The Octopus 

Exactly. And unforeseen.

My $0.02 is that terrible weather is almost never unforeseeable to the extent that extra time is warranted on a brevet. Assuming, or course, that the ride is started in the first place. I've seen one brevet in 8 seasons rescheduled due to weather -- a brevet in Oregon was postponed a week due to the risk of freezing rain (which did materialize and which also made for an awesome powder day in the mountains!).

But, once a brevet is underway, my experience has been that all's fair and there's no extra time for anything. I've ridden in snow. One 1200K was done in record heat and humidity (which DNF'd nearly half the field). I think the worst weather -- meaning most dangerous to riders -- is 30s and rain. Hypothermia is a killer. But if brevets were cancelled for that, or riders were given extra time, then whole swaths of the U.S. would pretty much be chucking the date-certain and time-limit rules of randonneuring out the window. As much as the weather at PBP in 2007 sucked -- it's just not a lot of fun to be wet for 4 days -- it was hardly unusual or unforeseeable. Brevets in the PNW and upper midwest typically are run in that weather. (Note, though, that Floridians had about the same finish rate in '07 as SIR did, which is pretty hilarious.) I imagine most Englishmen ride through that crap, too, on a fairly regular basis.

To bring all this home to the OP, as you get stronger on the bike -- and I don't necessarily mean "faster" -- your ability to endure lousy conditions will increase. You'll have greater physical reserves. The things that slow you down -- wind in your face; cold in your legs and lungs -- won't be as much of a factor. Greater physical reserves give a rider a better positive mental outlook on the ride: the rain isn't as wet, the cold isn't as cold, and the hills aren't as steep or long if they're not beating you up as much. You'll still have plenty of low points -- it's not all fun and roses out there all the time -- but the conditions that put you to the point of wanting to quit, or put you to where you're going to be out of time will be a lot worse than they were when you weren't as strong a randonneur.

There is something to be said for being "fast," though, when it comes to tough conditions. You're exposed to the wind or cold or heat or whatever for less time and, at the end of the day, it's time exposed to the condition that wears you down.