training in the winter months compared to the summer months

This could all be in my head, but does anyone else notice training
improvement differences in the colder months compared to the warmer
months?

I've noticed that I can't seem to improve muscles growth through
weights or endurance throught running/swimming/biking during the
winter as well as I do during the summer months?

Does anyone else have this same problem as I do? 

Does anyone know why this might be?

derg