Many athletes mistakenly believe that their hydration requirements decrease in the colder winter months. Not so! Of course, if you are properly clothed for training sessions then you will still be producing excess body heat and sweating as in any hard workout. In many places, the winter air is drier and your bodies natural moistening of the air as it takes it in results in further water losses via respiration.
What's more, your thirst response is diminished in the winter - so your drive to drink is less.
Athletes and coaches...you need to consciously choose to hydrate during winter trainign sessions for optimal performance.