Making realistic clouds is a lot easier than you might think at first. You might try putting a texture-mapped dome over the whole scene, but the perspective will be off when you move the camera. You might also try modelling individual clouds, but that would take up a big amount of memory. The best way I've found uses only one polygon with the following properites:
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Although the Earth isn't flat, it's so absolutely huge that it might as well be when you're sitting right on it. The big flat polygon is so big that it stretches almost to the horizon. It's less than a pixel away from the horizon, though, so it looks fine! Best of all, when you move the camera, the perspective stays exactly the same.
Here's an overhead view of the cloud texture: