basically you might use hdri - this gives nice light too when FG is active. in case you work with maya8 you might setup a physical sky. in the mia_physicalsky node you can map a greyscale file of your image into the haze channel. if you do so you have to raise the Alpha Gain level to about 3 until the clouds get visible. the funny thing about this procedure is you can animate the sky then - create dusk or dawn and the clouds get the colorinformation from the sky. those clouds are also interactive with the sun (sunDisc illuminates the clouds). if you have a series of skyImages you can animate the clouds as well. but there are also procedural clouds within the physicalSky - you wont need a file.

have fun!