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December 2014

December 2014

A Pair of Dogs, Beside an Open Fire

What could be more festive than a couple of colourful sundogs, in the Cirrostratus skies of northern Italy, seated on either side of a warm, fiery Sun?

Sundogs, which are also known as parhelia, are optical phenomena that can result from the sunlight shining through the tiny ice crystals of high clouds. When the crystals grow slowly, they can be ‘optically pure’, which means that they are clear enough to behave like minuscule prisms, refracting the sunlight as it shines through.

Sometimes, the crystals can form in the shape of regular hexagonal plates. When these are falling horizontally like autumn leaves and the sun is low in the sky, there is a chance of sundogs appearing. For them to appear, the sunlight needs to pass through the crystals as shown in the diagram below.

Photograph © Valeriano Perteghella & Michela Murano.

The sunlight has to pass through these sides of the ice crystals for sundogs to appear.
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