The area with fog/stratus is not easy to detect in the IR images. Also other cloud types can have similar grey shades. It is not the grey shaded area alone which helps to identify fog but it is much more the combination of a relevant greyshade.
Dark/black areas in the IR images might be so-called areas of "Black Fog".
Although HRVIS usually are a very good tool for fog detection there are situations with snow where a separation between fo and snow is very critical.
Can you solve these problems now with the fog RGB at least partly?"