Mulitple point density clouds - intersecting each other

Hello everybody,
for an animation I want to create a fluffy cloud scenery (with the internal render engine)
To to this, I created severeal cloud shaped meshes- each of them got a particle system. Over that I build a bounding box mesh with a volume shader and a point density texture to drive the density of the volume. No problem so far. I get a pretty nice cloud with this setup but after adding a second one, I get bad results. They are intersecting.

I could render every cloud on a seperate render layer, but this would be a tidy task. After all I have 26 clouds.

What is the problem here? Which option is unchecked? I cant find it.

Thanks for your help :slight_smile: