Indirect Illumination

When a cloud is in shadow - what illuminates it then? The question may seem odd because by definition of shadow there's no sunlight falling onto it - but of course we can still see the cloud, it is not pitch black. So there is some amount of ambient light also in shadows - where does it come from?

From comparing with images taken in vacuum, for example on the lunar surface, we can see that it must come from the atmosphere. The largest source of ambient light is the sky.

Many people believe that shadows are grey, but that isn't exactly true - they are colored by the sky-light. If the sky is blue on a bright day, so are shadows. If the sky is an overcast grey - so are shadows.

So what happens if the sky is colored?

Indirect lighting examples

The normal situation is depicted in the following sketch:

The direct light is blocked by a cloud that casts a shadow, but the air mass above the cloud an observer is looking at is illuminated, leading to blue Rayleigh light. As a result, the lower cloud appears in a blusih hue.

Blue shadows cast on the lower layer.

But there can be a situation in which a thin upper cloud layer is in direct light while a lower layer is not. The upper layer is then not predominantly illuminated by blue sky light but rather by red-shifted sunlight (because that is much stronger than the blue ambient light from the sky). This illuminated upper layer then creates a red hue for shadows beneath it because the sky color is now red.

This is not so commonly observed because it relies on three ingredients:

  • a low cloud layer that is visible by the observer
  • a shadow-casting cloud bank that hides that low layer from direct light
  • a high layer that is brightly lit and large enough to act as light source
The result may then look like this:

Violet shadows on the lower cloud layer.

The strong illumination by the bright red upper layer makes the low clouds not appear blue or grey as one might have expected but rather violet.

Here is a different, somewhat less clear example in which the upper layer is illuminated from below:

Interesting violet-pink colored sky.

Looking carefully in the upper right, there are some whisps of low clouds underneath the layer, and they appear shaded in a reddish brown because they receive most of their illumination from the red layer above.

Changes to the color palette

When blue skylight combines with indirect light of other color, new hues can appear on unlit surfaces. I suspect that is the mechanism behind the pinkish appearance of the hazy background in this shot, but it is hard to be certain.

Red shadows on the lower cloud layer.

Continue with Scattering on the view ray.

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