Halo , (Gr. Halo 0800275 a threshing floor, originally of a round shape), a term commonly used in meteorology to include all those phenomena in which a luminous ring, either colored or un-colored, is seen around the disk of the sun or moon. There are two distinct classes of such phenomena, called coronae and halos, and it is only for convenience that the latter term is sometimes used to include them all. Here we shall consider them under their several heads. The meteorologist Kaemtz includes under the term coronas all cases in which, when the sky is covered with light clouds, colored circles are seen surrounding the sun or moon ; also when a glory is seen around the observer's shadow on a cloud. Under the head "halos properly so called," he includes the great circles which surround the sun or moon, the diameter of which amounts to about 44°. The attendants of halos are: 1, circles having a double diameter; 2, parhelia or mock suns; and 3, various other circles.

Coronas are distinguished from halos in this fundamental respect, that the former are due to particles or vesicles of water in mist or cloud, the latter to minute crystals of ice. - Coronoe. All clouds which are not too dense to prevent the light of the sun or moon from passing through, produce coronae of greater or less intensity and regularity. When the clouds are irregular in outline, the corona) are incomplete. When the corona is complete, the following arrangement of colors can bo recognized. Close by the sun a dark blue circle can be perceived, next a white circle, and then a red; outside the series there can be seen under favorable conditions a second series, consisting of colored circles in the following order, proceeding eastward from the sun : purple, blue, green, pale yellow, and red. "More frequently," says Kaemtz, "we observe near the sun blue mingled with white, then a red circle clearly limited within, but confused outside with the others. If a second red circle exists outside this, then green is observed in the interval by which they are separated.

The distance of this circle from the centre of the sun varies according to the state of the clouds and the atmosphere; 1 have found it from 1° to 4°." The rings of coronae, the colors of which are those of the reflected series in thin plates, are fringes due to interference of rays which have undergone diffraction by grazing on either side of numerous minute globules of cloud or fog, that have for the time nearly the same size. An illustrative instance was first given by Necker of Geneva. When the sun rises behind a hill covered with trees or brushwood, a spectator in the shadow of the hill sees all the small branches that are nearly in the line of the solar rays, on either side, projected on the sky, not black and opaque, but white and brilliant, as if of silver; the effect of a small opaque body on the light being, in this class of cases, equivalent to that of a small opening in a dark body through which the rays should penetrate. Coronae exist around the sun more frequently than would be supposed; but they are often not observed, on account of the brilliancy of that orb.

At such times they may be detected by looking at the reflection of the sun in still water, or in black glass. - Anthelia. When the sun is near the horizon, and the shadow of the observer falls on any surface covered with dew, there can be perceived a glory especially round the head of the shadow. Anthelia are also seen, and more perfectly, when the observer's shadow falls on or near clouds that lie opposite the sun; or in polar regions when the shadow is cast horizontally upon a fog. Bouguer was the first to observe the phenomenon. He noticed that the shadow of his head, on clouds among the Andes, was encircled by three colored rings having diameters of 5 2/3,°, 11°, and 17°. Scoresby, who observed the phenomenon in polar regions, saw four concentric circles round the shadow of his head: the first was white, yellow, red, and purple, and had a semi-diameter of 1° 45'; the second was blue, green, yellow, red, and purple, and had a semi-diameter of 4° 45'; the third was green, whitish, yellowish, red, and purple, and had a semi-diameter of 6° 30'; the fourth was greenish, white, and deeper at the edges, and the semi-diameter of its internal edge was 3G° 50', that of its external edge being from 41° to 52° This fourth circle, commonly called the circle of Ulloa, or the white rainbow, is very seldom seen.

Anthelia are explained upon the same principle as corona), with the single exception that the diffraction in this instance does not occur during the direct transmission of the solar beams through the cloud, but during the retrograde transmission of rays which, having penetrated to considerable depth in the cloud, undergo reflection, and are then diffracted by nearer globules while on their return to the eye. - Halos proper. In the commoner forms, one or two rings, formed in the thin, feathery cloud overspreading the sky at a great height, extend vertically about the sun or moon. The diameters of these vary somewhat, and sometimes during the same display; but when one only is seen, its distance from the sun is very near 22 1/2°, or diameter 45°; the latter number being also the radius or distance of the second, when seen. Of these rings, the color of the inner border, when obvious, is almost invariably red; next to this comes green or blue; the outer edge is one of the latter colors, violet, or white. Very rarely the outer border is red. The breadth is usually slightly less than that of the luminary; sometimes a ring appears as if made up of two lying side by side, and crossing each other in very acute angles at certain points of their course.

