How cold is too cold for running outside?

I have been walking for several months and now that I've lost a good deal of weight and my knees can take the extra stress, I've started to take up running.  Alas, I live in Wisconsin, and winter is quickly approaching.

I like to run in the morning before sunrise, and, believe it or not, I'm still very comfortable running in shorts and a long sleeve T-shirt with the temperature in the low 40s.  I have also done some running in colder temperatures in insulated nylon pants and jacket, and I think that should be fine for me into the teens.

I dislike treadmills and want to continue running outdoors year round if I can, so I just wanted to know what other people wear once the temperature starts to dip where they live.  Also, at what temperature do you finally refuse to go outside and run?

I've run in -40C (about the same in F btw), ok it was part of basic training, but as long as you have the proper clothing, with a good base layer, warmth layer, and shell you should be able to do pretty much any weather in WI. Your body will be the ultimate judge, but make sure you know the signs of hypothermia and frost bite and keep your runs limited to close to home if it gets really cold.

I used to run in 20 degree weather in Germany...  It's all about layering and knowing how much clothing to put on....

There is some concern that once you get below 0 degrees or so farenheit, it becomes more difficult to warm inspired air completely before it reaches the lungs-that is fairly easily dealt with by wearing a mask or loosely-woven scarf, balaclava, etc--something that doesn't overly restrict breathing but provides that extra little margin. 

Other than that, your main concerns are footing in ice and snow, visibility in the dark, heat/moisture dissipation (not necessarily insulation), and blocking the wind. Non-cotton, technical fabrics are a must--fabrics that maintain their heat-insulating capacity even wet. Luckily, these are now readily available and relatively inexpensive. A breathable, water resistant shell will be the greatest cost, and even those are available at a wide range of prices.

I also recommend: A) always carrying a cell phone and B) modifying your route so that you don't get too far away from your car or house. Running generates a lot of heat, so you won't be wearing huge amounts of clothing, but if you must stop because of a fall, twisted ankle, etc, you can become chilled very quickly. Running laps, or a figure-8 course (where your car, house is at the center), etc are good ways to reduce your risk. (I'm talking about single-digit/below zero temps).

Running outside in winter has some challenges, but it can feel very empowering as well. And there is nothing like that first warm day when you get to throw off all the layers!

Too cold? Pshhh.

Okay, I lie. I run up till about -20C (about -4F, if my calculations are correct). I, too, have even hit the pavement at -40C (-40F), but that wasn't so fun and I wouldn't do it again. I'd say -20C is my cut off. The way I see it, we cross country ski in that weather, which is even more of an all over workout than running (as it involves the arms and the legs). It's just about wearing the right clothes and being safe. Find a route that isn't icy, and always go in the daytime (be mindful of shortening daytime hours). Don't expect to be able to go as long or as fast as you can in more moderate weather. Definitely carry a cell phone, and try and stay near people. Anyways, as for what I wear, I start off with cozy long underwear and a long sleeved technical shirt. Sweatpants if it's really chilly. Then a pair of waterproof type pants designed for activity. On top a sweatshirt, but only if it's really, really cold (as my middle stays warmer than my legs). Usually I'll just put another very thin layer on top, then a shell. A couple times a season I'll start off with a toque or mittens, but I always ditch them by the end. :P Oh, and by the end, my inner layer is totally soaked! XD 

Oh, and if it's really cold and the air is sharp, wear something over your mouth to breath into to sort of 'soften' the air. (: It's kind of unpleasant, but it helps. And don't let all this deter you. Yeah, lots of aspects of running in the cold kind of sucks. But it's also really rewarding! The world is just so beautiful in the winter, and lots of people miss that! And don't forget all the people in cars looking at you with jealousy (or like you're crazy - who knows? :P).

I used to run when it was -20C to -25C.  Me and my dog would go at six a.m but I live in the boonies.  When it was below -10, I always covered my face with vaseline to protect my skin from the cold.  Feels kind of gooey, but your skin will thank you.  I never wore anything over my mouth - it drove me crazy.  But make sure the tips of your ears are covered, especially if you're wearing earrings.  And don't let yourself sweat too much, meaning you should dress so you feel a little chilled at the start of the run.

I don't like running below about -5C (about 20F) or -10C (about 15F)--not so much because of the cold but because I find the pavement gets harder and the absorption in my shoes stiffens because of the cold so it's harder on my knees/hips/shins. When I did try to run in winter I ALWAYS got shin splints.

Snow and ice are a concern too. I like azdak's recommendations for safety--cell phone, modified route, etc.

Now I just stick with skating (have an outdoor speedskating rink 2 blocks away) or x-country skiing which is low impact but still great exercise, plus a great shot of fresh air...not always as convenient for most people as stepping out your front door and going for a run though.

I used to hate the treadmill but decided I love running more than I hate the treadmill. So now I just pop in my ipod and go for a long treadmill run--now I'm a treadmill convert!

I used to run in Minnesota all winter long. In addition to the right clothing and safety precautions, I found that a bandanna over my nose and mouth helped warm the air slightly and made breathing easier. I would also wear bright clothing, as the Minnesota grey winters don't help drivers see you when running on the side of the road.