Caelum is an Ogre add-on for atmospheric rendering. It is composed of a number of small mostly self-contained components and a big Caelum::CaelumSystem class which ties them all together in an easy-to-use way.
More information is available on the wiki page: http://www.ogre3d.org/wiki/index.php/Caelum
You can discuss and report issues in the forum: http://www.ogre3d.org/addonforums/viewforum.php?f=21
1.5.6