I recreated the Extends sky thing using DBP (As I had Win7 and at the time, Extends didn't run very well with it).
For the Sky, I had 3 spheres, one sky blue, One grey and one black with white dots (for stars). The blue was the smallest, the grey next and then the night one was the largest (Only about 100 units between each one, just enough so they don't clip each other). Then I'd fade them using alpha to change from fine, to gloomy to night or whatever I wished.
The next clever bit is another smaller sphere inside, textured with the cloud texture. This one gets rotated everso slowly on the y axis to simulate the clouds blowing in the sky.
You can fade the clouds in or out depending on how you want the weather to be.
You could also use multiple layers of clouds for different effects, or for different cloud types. There are plenty of seamless cloud textures available on the internet to use.
I'll try and post some code later for this if I can find it.