I live in Boulder, Colorado and I love long cross-country ski outings. I also enjoy riding outdoors even when the temps are in the 20s. How do I keep fueled? First, I know my nutrition requirements:
For rides of more than an hour, the American College of Sports Medicine (ACSM) recommends consuming every hour 0.3 gm of carbohydrate per pound of body weight (0.7 g per kg). In colder weather depending on conditions you probably will be burning more calories per hour to keep warm. Note that this is 0.3 gm of carbohydrate. Most foods also contain some protein and fat--check the labels to be sure that you are getting enough carbohydrate.
Your body can store limited amounts of glycogen (from carbohydrate), enough for a few hours of hard exercise. When you run out of glycogen you hit the wall and your legs feel dead. Your brain also needs fuel and can burn only glycogen. When you run out of glycogen you bonk and your brain feels fuzzy. Even if you start eating carbohydrate immediately when this happens, recovery takes time. You can probably suffer through these on a summer ride, but running out of glycogen in the winter can make for a very long, unpleasant outing.
More: Eating to Win: What We Can Learn From Pro Cyclists
While your caloric requirement increases in the cold, your hydration requirement decreases because you aren't sweating as much, although you still need fluid. Some physiologists used to think that the hydration requirement increased in the cold because you are exhaling moist, warm air; however, metabolizing glycogen (from carbohydrate) releases H2O, which more than offsets the water vapor exhaled.
In the winter you may be tempted to drink less than in the summer because bathroom breaks are harder when you are wearing multiple layers. Don't. Follow the rule the same year-round: drink to satisfy your thirst.
Per the ACSM I should consume about 55 gm of carbohydrate (220 calories) per hour. How do I do that in cold weather?
More: 8 Tips to Survive Winter Cycling