I second Frank about Matt. Flex your TG (cloud)muscles for us Matt
I reckon that in certain circumstances clouds are a pain.
Actually the current cloud shader is excellent for clouds with lower density settings.
Frank, I think this has to do with scattering indeed, although I do not share the same opinion on how to fix it or what the best sun position is.
What will come now isn't new to you Frank, but I'd like to share some of my insights on cloud parameters:
The trick is the scatter curve.
In TG2 the "fake internal scattering" has in my opinion a wrong label and should be called "scatter curve", because this is what it essentially is simulating. So what does it do then?
If you consider sunlight directly lighting a volume (cloud) then that light gets scattered.
Very simplified you can state that the light either gets scattered in the forward direction or in the backward direction.
Forward means that the scattered light will esentially have the same direction as the incoming ray from the sunlight source.
Backward means that the scattered light will esentially bounce back in the same direction the incoming ray from the sunlight source came from.
In real life this scattering is very complex and heterogenous or anisotropic.
This anisotropy of scattering is simulated in the cloud node and you can influence its weight with the fake internal scattering setting.
Depending on sun position this will either result in a glowing effect or a darkening effect.
The strength of those effects are also influenced by cloud density and the glow settings.
"Fake internal scattering" (FIS) is the setting which controls the 'curve' on how this behaves.
A setting of 1 means that both forward and backward scattering are equally balanced for all densities of the cloud.
A setting of <1 means that direct light will be scattered more in forward direction.
A setting of >1 means that direct light will be scattered more in backward direction.
In the case of having your sun in front of the camera this means that for low FIS cloud edges will glow stronger.
For FIS >1 these edges will tend to become darker (silver lining effect, for instance).
All in all you may understand now that it's all in the mix of cloud density, glow, FIS and sun heading/elevation.
A very potent but definitely complex mix!