As promised here are the tidied up nodes and tgd.
As stated previously I have not looked in any detail at the previous methods; not because there is anything wrong with them, the results look interesting just different from what I had in mind and I wanted to follow my idea through without getting sidetracked. A while ago I had posted a first thought on how a procedural hurricane could be made in a thread started by nvseal:
http://forums.planetside.co.uk/index.php?topic=3116.15I have more or less followed this reasoning although I have had to modify things a bit because there doesn't appear to be any way to manipulate the integral Get Position of either a Power Fractal or cloud Density Fractal.
The first step was to come up with a function that would start at zero, rise quickly to a peak then falloff slowly to zero again. I ended up using the combined result of two functions (I have included a POV render of a graph showing these). This falloff function is used in two ways, firstly as a control for the density of the clouds and also to vary the amount of twist with distance from the centre.
I ended up using a simple Perlin noise function for the large features because I could get at and distort the output from the Get Position. What I am doing is using a Get Position, rotating the output around the origin by an amount controlled by the falloff function as applied to the distance from the origin; there is also a control for the overall amount of twist. This modified point is then passed to the input of the Perlin 3D Scalar. I have included one of the tests showing this distorted Perlin used as a colour blend controller.
The final step is to take the output from the (Distorted), Perlin 3D Scalar, multiply it by the falloff function so that the hurricane is dense towards the centre and more wispy on the edges. This is then used as the Mix Controller for a Merge Shader merging a normal cloud Density Fractal and a constant zero (Clear sky). The falloff function is also added into this density controller and the combined result passed through a Colour Adjust, these additional steps kept the hurricane confined without the whole thing going wispy.
I have marked the main controlling nodes in the diagram.
Last but not least the tgd have fun