Creating Clouds

http://freespace.virgin.net/hugo.elias/models/m_clouds.htm

Cloud creation using perlin noise - if you have a perlin generator already, it shouldn't be too difficult to get in :)

Use the results as both the sky box texture, and the shadow map for you cloud cover.

There are other ways to generate cloud cover, as I'm sure others will point out, but this is using technology you already have. 3D perlin noise would also create dynamically changing cloud cover, rather than just scrolling clouds.

Spree

You could look into Jos Stam's simulations of fluids with temperature.

what? I just want some simple but realistic clouds generation. I tried the link above but the problem is that there is no code example so is really hard for me since Im new to this

As far as I remember :

Use 2D perlin to generate a random number (say between 0 and 1) for every pixel of your cloud texture
Choose a threshold value depending on the overcast, discard all pixels under this value (1 = blue sky, 0 = bad weather). You could also use a greyscale above the threshold to have the center of your clouds darker, and play with alphas

If you add time as the third perlin variable, your clouds will be changing with time (but to have some wind effect, you'll need to translate the x/y coordinates with time as well).

I guess you can also use the texture to get shadows.