If your winter riding schedule demands you venture out in any weather, make sure you and your riding buddies stay warm and healthy. Frostbite (damage to the skin caused by cold) and hypothermia (when your body loses heat faster than it's produced) can set in when temperatures are as high as 40 degrees Fahrenheit. And riding at a trot or lope/canter for any length of time can create skin-freezing wind chill.
If your face or other exposed skin (such as hands or wrists) feels numb or waxy, stop and take shelter. Also, to watch your riding buddies for signs of frostbite, stop every 10 to 15 minutes to warm up with a chemical hand-warmer or small blanket. Examine their faces for white or yellowish spots - a telltale sign of frostbite. (Tip: Metal jewelry, clasps, and glasses will freeze before your skin does. Keep any metal away from your skin to avoid frostbite.)
For more information, check out "Winter Weather FAQs" at http://www.bt.cdc.gov/disasters/winter/faq.asp#frostbite, hosted by the Centers for Disease Control and Prevention.