realistic clouds

Very realistic... depends on your needs: Do you want to be able to fly through them or just to walk around and take a look at the sky?

If you just want to walk around on a terrain and look at the sky then you can use perlin noise and some light scattering calculations. You'll find a lot of info in the Sky rendering thread.

If you want to be able to fly through the clouds you can do two things:
- Billboards using light scattering to compute brightness. Check out Harris's work

- Full volumetric clouds. this doesn't really work in real-time, i've seen some screenshots around done by Dmytry which seem to use this and they look great.

These images :-)

There is 2 parts of work needed to make maximally realistic clouds:

1, Programming: Renderer. I trace ray through clouds, integrate cloud density and lightness along ray to compute color of pixel. I use some ray-to-ray coherency for acceleration, and are making deep shadow maps support.

2, Procedural Artwork: Clouds model. I develop procedural volumetric texture for clouds using MojoGraph (see link to Pandromeda) . For most of clouds in my site, i used Monofractal with Perlin basis, with increased fractal dimension (that is, noise amplitude is changed not by 0.5 per octave, i.e. if first octave have amplitude 1, second have amplitude like 0.6 or 0.65) + spline controlled by altitude, that is used to change vertical profile of clouds (to make "layers" of clouds like in nature). This spline makes density be entirely negative at too low and too high heights. Also, i add some perlin noise to altitude before passing it to curve, so result looks better.

In all images except recent one with rays (it is test render for my shadowmapping), i did shading pretty much `manually', as anouther procedural texture, with different fractal settings. I found that such tricks often looks better/nicer than semi-phisically-accurate shadowing. First time i started working with it, i had to tune clouds many times for maximal realism, currently i have more experience with clouds modelling but it is still necessary to do some test renders and ajust settings, especially coloring.

Also, it is rendered on planet, with accurate planet curvature.

Yes I dont need real time so option 2 is it.
Thanks for the information guys.
Apreciate it.

why don t you use 3d textures and create clouds out of a combination of 10 or more 3d textures

There was a great article on this in Game Programming Gems 2. If you have a copy, take a look. If you don't, consider getting one (it's very good) or at least stop by your local book retailer and take a look at it.