Hi
It depends on how you define "bad weather", which includes a specification of how bad for how long. You appear to have three different definitions in mind:
1. A "bad weather week", which is an event that stops work. (This happens in roughly 8% of weeks; or somewhere between 2 and 9 times a year).
2. A "bad weather year", which is a year containing at least one bad weather week. (the probability of a year being a bad weather year is presumably near 100%)
3. A "really bad year", which we don't have a deinition for (but we know that its probability is 1/50 or 1/100). Presumably it features a week with weather that is far worse than a mere "bad weather week".
James