An experiment using Image Shader for clouds. What you can see here are so-called "Chemtrails". You can see them up in the sky when some planes flew across the sky leaving these trails.

Here I used an image I quickly made in MS Paint (diagonal lines in different widths), resized it to the double size. Then I used Gimp to make the lines more smoothy and fraying. In the next step I created a cirrus layer in Terragen 2 TP and used the Image Shader in that this image being loaded. And another cirrus layer above it to create some clouds above the trails.

The terrain and surface shader created quickly as well and don't look very good.

Terragen 2 Technology Preview, MS Paint, Gimp 2.2