This site has a number of tutorials on planets however I can't check right now to see if it has one on clouds specifically as they were hacked and are down temp.  http://www.solarvoyager.com/

Here is a tutorial may be the same one on SolarVoyager http://www.3dtotal.com/team/Tutorials_2/painting_planets/planets_01.asp  they posted about 100 tutorials some are quiet interesting http://www.3dtotal.com/ps100/pstut100.html