Click on the image on the left to load the canvas demo (requires FireFox or non-IE browser)
Perlin Noise is a technique for generating procedural textures, commonly used for smoke, clouds or to add a random looking realistic roughness to surfaces.
The algorithm is basically to generate noise, then scale that noise randomly across an image at different sizes and levels of transparency. This creates a self similarity which mimics the appearance of some natural processes.
I found a canvas implementation by iron_wallaby and made the noise be the transparency of pure white, giving a cloud like effect. This was originally going to be just the background for another canvas demo I was working on, but that is delayed as I'm currently doing bioinformatics study/working on a project in my spare time.
Things to improve on this are restricting how cloudy it is to a range, and removing some visual artifacts (lines/blockiness)