I've posted on concerns with both excessively hot weather and cold. This is something I think you'll find helpful.
The Tufts Animal Care and Condition scale (TACC) gives a really good approach. It addresses the size of the animal, its physical condition, the condition of its environment and housing, as well as the weather and temperature. It gives guidelines for both hot and cold.
Here's a link to one publication. If the link doesn't work, just Google it. Lots of humane organizations and animal welfare agencies use this as their standard.
Thanks for the above information. I had never heard of this scale before. It seems most of the attention goes towards not keeping a pet in a hot car, but that is really just one aspect of keeping your pets safe in all kinds of weather, so your article was very helpful.