As in the rainbow, the red border is most defined, the opposite being lost in a diffused light; and the sky just within these rings usually appears more dark than elsewhere. The third and largest ring, having a radius of about 90°, is usually white; but it is seldom or never complete, even in the visible portion of sky. The same obvious atmospheric conditions that show these more ordinary forms, may also give rise to a white circle having the breadth of the solar disk, through which it passes, and extending completely around the sky in a plane parallel with the horizon. A second white band of the same breadth may pass through the sun's disk in a vertical direction, while at the points in which this intersects the two ordinary rings above, tangent colored arcs may be seen curving away from the rings respectively, and tending to include the zenith. - Parhelia and Paraselenoe. A parhelion is an attendant image, more or less distinct, of the sun's disk, which may appear with any halo, at one, two, or more points near the sun; but which is more frequently formed in the course of the horizontal or vertical white bands, or in both, at or slightly without the intersection of these with the ordinary halos.

Parhelia show the usual colors of halos, in the same order, but more brightly, and even when the latter are not perceptibly colored. They seldom appear at once at more than three or four of the intersections, and sometimes present a sort of tail in the direction opposite the luminary. Popularly, they are known as sun dogs, or mock suns. The corresponding appearances about the moon are termed paraselenae. The more complicated phenomenon, showing both the halos and mock luminaries, although somewhat rare, is still much varied in form. Among the earlier clearly recorded observations may be named an example of the solar observed by Schemer at Rome in lf>29, and of the lunar by Hevelius of Dantzic in 1660. Very brilliant halos were seen about the sun for several days in succession at Moscow in 1812, during its occupancy by the French; the most splendid instance on record occurred at Gotha, May 12, 1S24. - In high northern latitudes, halos and parhelia are very frequent; Capt. Parry always saw the former about the time of full moon.

But whether in higher or lower latitudes, they are only seen when there intervene between the luminary and the observer those highest thread-like forms of cloud, the cirrus or cirro-stratus. The cold prevailing in the elevated regions occupied by these clouds renders it quite certain that their particles must be in the frozen condition - a fine ice mist -such as we experience in the coldest days of winter, and which, driven against the face by a wind, actually prick the skin. These crystals incline chiefly to the form of hexagonal prisms; and to refraction and decomposition of light passing through certain angles of these, Mariotte was led to ascribe the production of halos. For any refracting angle of a prism there exists a minimum angle of deviation, dependent on the density and the angle. Now, the minimum deviation of a decomposed ray occurs when the angle of refraction is just half the refracting angle of the prism used. The refracting angle of the ice prisms being 60°, the angle of refraction giving the least deviation for the red ray from the original course of the light must be 30°; and, the refractive index of ice being 1*31, the angle of incidence must be 41°. Then the deviation, being equal to twice this angle less the refracting angle, or 2 x 41° - 60°, is 22°, a result very closely agreeing with that of observation for the smallest and most common form of halo.

To produce this ring, then, it is only necessary to imagine the minute prisms of ice floating or descending through the air in all positions, but, owing to the resistance presented by the air to the action of their weight, taking especially horizontal and vertical directions; then, near to the position giving a minimum deviation of the transmitted rays, a considerable turning of the crystal about its axis gives only an insignificant change in the direction of the emergent light; and hence, a far larger number of the crystals will transmit red rays deviating from their previous rectilinear course by exactly or nearly this angle of 22° than by any other. The rays from the sun or moon being in effect parallel, there should therefore be seen, at nearly this angle with the luminary, a dim circle, red and defined within, but beyond this having the colors overlapping one another, and indistinctly seen or resulting in white. The halo of 90° or 92° may be accounted for by the refraction occurring through the angles of 90° at which the sides of the ice prisms meet their bases, the minimum deviation for red in this case being about 45°. The partial polarization of the light in a plane tangent to the ring shows it to be refracted light; that of the rainbow being polarized in a plane normal to the circle, and mainly due to reflection.

Musschenbroek saw large colored halos about the moon, by looking through plates of ice formed on the panes of his window. The white horizontal and vertical bands can be explained by reflection from the vertical faces of crystals, descending in a calm air and in all possible azimuths. The parhelia may be considered as being the intensified effect at certain points of a greater condensation of the dispersed rays at the angles of minimum dispersion; so that they are to the halo what the halo itself is to the diffused light thrown on the surrounding cloud. The fact that they are usually a little without the rings has been supposed due to the greater obliquity of the crystals, at the points where they appear, to the plane of refraction. But while the explanation of these simpler parts is quite satisfactory, that of the more complicate and peculiar phenomena becomes extremely difficult; and we can only in a general way refer these to the variety of changes, including reflection and simple and double refraction, of which light is capable, and to the probable effects of extraordinary forms and combinations of the crystals.

If the views taken of the phenomenon be correct, then, by consequence, halos prove what is the temperature of the highest cloud region, and the condition of cloud occupying it. Certain it is that they are closely connected with peculiar meteorological changes; and that, occurring in summer, they indicate rain, and it has been said wind, while in winter they precede snow, or it may be also frosts.