The problem is that clouds are rendered with several layers, so, if the cloud fills all the screen, the engine have to render every pixel in the screen as many times as layers. That is a extremely lot of pixels. 480 is a bit obsolete for this if you play with high resolution. You can low the resolution, buy a more powerful card or avoid clouds filling all the screen.
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Win 7 64
Quad core
4Gb ram
GTX 560
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