Here is a wee bit of a tuned version of the two cloud layers I used in this image:
http://forums.planetside.co.uk/index.php?topic=10172.msg105547#msg105547 The 'cirro' cloud layer in that image differ from this default clipfile setting in two ways; the cirrocumulus layer is lower than these, and it is also shadowed. * The cirrocumulus layer will
not self-shadow, nor will it cast visible rays in the atmosphere at default clipfile settings. This is true of a real cirrocumulus cloud.
You can, however, recheck their box 'enable secondary' to have them cast shadows and rays across the scene.
Here, I've done just that, and lowered their altitude(image 3), the result makes another nice impression of an altocumulus layer:
Altocumulus + Cirrocumulus Layers:
[attachimg=#]
Cheers!
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Edited: Replaced multiple demo images with one composite.