Just an idea for those of you stretching in colder climates: take a little more time to stretch. When it is cold out, there is less blood supply to the smaller areas of the muscles (specifically, where the muscle attaches to the bone at its origin, and where it attaches at its endpoint, the insertion.) Go a little slower, take a little more time, be patient. If you don't have more time, do fewer stretches for longer periods.
One of the silly things I did when I was about 16, was to train in karate in the snow. Barefoot. Just wearing my uniform. I wanted to be like those Buddhist Monk Martial Arts Masters you see in the movies. I pulled a few muscles, and that was that!