Wind Chill and Hypothermia are biggies to combat. I was stationed in Maine for 4 years, and have seen way to many cases of frost bite and such. Dressing in layers than allow sweat to leave and keep the core warm is essential. When the extremities start to tingle, it is time to re-evaluate and check them. It doesn't take long to get a really good case of frost bite at any temp below 20 degrees, and in the wind, and you can drift into real trouble quickly. Once frost bite sets in, you need to warm up slowly, or you will do extensive damage to tissue. Left untreated or treated wrong you'll be visiting the doctor at the least, and in some cases the hospital. Know whatyour exposure time is for the conditions you will be in.