Pittsburgh Pennsylvania is where I live and right now it is cold. Very cold. I don't like the cold and as near as I can tell, the cold doesn't like me. One difference between the hottest days of the summer and the coldest days of the winter is that in the summer I never walk outside and immediately begin to cuss. Man oh man, I stepped outside this past week and before I had taken one full breath heard words come out of my mouth that would offend Lord Voldemort.
We've had a run of cold cloudy days here, punctuated by one frigid, bright, sunny and cloudless morning. It was one of those days that invites the unsuspecting preoccupied amongst us to casually wonder outside for a newspaper. Sunshine is warm, right? Brightness is pleasant - isn't that the unwritten rule? Aren't we zoned for sunny = warm? I later reported to my coworkers that there was an unfamiliar bright yellow ball in the sky, but not to worry as it didn't appear to be giving off any radiant energy! In fact, I think it was giving off anti-heat.
The cold weather presents unique challenges and opportunities for martial artists. For one, it gives you the opportunity to experience life about twenty years into the future. You get to feel the stiffness, lethargy, reduced flexibility, debilitating pain, and of course, general crankiness that will be you in two decades. Who needs a time machine when you have winter!
What this means is that your pre-workout warm-up must be approached with caution to prepare your body for activity. You need to take more time, stretch more slowly, take more care in limbering the joints, tendons, and muscles. Cold weather isn't just the time when you risk strained muscles, you risk pulls and tears.
All that being said, winter is also the time when you can really work on endurance. The body doesn't tend to overheat easily as it might in the summer months, so you can "up your reps", burn a little more fat and calories. Yes, once the body has been properly prepared, winter is a great time to push yourself aerobically.
Teachers can adjust their classes accordingly. Your students will come in from the outside and still be chilled. Start them out slowly with joint flexes, deep knee bends, jumping jacks, and other easy non-impact motions. Once the class seems to have warmed up, have them do 10 reps where you'd normally do five, 20 where you'd do 10. We typically will execute one technique repeatedly, progressing across the training room. The class then reverses direction and performs a different technique progressing back. In the cold winter months, I'll have them do the same technique (hand block, stepping punch, kick, whatever) back and forth across the room several times. Whereas a student might perform the technique 5 times right-to-left across the room, I'll have them do it 15 times, back and forth. Not only does this improve their stamina, it also improves the technique, and subtly - their concentration.
While I'm not going to say I like the cold (ever, ever, EVER!), I will admit to using it to my advantage.