Thursday 22 January 1998
While less gaudily impressive than a rainbow, a halo of light around the sun or the moon is, from a physicist's point of view, just as impressive a demonstration of the laws of optics, and, for a weather-watcher, it has considerably more predictive value. Rainbows occur only when it is both raining and the sun is shining; haloes may be seen in advance of the rain.
The halo of bright light about the sun or moon occurs when a thin layer of high cirrostratus cloud has formed. Sometimes the cloud is so thin that it cannot be seen - the halo it causes is the only indication it is there at all. The optical effect is caused by the refraction of light through ice crystals in the cloud. The ice crystals are hexagonal prisms and, most commonly, light entering through one face emerges from the opposite face after having been refracted through an angle of 22. In such cases, you see a halo surrounding the sun or moon with halo-eye-moon (or sun) forming a 22 angle. More rarely, a 46 halo may be seen, formed when the light enters one side of the ice crystal, then emerges through the bottom at a 46 angle to its original path.
The perfect conditions for all this are when a warm front is on its way, and its moist air shelves gently and for long distances above the preceding cold air. First you see the halo, then comes the rain.
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