SUN HALO EFFECT

Taranaki Daily News, 5 September 1933, Page 10

SUN HALO EFFECT

ICE CRYSTALS IN ATMOSPHERE. UNUSUAL PHENOMENON. ' Christchurch, Aug. 31. A sky effect not often seen in Christchurch was visible during part of yesterday afternoon. It was a 22-degree solar halo. Unlike the small haloes often seen round the sun, this halo had a radius subtending an angle of 22 degrees, so that the diameter of the halo extended over about one quarter of the vault of the heavens. The halo was distinctly coloured, from red on the inside to violet on the outside. At the time that it was best visible between 3 and 4 o’clock—the sun (at its centre) was high enough in the sky for the whole circle to be seen above the horizon. The small haloes which are commonly seen are due to diffraction by fine water droplets in the atmosphere, and their radii usualy vary from 2 to 5 degrees. But large haloes such as the one seen yesterday are due to refraction by ice crystals suspended in the atmosphere. In New Zealand they are more often seen round the moon. , Owing to the definite shapes of these crystals, the haloes thus formed have definite radii for approximately 22 degrees and sometimes of 46 degrees. As in a rainbow, the band of colour is due to the differing amounts by which the various colours which make up the white light are bent in refraction. Unlike the ordinary rainbow, which has red on the outer side, the 22 degree ice refraction halo Iras red on the inside. Phenomena such as this are common in the Arctic and Antarctic regions, where, in addition to 22 and 46 degree haloes, are to be seen horizontal bands, vertical columns, tangential arcs, and mock suns—caused by the refraction of light by ice crystals high up in the atmosphere. These effects in polar regions have nothing to do with the auroral ilghti.