A fog animation based in webkit/CSS3 keyframe
Feel free to use it on your website.
The animation is continuous, you can change the opacity in fog.css.
If you want to add it in your html, just paste the fog html string into your previous background string, add the /* FOG ANIMATION */ strings in your css file and copy the fog#.png files to the root directory.
If you are having troubles with scroll bar, add "overflow-x: hidden;" in body properties (css)