for clouds, tg is limited to one layer, if you wanted to do multiple layers you need to go up to tg2, which comes with SOO many nice new features its worth a computer upgrade.
to get stormy clouds, im sure theres some tutorials out there if you google them. terragen had many great tutorials out there still.
storm clouds:
http://www.terranuts.com/index.php?page=storm_cloudsclouds in general and how to use them:
http://www.basinski-net.de/tutorials/clouds/clouds.htmwhenever they say 3d, just use 2d instead and it will be mostly the same. except slightly lower quality but much better render times.
just out of curiosity what kind of specs does the machine have that you are doing this on?
what resolution are you attempting to animate at? to you what is a "long time" to render?
i just ran quick renders testing tg demo speeds on my core 2 duo 2.4ghz laptop. keep in mind origional tg doesnt use both cores. with a very simple scene, 3d clouds, everything on highest details i could bump it to 640x480, took exactly 4 minutes to render. with only switching the 3d to 2d made my render take 55 seconds. 2d with lower quality took 20 seconds or less.
so even on this machine, lets say i run 2 tg's at the same time each doing half the work so both cores get used. with 2d clouds, thats 2 frames every ~minute. so in 15 minutes of render i could have 1 second of video at 30frames/second. one minute of video would be 15 hours of rendering this very very simple scene. and thats with something very simple (just loaded in, generated terrain, and made some 2d clouds, and set it to highest quality render). Now granted, by cutting the quality down a notch or two, you can get a frame done in 20 seconds or so decentish quality. but still, that's 5 hours to do 1 minute of video.
All this on core 2 duo running 2 copies of terragen at same time each doing half the work. so double that time to get it on one core alone, and then also a lot longer depending on your processor speed.
Animation is really best set for higher end machines. With i7's being out now, core2 machines are starting to get price slashes, and arn't to expensive, and if you're using a computer that can't run tg2, its getting to time to upgrade, no offense.
