Transparent textures and shadows with Cycles

Hi everybody,

I’m currently working on modeling a planet with Cycles, and I’m running into a problem trying to add a layer of clouds around the planet.

I found a great grayscale image of clouds to use as a UV-mapped texture (white areas are clouds, black are empty space), and after quite a bit of messing around got Cycles to show the white areas and make the black areas transparent so you can see the ground underneath. It might not be the most elegant solution, but it seems to be working so far - the object is using a mix shader (transparent and diffuse) with each shader using the cloud image.

My problem is this - I can see through to the ground below pretty well, but the clouds aren’t casting shadows onto the ground. I see you can set the ground layer to Receive Transparency if you’re using Blender Render, but I don’t see a similar option in Cycles. Is there any way to do this with a grayscale image, or do I have to use something like a PNG with white clouds and transparent gaps to map onto the sphere?

Anybody? I can try to clarify if I’m not explaining the problem very well.