It IS harder! I try to ride in all weather, but take the indoor trainer option more frequently these days. A bout or two with frostbite in the last couple of years has tempered my enthusiasm.

Less miles, less time in the saddle, but I'll bet the caloric output is at least the same as Summer. There's increased effort frrom road surfaces and wind, calories burned to keep warm, as well as the sheer terror of a car sliding into me on the icy roads (bet that burns a few extra!)