I scanned this from the inside cover of the most recent Aerostich catalog. Naturally, its an advertisement for how good they think their gear works. But it is based on some concept about what is feasible weather for riding a motorcycle. It also emphasizes that hot weather on a bike can be a real pain, especially if you want to wear protection on your body.
Personally, I'm not up for as much cold weather riding as they seem to sign up for. Even accepting a little cold weather fanaticism (Aerostich is in Duluth, after all) on their part, I was very surprised to see them identify Phoenix as the place with the fewest riding days per year.
It does bring up the question, how cold are you willing to ride, especially a full day ride? And how hot is too hot for you on that same full day?
For me, the answer is somewhere around 40 deg F on the low side and somewhere in the mid 90s on the high side. I have ridden above 95, even over 100, but I didn't like it and it just wasn't fun. I have also ridden on days that started in the 30's, but it was never fun, just getting home because there wasn't much choice.
I'm sure others have a wider range. I'll put mine down to a naked bike with me out in the wind. That, and not being as macho as I once was.
Finally, I thought it was interesting that San Francisco has 365 days, one day more riding than San Diego, 4 days more than LA. I guess rain and fog don't count at all. That seems to be confirmed by Seattle having 356 days.