OK, back on topic. First off, assuming that you know where you are, map and compass still work in the fog. One way of dealing with poor visibility if you are not solo, is to "split up" and share sight distances. Lets say there are two hikers, person A and person B. You are both standing at a cairn and you need to find the next one and can't see it. A and B go forward in the desired direction (compass) only until you can still see the cairn you just left. At that point, A stays there and keeps the prior cairn in sight. B then goes forward keeping A still in sight. B does a sweep in an arc left to right until the next cairn is seen. A then joins B and both go to the next cairn. Repeat. If you can not find the next cairn then at least you can still see the previous cairn and retrace your steps.
Cairns in the Northern Presies are generally 50' apart. With 2 people, you should be able to make your way fairly well. 3 people can split the gap and overcome just about any fog imaginable. Even if visibility drops to 25', then one person solo can still make their way. The key is to never leave sight of your previous cairn until you see the next one. Take your time though. Visibility in fog generally varies as the wind blows. If you can't see, stand still and wait until you can see a little better. 5' or 10' can make a big difference sometimes.
- darren