Yes, they are a nasa data set.

I took the highres greyscale image and converted it to a PNG.  I loaded a 
mask into the image so that the source luminence of the image is the mask's 
transparency.  That way the black is fully transparent, showing the ground 
below.  And the grey clouds (the thin, wispy ones) allow for partial 
transparency.  I then used the original greyscale image as a bump map on the 
cloud layer.

In my original image I made the clouds procedurally.  While it is nice to 
have a resolution independent method, I found the NASA data (now at 
impressive resolutions) much more realistic.

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