This is a very basic way to make clouds by blue blobs. Jon West (Hetzen) gave me the node setup some years ago, and now his kind permission to share. So all credits go to him!
You can add (many) more blobs if you like. Beware (of course) that the blobs should stay within the total cloud height and level, especially if you check 'taper top and base'. For some really big thundering clouds they should be miles away and a lot higher. This is just a local one.
Have fun!