There is a live webcam to check the weather conditions, which at the moment look like this:
First I thought that the webcam was broken, but when I clicked on the views from other angles, I noticed that there is a little bit of landscape hidden among the fog. This doesn’t seem to be an extraordinarily bad day, because below the webcam it says: “During periods of bad weather in the Ben Nevis area, you may only see a grey picture. Don’t worry – this is genuine and is a natural phenomena called Scotch Mist!”
If this is what it looks like in the middle of summer, I am curious about autumn.
UPDATE: I am back from my walk across Scotland and on top of Ben Nevis and I had a wonderful week (from 4 to 11 July). It was beautiful weather almost for the whole time, with a lot of sunshine and great views. I did not have to wear my raincoat a single time.