Halos and Coronæ. Halos are circles of light surrounding the sun or moon, and are due to the presence of ice-crystals in the air. The commonest and usually the brightest has a radius of about 22 degrees—i.e. this is the angular distance from the sun to its inner edge. This size can be computed from the hexagonal shape and known refractive power of ice-crystals. The calculation shows that light passing through the sides of such a crystal is bent at an angle varying with the direction in which it falls on the crystal, but never less than degrees, which is therefore called the angle of minimum deviation, and in the greater number of cases not greatly exceeding that angle. If, therefore, the air between the observer and the sun or moon be filled with such crystals the light will be thrown outwards beyond the angle of minimum deviation, and will mostly appear at about 22 degrees distance from the sun or moon, forming a circle round it. As blue light is slightly more refrangible than red it is thrown farther out, and the halo appears coloured red inside and blue outside. Some of the crystals may, however, be lying so that the light enters at a side and leaves at one end, or vice versa, in which case the angle of minimum deviation is about 46 degrees, at which distance a second fainter halo is frequently seen with colours in the same order as in the first. These colours are generally well seen in solar halos, but not in lunar, as the moon's light is too faint to give distinct colour to each part. In addition to the above, a third still larger halo has been seen. There are only four observations of this halo on record, and the radius has been estimated in the different cases at from 81 degrees to 90 degrees. The cause of this halo has not been ascertained. It is not coloured, and may be due either to some more complex form of ice-crystal or to internal reflection from the hexagonal crystals.
Another phenomenon sometimes seen with halos is the Parhelic circle, which is a white circle passing through the sun and parallel with the horizon. It is caused by light reflected from the surfaces of ice-crystals falling vertically through the air. When the sun is near the horizon this circle is intensified at distances of 22 degrees and 46 degrees from the sun, and forms parhelia or mock-suns, and another mock-sun is sometimes seen on this circle directly opposite the sun. A similar circle is also formed passing vertically through the sun by reflection from the upper and under surfaces of the ice-crystals. Halos are sometimes accompanied by contact arches, which are arcs of circles touching the halos of 22 degrees and 46 degrees; they are formed by long hexagonal prisms floating horizontally in the air, and are curved away from the sun when it is below 30 degrees altitude, but are concave towards it at greater elevations. Several other more complex forms of halo have been seen in the arctic regions, but are of rare occurrence in Britain.
Halos must not be confused with Coronæ, which are smaller coloured circles that appear round the sun or moon when they shine through thin cloud or mist. In these the red is the outermost colour, and several successive sets of coloured rings are usually formed. They are due to the diffraction the light undergoes in passing among the drops of which the cloud is composed. The radius of the first red ring of a corona varies from 1 degree to 3 degrees, according to the size of the drops, and the radii of the others are successive multiples of that of the first.
When the sun shines on a bank of fog a large bow of about 40 degrees radius, resembling a rainbow, but not so brightly coloured, is seen. It is often double, like the rainbow. Owing to the smaller size of the water-drops in a fog than in falling rain, the Fogbow is wider and fainter than the rainbow. The law determining the order of the colours—whether red inside or red outside—has not yet been thoroughly worked out. If the observer is standing on an elevated point so that his shadow falls on the fog, coloured rings called Glories or Anthelia are often seen. Five or six sets of colours have been observed, the outermost having a radius not exceeding 12 degrees. In each ring the red is outside, showing that it is a diffraction effect like a corona, but the exact cause has not been determined. If the fog is very near, the observer's shadow is visible, forming what is known as the Brocken Spectre (see MIRAGE); and if the fog is thin the shadow looks farther away than it really is, and is therefore supposed by the spectator to be of gigantic size.