December 2014

Cloud of the Month for 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.
The sunlight has to pass through these sides of the ice crystals for sundogs to appear.
3 Comments
  • Johanne Paquette

    December 16, 2014 at 9:59 pm

    Wonderful image ! What a view, tks for sharing with us.:)

  • Kathryn

    December 24, 2014 at 9:54 am

    Many thanks for the explanation of how sundogs are …? formed…? made… or just happen.. I have wondered whenever I see them exactly how they come to be. I LOVE the clouds and spend time, though not enough, looking and wondering !!

  • Kathryn

    December 24, 2014 at 9:56 am

    Just had another thought, as I looked up at my cloud calendar, this month of December, there are sundogs, and a third one of the hairy kind. Just coincidence ?

Post a Comment