Maybe I'm wrong here but even if it is set out as such in theory it definitely isn't that way in practice.

There has to be some sort of line somewhere as to what is extraordinary or not - rain or fog at Heathrow is not extraordinary - at least I can't possibly consider it so - since it's pretty ordinary. An Icelandic volcano erupting...now THAT is extraordinary.

But where is the middle ground?

And this type of fogginess can be applied to all the cases.