If you remember my first post about Cold Weather Riding, it originated from a USA Cycling Coach that was posting the question. It talked about why the rider was slower.
Since then, I have noticed the same phenomenon and just attributed it to being a little out of shape at the start of the off season workouts. While that is partially true, it is not the biggest reason. I stumbled across a thread on Bike Journal that referenced this article:
Slower in Winter?
Basically, due to increased wind resistance from wearing more clothes and the higher density of air at colder temps, we have to work harder in order to achieve the same speed. The factor the article states is anywhere from 15 to 30 percent. For me, I think the difference is less than 15 percent but more than 10. Check out the article; it is an interesting read. It certainly makes me feel better about my recent workouts.
Also, check out the Cold Weather Riding article on my club's website:
32 Suggestions for bicycling when it’s Thirty-two
Lastly, check out another tidbit about Cold Weather Riding that is especially important to those looking to lose weight:
Nutrition for Cold Weather Exercise
The crux of the article is that fat utilization is decreased when riding in cold temps. The article describes why. So, if you are trying to lose weight, consider using the trainer more than outside rides in the winter in order to maximize fat loss.