Caelum is a plug-in/library for Ogre targeted to help create nice-looking (photorealistic if possible) atmospheric effects such as sky colour, clouds and weather phenomena such as rain or snow. 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.

For more information see the big Caelum thread on the ogre forum:
http://forums.ogre3d.org/viewtopic.php?t=24961
The thread is very long; you should try reading it backwards.

There is also a page on the ogre wiki (might be out of date):
http://wiki.ogre3d.org/Caelum

Features

Sky colour; depending on time of the day.
Drawing the sun and moon (including phases).
Multiple animated cloud layers with controllable intensity.
Astronomically correct sun and moon position based on calendar date and geographic position.
Point-based starfield based on bright star catalogue.
Precipitation compositor. It's a non-particle implementation of precipitation which is very fast on larger displays.
Depth-based fog compositor. It can provide more complex fogging without requiring changes in your own materials.
Easy to integrate into your own project.

Usage

See CaelumDemo's source code; it should contain some minimal initialisation and tweaking. CaelumLab is more complicated and can be used to visually tweak different sky elements.

There is no Caelum "SDK"; the recommended way to use it is to build it yourself.