As a rule, the various circles, with the exception of the halo circle, are only partly visible; and in many cases the unusual arcs seem to have no connection with the halo. Many interesting illustrations of complex halo circles have been published. Usually these have the circles of 22 degrees, 46 degrees, and the mock-sun circle in common; otherwise they are unlike.
Occasionally a vertical column of sheen extends above and below the sun—perhaps more frequently the moon; it is known popularly as the pillar of light. Rather infrequently a horizontal bar of sheen may be seen forming the popularly named “heavenly cross.”[1] Sun pillars, varying in color from white to red are occasionally seen at sunset or at sunrise. Patches of color occasionally are observed in cirrus and cirro-stratus clouds at a considerable angular distance from the sun. They may be due to causes similar to those which produce halos, but the causes are not known.
Cirrus or cirro-stratus clouds, or ice mist, in front of the sun or the moon are necessary to the production of halos. Some of the ice crystals are tabular; others are columnar and prismatic in shape. It is thought that both reflection and refraction of light are involved, each depending on the character of the crystals. Spectrum colors which abound in halo phenomena are explainable as a result of refraction; white-light surfaces may be due to reflection.
Rainbow.—The rainbow against a dark gray background of cloud is one of the most beautiful objects in nature. It may be seen as a full circle against spray thrown into the air, or against a mist. The rainbow of the summer shower consists of a bright arc near the horizon and usually a fainter arc above. The radius of the bright, or primary bow is about 42 degrees of arc; that of the upper or secondary bow is not far from 52 degrees of arc.
- ↑ This effect may be produced by looking at the moon through a piece of polished copper screen-netting held at a distance of 20 feet. It is an effect of diffraction.
