Living here in Canada I've run in some pretty darn cold weather. This morning was no exception as I went out in 40F to run 8km. It got me curious though, how cold is too cool, or unhealthy to run in? I wear the proper clothes but there are days I only have my eyes uncovered. I'm just curious if there is a rule of thumb out there that I haven't heard of. Thanks for your suggestions/help!
I've wondered the same thing. Last night I ran approx 8 miles in 15 degree (Fahr) weather. The activity kept me warm throughout the run. However, after the sixth mile I had to stop for a couple of minutes at a very busy intersection to wait for the walk light. At that point, the wetness of my clothes (I sweat more than most people:)) caught up to me, and the next two miles had me shivering even though I was in full activity.
It had me worried had I been running a longer distance, if symptoms of hypothermia would have set in and at what point, and would I have noticed when they did set in?
Good news, the simple answer is NEVER! With that being said, you need to dress for the weather and use a little common sense. If you're running at -32F or 0C any exposed skin has a pretty good probability of getting frost bite. Most importantly wear clothing that wicks moisture from your body and dress in layers. I've heard that dressing like it's 20 degrees warmer is a good rule of thumb (albeit at -12F and -32 are both brutal). You're also more prone to exercise induced asthma at extremely low temps. So, if you experience this at warmer temps it might be a good idea to hit the indoor track or treadmill. Furthermore, watch where you're running when it's that cold. What I mean is this: What happens if you twist a knee 8 miles from civilization? I'm sure there's more, but these are the things I've discussed with others and have read. I personally used to run at sub zero temps in Montana and I always appreciated a warm shower afterwards.
I ran the Wicked Frosty Four in Salem, MA on new years day when it was 5 degrees Farenheit and 20-30 mph wind. It snowed the night before too, so there was a lot of blowing snow. With a good base layer, thick sweater, polyfleece-lined parka and goretex pants, my body was fine. Regarding the breathing of cold air, I brought my Wigwam ski mask, too. I don't know how, but the mask warms the air a bit before you breathe it. I have also found that it is important to have (and use) a wide chin strap on your hood. My chin has threatened to freeze up many times before I fastened my chin strap. It works wonders.
So personally, I dont think a 4-5 mile run even in single digits need have a negative effect on you. Run slower though, because although your body might feel as warm as in the summer.., it isn't. Warm up twice as long because the cold and hard roads can wreak havoc on your muscles.