Gradual transition between environment textures

I have made an environment texture and would like to create a night time version so as to gradually blend the two as time goes on. It may be a 10 or 20 minute process, but I would like the sky to get darker as day becomes night, and then lighter again as night becomes day. I am also considering a version with four textures: dawn, midday, dusk, night - but first I would like to know if it is possible to transition between even two environment textures.

If possible, I am thinking that if I could have a sky with alpha I might be able to layer a sun and clouds texture behind it and rotate that texture across the sky as time passes. But since it is my understanding that nodes are not supported for World materials in Blender Internal / Blend4Web, I don't know if it can be done.

Any advice is welcome. Thanks in advance!

Hello!

but first I would like to know if it is possible to transition between even two environment textures.

In blender internal it can be done with texture's "blend" parameter but for now blend4web doesn't support such transitions. It is in our todo list of course but not in the nearest future.

If possible, I am thinking that if I could have a sky with alpha I might be able to layer a sun and clouds texture behind it and rotate that texture across the sky as time passes. But since it is my understanding that nodes are not supported for World materials in Blender Internal / Blend4Web, I don't know if it can be done.

These effects may be implemented using some kind of covering object (like large sphere) and node materials.
Our artists will provide you more detailed answer

But since it is my understanding that nodes are not supported for World materials in Blender Internal / Blend4Web, I don't know if it can be done.

You can make this using animated in node material. Just create a sphere around your scene with node material with sky and sun textures in it, then using Value (or Time) node you can aniamate UVMap for this textures as it done in this tutorial with lava material. Or you just can wait till the end of this week, and you will get and example of how to do it. We're prepearing something interesting at the end of this week. It will be an answer for all your question about an environment creation.

Thank you both!

@Konstantin: Of course - I can just model an object to be the sky! I've been so focused on World materials lately I missed the most obvious method.

@Mikhail: Oh, cool. I hadn't got that far with the Pyatigor's Tale tutorials, but I had wondered how the lava was created. Animated UV maps… cool.
Looking forward to the material you plan to share at the end of this week!