Cold weather vs. power output

I have a Kurt Kinetic trainer set up in my garage and wonder if there is a relationship between colder temperatures (let's say 32F/0C versus 50F/10C) and power output. I try to keep everything consistent on my KK setup (tire pressure, wheel resistance/coastdown) yet experience 10-15 watt variance in my workouts. Perceieved exertion and heart rate remain similar. I know there will be some vagaries associated with the KK wattage and HR consistency, so enquire to those of you with real PMs if you notice any differences associated with colder temperatures. I do remember from my days of following horse racing that horses run slower times in the winter; any similarities to what I am describing?