Thank you, folks, for the compliments, encouragements and welcoming words :-)

I`m glad you like the clouds. Would be interested to know how well they integrate into your atmospheres, along with volumetric clouds.

Thanks, ArtPearl, for the advice re: uploading. I probably prefer to upload the stuff to my site and attach a link to avoid any potential confusion.

Yes, the picture with one of the functions used to generate cirrus clouds is a bit too small to unveil the secrets. This is intended ;-)

Here are 2 key techniques. They are basically very simple.

1. Take 2 identical fractal noise nodes. Mix them with a combiner node in "add" mode . Now set the amplitude of the second input to -1 while leaving the first input at 1.
   The result will be 0. Complete phase cancelling. You should see uniform grey.
   Offset  one of the 2 noise nodes by slightly shifting the origin in x or y direction. The result is a new pattern that has desity peaks at the edge instead of being in the center.

2. Again, take 2 identical (fractal) noise nodes. Connect the second node to a "Composer 3" (math node). This node has 3 inputs (x,y,z). Connect it for example to x.
   Now connect the output of the composer to the origin connector of the first noise node.
   This should push the peaks of the noise in one direction. 

Of course you can combine both techniques.

The reason why I use those techniques for cirrus clouds is because it approximates the effect the wind has on them. By pushing the clouds in a certain direction, the maximum density of the clouds is no more in the center but is shifted in the direction where the wind goes.

I hope this was not too confusing :-)

Similar techniques can be used to create desert dunes and ripples.

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