CompuServe Messages

I'd welcome some tips on how to create a "transparent" clouds material. I have a spherical object whose material is a nice GIF map of the earth's surface (I think I downloaded it here some months ago: BIGWLD.GIF a 330k file containing composite satellite pictures of the earth's surface). I've created a slightly larger sphere to surround the globe object. Its surface material is a map of cloud formations as seen from space. (I think I downloaded that file from here as well: EARTHC.GIF). This cloud graphic contains a lot of white swirls on a black background. I'm trying to make the black parts of this map completely transparent so only the white cloud sections appear to "cover" the globe object. I've experimented in the Materials Editor with various transparency and opacity settings, but I still find that the black sections still significantly darken the underlying globe surface. I've even experimented a little with some RGB masking in Video Post but without much apparent success. I'd welcome some tips from more experienced users who have worked with something like this. Thanks, Lee