--- Make some clouds, colorize them (control + U).
--- Copy the layer. (Control+J)
--- Go to Filters > Textures > Patchwork -- make it look decent, like small squares.
--- Set that layer to soft light.
--- Copy that layer, Control+J
--- Go to Filters > Blur > Radial Blur; set it to zoom, 100 strength, best quality
--- Set that layer to screen.
That's just from memory, I dunno how it's gonna turn out, but it can't be THAT bad.