Friday, 1 May 2009

Crepuscular and anti-crepuscular rays

Crepuscular rays are areas of brightly-lit air that appear to radiate outwards from the sun when it is setting or rising. They are caused by clouds or mountains near the horizon casting shadows across the sky. Under good conditions the rays can be seen crossing the sky all the way to the point directly opposite the sun, the anti-solar point, where they appear to converge. In fact both the apparent divergence and the apparent convergence are illusions caused by perspective. The sun's rays are parallel but we perceive them as converging in the same way as we perceive a railway track as converging in the distance.

A full explanation, diagrams, and examples, can be found on Les Cowley's brilliant Atmospheric Optics site, here.

There was an amazing sunset here in Christchurch on January 17th. Clouds near the setting sun cast shadows across the sky that resulted in dramatic contrast between sunlit and shadowed air.

This was the view towards the sun:

This was the view to the north. You can see the rays' paths across the sky, travelling to the east...

At the anti-solar point in the east, the rays appear to converge. Around the anti-solar point these are known as 'anti-crepuscular' rays:

Crepuscular rays and anti-crepuscular rays are different ends of the same phenomenon, as can be seen in this wide-angle image taken of the same January 17 sunset at Birdlings Flat, some 50km from where I took my photographs.

I photographed a dramatic single crepuscular ray on April 5:

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