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   Formation & Perspective 

   
  Rays and shadows as seen by the eye.

They appear to radiate away from the sun as though it were nearby, close to the clouds.

Instead of perceptions, let the mind picture them truthfully - long parallel columns of light and darkness streaming downwards towards you. As they get closer they appear wider and wider.


Muir Woods, USA              ©Les Cowley
The trees in this redwood grove are roughly columnar and parallel. Though solid wood rather than shadowy air, they show the same perspective effects.
 

Sunrays need clouds to shadow the sun and divide the sky into contrasting columns of shaded and sunlit air.

The sun's rays reaching planet Earth are all parallel* and the cloud shadowed regions are parallel too. They are tangible things, immense columnar volumes, dark, bereft of sunlight.

When the sun is still above the horizon the shadows and rays must always slant downwards but sometimes they appear instead to shoot upwards from clouds. This is purely a perspective effect.

Shadow volumes are real three dimensional objects and they are always seen in perspective. Take the shadow from cloud "a". It reaches the ground between the sun and the eye. The shadow is seen below the cloud. The shadow nearest the ground is closer and so appears wider. In contrast, shadows from clouds "b" and "c" slant downwards at exactly the same angle but reach the ground past the eye. To the eye they appear to radiate upwards from the cloud and would, if they were still visible, pass high overhead.

Next time you see sunrays, imagine them for what they really are, miles long columns of sparkling sunlit air highlighted by the darkness of adjacent unlit voids. Let the mind fly around and through them to give them solid form that replaces the flattish way we normally see the sky.

Rays and shadows are most visible near to the sun. There the shadows are darkest because the eye is looking along their length. Elsewhere the path lengths across the shadows are much shorter. There is a second reason, larger dust particles scatter sunlight sharply forward and so the air shines brighter near the sun.

Another direction where the eye looks along the shadows is in the opposite direction to the sun. There the shadows again darken and the contrast with the sunlit sky increases. Rays converging towards the antisolar point are called anti-crepuscular.



* The shadows are not quite parallel because, although the sun is ~93 million miles away, it is so large that its globe appears 0.5º across. Rays consequently spread up to 0.25º each side of the mean direction. This causes the 'umbral' or absolutely unlit parts of cloud shadows to narrow slightly with increasing distance from the cloud.