My friend Bill and my dog Bailey laying the first tracks in the snow
It is very important to keep warm while outside, especially the hands and feet. Most of the body's energy will be used to keep internal organs warm and will therefore draw blood away from your extremities. This will increase the risk of hypothermia and frostbite. There is also a risk of post exercise (climbing) hypothermia since the heat production has decreased and heat loss is still high (Cold Weather Activity). So how does one stay warm while climbing without piling on too heavy of clothing?
Baselayers, hoodies and beanies: typical bouldering basics in cold weather!
Having extra weight to carry can be just as exhausting as climbing with extra weight. What should be carried with to the crag or boulder field? The answer: only the essentials. The utmost essential is water and plenty of it. Most people don't realize how much water they lose when outside in the cold. Every breath you exhale is a small amount of water lost. At extreme temps (-10 degrees Celsius/ 14 degrees F) water loss through breathing can be as much as 20 mL per hour during non-activities (Study abstract: How Much Water is Lost During Breathing). Much more water is lost during physical exertions, so bring plenty of water! Water can be exchanged with something hot as well to keep you warm (tea, hot chocolate, soup broth, etc.). Try to keep the water close to room temperature (use a thermos) since the more coldness the body has on the inside, the more heat that is lost from the body to the cold water. Snacks can be kept to a minimum by carrying complex carbohydrate foods (granola, fiber bars, whole wheat items, etc.) that sustain energy levels, while not adding too much weight to carry (Hydration and Nutritional Considerations in Cold Weather).
Climbing in the winter time can be great and lead to a lot of hard ascents. It also must be done with careful consideration of the conditions that will be put upon the body. Apply the layers, stay warm and hydrated, and climb hard!