On Tuesday, there was overrunning of cirrus over the Lake. On Wednesday, there was rain. Lorraine Symmes took a picture to the west and sent it to me. It showed a halo complex, something that often precedes rain.
All of the optical phenomena in the picture are the result of tiny ice crystals in the sky bending the sunlight. While popular consensus is that ice crystals falling from the sky are uniformly the beautiful stellar crystals, most are not. Most crystals are either simple hexagonal plates or columns. And these fall differently. The small hexagonal plates fall nearly horizontal rather like dinner plates spread haphazardly on a table. Small columnar crystals fall at right angles to the plates, rather like endless pencils spread on the table. This halo complex is made of both types of crystals: plates and columns.
We start at the top of the picture.
Circumzenithal arc: This is caused by the refraction of the sunlight through the 90° edges of the oriented hexagonal plates. This one is a tad faint, but is the portion of the circle with the zenith (off the frame) at its centre.
Supralateral arc: This arc is tangent to the 46° halo with which it is often confused. It touches the circumzenithal arc but is curved down. This is caused by the refraction of the sunlight through the 90° edges of the oriented columns. It is quite bright in this picture and its centre approximately on the sun.
Upper tangential arc: The optical phenomena in the lower portion of the picture appears separate, but is caused by the same types of crystals. The upper one appearing like a graceful bird’s wings is the upper tangential arc. This is caused by the refraction of the sunlight through the 120° edges of the oriented columns. The 120° is accomplished by light passing through alternate sides of those separated by 60°. This arc evolves quickly depending upon the sun’s height.
22° halo: This is the most common of all the phenomena. It is located at an angle of 22° from the sun. Like the others, it is the result of refraction but of randomly-oriented small plates or columns. It is caused by refraction though 120° prisms.
A halo complex usually presages rain.
Photograph courtesy Lorraine Symmes.