At what temperature does it become "too cold" to let them out?

It sounds like a strange question, but here is the situation:

There is a relatively skittish cat that has been coming around at night since the middle of summer to "fight" with my cats through my sliding doors. I guess I am wondering at what point weather-wise I should trap him/her and take it to a shelter...I don't want to take someone's pet but I also don't want it to suffer in the cold...

When my cats went outside they would be able to come and go as they pleased. They would still go out when it was very cold, just not for as long maybe. Sub zero (C) was fine for them. They loved snow. When I walk my cats, some like to be out when it's cold and snowy but others do not.

Not sure what the temperatures are like by you so it may vary from country to country. My cats just had a thicker winter coat when they could go outside. I never restricted their access because of weather.

Is the cat well fed? Is it likely it has a home? Could you pop a collar on with a note attached to see if anyone reads it, maybe ask them to call you.

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Lollo

The cat could have a home and just be an outdoor kitty cat who likes to visit your house. There is a ginger kitty that comes around multiple times during the day to try to eat the birds at the feeders. My Bengal has tumbled with this cat many times until I took ownership and no longer allow him to run outside at will. Still, he has a large enclosure on the deck and Raiden is always out there looking for Tom (the ginger cat). They will howl at each other, but there is no way for them to tangle any more. On our walks, Raiden is always on the lookout for Tom and sometimes they see each other, but they keep their distance. As much of a nuisance that Tom can be, I would never trap him. I know he has a home and a family and he's just behaving like a cat. First of all, you might want to go around to your neighbors and ask if they own this kitty cat. Even though I've decided to keep my cat on the end of a leash doesn't mean most cat owners would do that.

I grew up in a small town in Northern BC, Canada, and ALL my cats where outdoor cats. It at times got below -40C and sometimes my mom would allow them to be in a cage in the house at night, but for the most part they were always outdoors. Personally I hated it, but my mom was not an animal lover and did not allow the cats in the house except for a tiny area for us to play with them.
It is mostly important that the cat has a warm place to keep cozy in, for example a box loaded with hay.
On average cold winter days were -20 to -30, and the cats never had issues if they were given a sheltered, enclosed bed.

Also keep in consideration the cat's coat. In the north lots of our cats had thick coats as most people there kept their cats outdoors and like us brought them in at night when it was very VERY cold.

If you are worried you could try putting a nice cozy enclosed bed outside for the cat, but it should be fine :)

If concerned enough to act,
I believe using (free from ASPA?) a secure and safe animal trap and some yummy food, would be a good deed to do, to get this kitty scanned for a microchip.
If not chipped and owner untraceable? in this case I would be finding a no-kill shelter for kitty
..to be given chance of being claimed/rehomed in a new forever home.
If your area's animal charity/assoc supply free animal traps on request?
you can also ask for their advice about when (cold and wildlife) its most dangerous for cats to be outdoors in your area to get a second opinion you can trust, to help you decide what to do you.

As long as there is shelter of some kind from the wet and the cat has had time to build up a nice thick coat, (ie it was not an indoor cat), and it has a supply of food, then it will survive just like any other wild animal does.
Very severe weather with lack of food and shelter may kill it just like it kills some wild animals too.
http://fivercats.com/

Start feeding it under the back light, that way you will get an idea of how hungry it is. A real stray will be ravenous, whereas a free ranging owned cat will eat it, but will not gulp it down.
Obviously it has survived all this time, but if it is feral, its supply of wild prey will be diminishing, so it may now be getting very hungry.

Of course it may not be from your area at all, some pet cats sneak into vans and lorries and get transported far from home, or there are any number of scenarios where cats become lost, lose their sense of direction and end up stray. You may find like I did with a stray, that once food is on offer, the lurking about in the shadows stops and it is very tame indeed. :)

Someone I work with runs a rescue and will lend me a trap of I need it. I haven't been feeding it. My hope was it would only be around during nice weather and presumably be at home indoors when the weather turned. I didn't want to encourage it to keep coming around by giving it food. As my husband said, it already probably thinks this is a nice, cozy, cat B&B, food will just make it move in to our yard.

Trying food may be a good idea at this point, and like Elaine said, it could be the best indicator of its current situation.

Unfortunately, most people around me don't microchip their cats. The vet was surprised when I asked about microchipping my special needs cat. He said he was more than happy to do it but that rarely do people ask for it. Of course, it is worth a shot. The one positive thing about this area is that I really can't think of a shelter that isn't no-killl. There are also lots of rescues, two I have fostered for in the past.

The weather is rainy and cold tonight again. I am sure the cats will let me know if the kitty comes around. :)

Oh my...I typed that wrong. ALL the shelters around me are no-kill, I can't think of one that IS!

Sorry!!

That is what I wrote the first time!

No kitty sighting last night...hopefully he was warm and toasty inside his house! :)

:)

Tons of no-kill shelter around here. Even the city "pounds" are no-kill. Toms of rescues too. One of the few redeeming qualities about this area.