Assuming you have already seen what can be done with 2-D "psrdnoise" by warping texture coordinates along the gradient direction, we present the straightforward "puffy smoke" shader in 3-D without explanation. It's just the same function, only with 3-D noise instead. This kind of effect was demonstrated by Ken Perlin and Fabrice Neyret back in in 2001, although they did it only in 2-D. Three-dimensional flow noise is a novelty, but this pattern is familiar. The next example will be a little more interesting.
// Fragment shader code goes here