I have a 10/20/30 rule for my enviromental envelope.

Wind more than 10mph.
Temp less than 20 degF.
Precip more than 30% chance.

If it's all of those, I don't ride. If it's none of those, I definitely ride. If it's some combination, I play it by ear, but more often than not I ride. I'm lucky that I can take the train for most of the trip on those less than nice days.

I may take a 2 out of 3 as an excuse to take the car and run all those hardware store/auto shop/warehouse store errands on the way home.