Some of you may be into extreme exercising, trekking for miles in cold weather no matter the conditions. Others might take winter as a sign to hibernate. Exercising during the winter can be fun. Just be sure to be safe and smart.
First of all, choose your weather carefully. We could toss around some numbers for optimal temperature limits, but if you're like my wife and other dedicated runners, you won't listen. These people will run when it's snowing, when it's raining, no matter how hot or cold. The only things that will keep my wife off the road are ice and lightning.
I don't know if I would recommend this approach for everyone, but I do recommend that athletes be smart. Refrain from playing tennis in a blizzard, even with neon yellow tennis balls. But don't stay indoors all winter — remember, 20 minutes of sunlight can provide the daily recommended amount of vitamin D. Even in the winter, go for a run if you're comfortable. Take a brisk walk at lunchtime to break up the workday and revive yourself. Just be sure to dress in layers for the weather: wear a hat, consider gloves (or tuck your hands in your sleeves or pockets) and wear a layer or two on top and bottom. If you can, warm up with a nice warm shower upon your return.
Extend your warm-up. Make sure you're warmed up before you start. Spend a little more time loosening your muscles before exposing them to the cold because it will take longer to get from zero to 60 in cold weather.
Finally, keep an eye on your surroundings. Watch where you're going when there's ice on the road. When you go off-trail in the snow, be careful to avoid even the smallest of snowpiles, which could be covering roots and other road hazards. Remember that low-hanging branches could be frozen solid or harbor ice that can scratch and cut. Trust me, it will take quite a while to recuperate from a bad fall or an ice cut.
If the great outdoors is too much of a challenge, don't take it as a sign to slack off your fitness. Take a brisk walk around the mall or walk the stairs at work or school. Go to the gym or work out at home. Find opportunities — they exist.
One final note: numbness in your extremities is bad. It may take a few minutes to get warmed up during a hike or run — however, if you can't feel your feet or hands even during rigorous exercise, come in from the cold. Frostbite is very dangerous and can do permanent damage.