From BlenderWiki

Animating World

Before reading this page, you should know about Blender’s world – if not, read this chapter first!

World, like materials and lamps, can use textures – so this Ipo type also has the small “texture channel” selector to the right of the type selector, in the header, as well as a subset of the material’s mapping controllers…

A bit like texture ones, world Ipos do not cover all properties – in fact, even obvious things lack, like e.g. curves to animate the ambient light color!

All in all, these curves are quite similar to material ones. And as with material, you can indirectly animate texture mapping via the Object input coordinates type…

There’s one thing you should know, though. The Sun lamp has now a special feature, called “sky and atmosphere”, which, among other things, “paints over” the world’s background to generate a simple but realistic unclouded sky. The interesting point for animation is that, depending on the orientation of the sun lamp, the sky is “mid night”, “mid day”, “sunset”/“sunrise”, etc. So you can animate a “blue sky” world background across whole day, just by animating the rotation of a sun lamp! See this page for more details.

Options

HorR, HorG, HorB

These curves control the horizon color of the world (values range in [0.0, 1.0]).

ZenR, ZenG, ZenB

These curves control the zenith color of the world (values range in [0.0, 1.0]).

Expos

This controls the exposition parameter of the world (value ranges in [0.0, 1.0]).

Misi

This controls the mist intensity of the world (value ranges in [0.0, 1.0]).

MisDi

This controls the mist depth (Dist property) of the world (value ranges in [0.0, 10000.0]).

MisSta

This controls the mist start distance of the world (value ranges in [0.0, 10000.0]).

MisHi

This controls the mist height of the world (value ranges in [0.0, 100.0]).

StarR, StarG, StarB

These curves should control the stars color of the world (I guess), but this seems to be unimplemented.

StarDi

This controls the average distance between world’s stars (i.e. their density, value ranges in [2.0, 1000.0]).

StarSi

This controls the world’s star size (value ranges in [0.0, 10.0]).

The other curves are texture channel-specific (i.e. mapping ones), and are a sub-set of the same ones for material Ipo datablocks.