Meteorology: The Science of the Atmosphere

by Charles Fitzhugh Talman

(After Besson, Monthly Weather Review, July, 1914.)

1. (Upper). Perspective view of the sky, showing the sun (S); ordinary halo of 22° (a); great halo of 46° (b); upper tangent arc of the halo of 22° (c); lower tangent arc of the halo of 22° (d); ordinary parhelia of 22° (e, e’); Lowitz arcs (f, f′); parhelia of 46° (g, g′); circumzenithal arc (h); infralateral tangent arcs of the halo of 46° (i); the parhelic circle (m); a paranthelion of 90° (q); light pillar, (u, u′); the observer (O). 2. (Lower). Perspective view of the sky, showing the observer (O); the parhelic circle (m): ordinary paranthelia of 120° (p); the paranthelion of 90° (q’); the oblique arcs of the anthelion (r, r′); and the anthelion (n).

The commonest halo is a circle of 22 degrees radius (the 22-degree halo) about the sun or moon. When formed by the sun it generally shows a distinct reddish inner border and traces of other spectral colors. The lunar 22-degree halo usually appears colorless. This halo is visible, in whole or in part, to the attentive observer about once in three days, on an average. Less common, but by no means rare, are the parhelia or “sun dogs” of 22 degrees (called paraselenæ or “moon dogs” when formed by the moon), the beautiful circumzenithal arc, and a few other members of the halo family. Most forms of halo are so uncommon that their appearance is an event of some scientific importance.

The accompanying diagrams, by Dr. Louis Besson of the Observatoire de Montsouris, show the positions, with respect to the sun (or moon), of the majority of known halo phenomena. The upper diagram shows the halos that occur on the same side of the sky as the sun (or moon), and the lower those that appear on the opposite side. Most of these halos, when bright, show the spectral colors. The circumzenithal arc, h (commonly described, by the uninitiated, as a “rainbow”), and the parhelia of 22 degrees, e, e′, are especially brilliant in their coloration. The parhelic circle, m, which sometimes extends entirely around the sky, is white, and so are a few of the rarer forms of halo.

The upper and lower tangent arcs of the halo of 22 degrees, c and d, undergo striking alterations, with changes in the altitude of the sun. When the luminary is more than about 40 degrees above the horizon, these two arcs become joined at their tips to form the circumscribed halo, and at still greater solar altitudes this halo contracts from an elliptical to a circular form, thus blending into the 22-degree halo as shown on the next page, where the solar altitudes corresponding to the different forms of the halo are indicated. The positions of the parhelia of 22 degrees, e, e′, also depend upon solar altitude. When the sun is on the horizon these “sun dogs” are 22 degrees from the luminary, and therefore lie in the 22-degree halo; at greater solar altitudes they lie outside this halo.

The reader who wishes to acquaint himself further with the different forms of halo and the methods of observing them will find a comprehensive article on the subject (devoid of mathematical discussions) in the “Monthly Weather Review” (Washington, D. C.) for July, 1914.

SUCCESSIVE STAGES OF THE UPPER AND LOWER TANGENT ARCS OF THE 22° HALO

When the sun is high they unite to form the “circumscribed halo.” (Altitude of sun shown in the center of each figure.)

The ice crystals that produce halos consist of hexagonal plates or columns, occasionally including complications of structure, such as pyramidal bases, combinations of plates and columns, etc. These have the well-known effect of prisms in refracting and dispersing light that passes through them. It is evident that there are many possible paths for the light rays through the sides and bases of such crystals, resulting in different deflections and corresponding differences in the forms and positions of the halos produced. The attitudes assumed by the crystals as they slowly sink through the air, and the oscillations they undergo, are further points to be considered in working out the theory of each form of halo by the application of the laws of optics. Nearly all the known forms have been fully explained. A few species of halo—notably the parhelic circle (called paraselenic circle when formed by the moon)—are due to simple reflection from the faces of the ice crystals, and not to refraction.