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A sundog seen near sunset, 21st August 2005
Credit: Megan

Here is an image of a sundog that appeared near sunset today. Sundogs, also known as parhelia, appear when light is refracted by ice crystals in the atmosphere. They are usually only seen at sunset or sunrise and, like halos, always appear at an angle of 22 degrees to the Sun. For sundogs to be visible, the ice crystals have to be in the same plane as the Sun and the observer. If the crystals are long, thin and oriented randomly then a halo is seen, if they are flat and mostly arranged horizontally then we see a sundog instead. The flat, hexagonal crystals act like tiny prisms which refract the light, splitting it into the component colours, with red appearing closest to the Sun.

The old wives tale that a halo indicates bad weather to come has some truth to it as they are caused by cirrus clouds which often indicate an approaching weather front.

There are many other phenomenon caused by ice and water vapor in the atmosphere, sometimes there are halos around the Moon, and sometimes there are Sun pillars (not to be confused with crepuscular rays) which also look quite spectacular.

Posted by Megan on Sunday 21st Aug 2005 (22:57 UTC) | Add a comment | Permalink


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