and depends on a backing for the water droplets. The heiligenschein also makes a luminous halo around the shadow of your head on, say, dewy grass, but there are no colored rings. In German, "heiligenschein" is the usual word for halo, so there a distinction has to be made.
The glory was reported before the wave theory of light was discovered, and was originally thought to be similar to the rainbow. The clouds were supposed to contain ice crystals, whose plane faces could reflect light. A persistent idea, which survived until recently, was that the cloud reflected light, and the glory was produced in this reflected light by particles closer to the observer. With the wave theory of light, the glory became a diffraction phenomenon in this reflected light. Theory succeeded theory, but none was sufficient to explain the glory satisfactorily.
It is becoming clear what causes the glory. Light entering at the periphery of the drop is refracted into the drop at almost the critical angle, reflected at the rear of the drop, and then refracted back at almost the critical angle the way it came. With ray optics, it does not quite make it, since the critical angle for total internal refraction is 48.5°, not the 45° that would be required to make this perfect. However, for angles of incidence close to the critial angle there is a strong interaction with a surface wave that travels over the drop, spewing light like a rotating lawn sprinkler. This means that a drop in a beam appears as a ring of light from directly in front. There is also some light reflected from the convex surface of the drop that looks like a point of light from this direction, and adds to the annular wave. The diffraction pattern of randomly arranged ring sources looks like the glory.
The annular wave was first used by Tricker to explain the glory, and we now know that this is essentially correct. Polarization effects are important, so in general it is not an easy problem if exact results are desired. Of course, electromagnetic theory can be used, but this is extremely complicated. Electromagnetic theory seems to support the optical theory, however. The intensity found by Tricker was of the form coJo(x)2 + c2J2(x)2, where x = (2π/λ)r sinθ, co = 0.634 and c2 = 1.450. This agrees with the observed intensity, in which the first dark ring is not very dark. The effect of the directly reflected light is not included. Tricker regards the glory as an extension of rainbow theory, and this may be valid for very small drops.
The drops on which the glory is seen are smaller than raindrops, less than 0.5 μm in diameter. The smallness of the drops explains the frequent appearance of the fogbow in connection with the glory. For drops this small, the Airy theory of the rainbow is not applicable, and new methods borrowed from quantum mechanics are necessary. The drops are still large compared to the wavelength of light, however, which complicates the electromagnetic theory.
R. A. R. Tricker, Introduction to Meteorological Optics (London: Mills and Boon, 1970). Chapter VII.
H. C. Bryant and N. Jarmie, The Glory, Scientific American, July 1974.
M. Minnaert, The Nature of Light and Colour in the Open Air (New York: Dover, 1954). pp. 224-225.
Composed by J. B. Calvert
Created 31 July 2003