The trick to making this look real is to use multiple 3D layers and put some of them at an angle so the camera is flying over something like a ground plane that is a semi-transparent cloud layer. Except for some atmospheric haze or other atmospheric effects you shouldn't need a particle system. Just several layers of clouds arranged at angles should do it.
When you are flying a plane, and I've got many thousands of hours, there's really not much of a 3D experience unless you are flying between a lot of cumulous clouds that are very close together. It's mostly fog (in a cloud) or shapes above, below, or to the side. There's very little perspective change on clouds you are approaching until you get very close.
Thank you. This makes sense, and I have been able to pull off some nice effects with 3D layers spaced as you suggest, also, a great tip I saw was to always tilt your camera slightly downwards, it adds a lot to the effect. Does anyone have any suggestions where ot get some cloud JPEGS or something? I grabbed a bunch off the net, but keying them can be quite tricky. I get results that are not terrible as such, but most are quite small, and scaling causes the obvious softness, and all depth is lost.
Flying sideways works well, spinning works well, it is the question of "How many layers will I need to simulate flying towards the camera for a few seconds?"
This is also why I am so interrested in the VIdeo CoPilot skyscapes. I would love to know what they look like...