Cloud Shadows

In the previous section we have seen the effect of cloud layers casting shadows on themselves, not we look a bit into what happens when clouds cast shadows on other clouds. We're used to thinking that higher clouds cast shadows on lower clouds - but during sunset that is not so as the light comes near horizontal, so the basic situation to be studied is like this:

The light falling onto a cloud layer above the observer depends on the presence (or absence) of the cloud bank marked with a '?' - which the observer may not be even able to see directly as it could be below his horizon and which may be tens of kilometers away (as you can imagine, for trying to work out computational techniques for rendering sunsets, objects far outside the scene crucially affecting the scene is a very dissatsifying state of affairs...)

Dramatic differences

To see the effect, consider the following two sunset scenes. In both cases, the lower layer is a scattered, dissolving Cumulus-type. In both cases, high clouds are seen well-lit, and the horizon is bright, the overall light level and sky brightness is fairly similar. Yet in the first case the lowest layer is lit, in the second scene it lies in shadow, and the visual difference is dramatic.

Illuminated lower cloud layer

Lower cloud layer in shadow.

Here is another example with fairly similar cloud layers and nearly comparable light level - in this case the shadow-casting cloud bank is visible on the horizon:

Illuminated lower cloud layer

Lower cloud layer in shadow.


Of course, the shadow-casting cloud bank does not have to be one compact block - if it has gaps, light can fall through gaps and this leads to really surprising illumination near the observer. For instance, single bits of a cloud can be in bright light while the rest is in shade as in the following example:

Light falling through a gap.

Here the light is constrained in elevation by the shadow-casting cloud - it reaches the lower filaments of the layer, but not its main bulk:

Shadow on the upper cloud layer.

Continue with Indirect Illumination.

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