Dear Magnus,
I know you're busy, I just have a request if you think to introduce the altostratus opacus.
this type of cloud is present on a low pressure system, his layer position is always above the other clouds. this layer is also a barrier to the sun rays and the clouds that are below had always the grey color and the dark of pre raining weather situation.
I think if you can create this layer when weather conditions are adequate will add to a xEnviro a theme reproducing a rainy or pre-rainy conditions.
I really hope you will be interested .
Thank you ,