If you must allow your cats to be part-time outdoor cats, set up a heated house for them--an ordinary doghouse can be rigged with a soft, padded mat on its floor and a lightbulb (15 watts will do very well, but it needs to be an incandescent bulb, NOT a CFS), in an upper corner. This will keep that house nice and warm so that they have a refuge, should they need somewhere to go in the middle of the night. But the best thing to do for your cats is to make them indoor-only. The life expectancy of indoor cants can be 20 years or more. The average outdoor cat life expectancy is only 2-3 years. Yes, some cats live longer, but they're the exception. Outdoor cats must deal with predators, from coyotes, dogs, (even wolves), raccoons, and other cats, to the sick humans who enjoy hurting and killing small creatures. It's true that it's hard to get cats to get used to the indoor only paradigm, but they can. And I'm able to give my cats some SAFE outdoor time--on a harness and leash. Yes, you can walk a cat on a leash. Just understand that it's a learning process for both, and it isn't like walking a dog. You kind of have to go where the cat wants to go (except up trees, LOL).
I've heard once the temp outside becomes 32 or lower, there is a high risk for frostbite and hypothermia. That is for healthy cats. Kittens and unhealthy cats will have a harder time dealing with 32 or lower. But it also depends on where you live and if the cat is used to the weather. I'm in nj, and we have some bad winters at times, and I've seen the same outdoor cats around the last 3 years.
Ferrel cats 'can' live even in freezing temperatures.. I don't know how they survive, but they do. They find warmth where ever they can, and eat only what they can catch. It's a horrifying and extremely difficult existence. A domestic cat, however, would find it even more difficult.
If you EVER see a stray .. do something to capture it and give it a better like than a cold torturous existence.
My cats only come in when hungry, and they won't stay in any longer than neccessary to let me know their hungry. They are both shorthair cats and stay outside year long even when it's in the teens or lower. They were raised inside for their first year and I can't change their behavior. I live on a mountain in Arkansas and their both extremely healthy, happy cats. I have no idea what their doing or where their staying out there since I'm surrounded by forrest there's no way to track them.