Strangely enough, I'm working on a project right now where we take in clouds and visibility. The easiest would be to use NOAA's "experimental" XML feeds, but because they don't include visibility data for that, I'm just using their good old METAR files available via FTP. Find your URL for the one you want. The one in my code is for Fairbanks.
So, like I said, I'm using the METAR files. You can google for the specs. I made some regular expressions to find the cloud and visibility data.
Hope this helps. Perhaps Wally the bartender can move this thingy over to the Java thread?
In case you don't want to scroll through my ugly code, here are the regex's (note that they're double-escaped for use in java string literals):