I've worked on a few cloud icons and I can tell you it's a tough problem. The two things that most describe the concept of a cloud in an image are color (white on blue) and shading to suggest 3D form. Both of those are unavailable when working to create a simple b/w icon. You're left with describing the shape, something that's very fluid and doesn't have telling details like the ears, tail, and feet of a cat, for example. It's not surprising to me that we're working to agree on a common representation. Circles on top, flat bottom to suggest something above us.

Deviate too much from the agreed upon shape, and you're likely to get a lot of "What's that?"