Here is a post from a couple of years ago that I found very informative:
The thermodynamics of coat, heat and humidity from an engineer and owner of an Old English Sheepdog, another breed with an undercoat. forum.oes.org ? View topic - Summer Heat Exposure and Bobtail Coats (the whole post is at the link)
Quote: "Now let's consider what happens when the surrounding temperature matches the dog's body temperature. At this point the dog cannot cool himself/herself by rejecting heat through the skin/coat, nor from "convective panting" as the air passing over the tongue is the same as the body temperature. The only cooling that can be accomplished is through the saliva evaporation described above. While this is becoming serious the evaporation of saliva removes a lot of heat and the dog can survive, albeit uncomfortably, for several hours without fatal distress as long as water is available to prevent dehydration.
As the temperature continues to rise and exceeds the dog's body temperature things quickly become critical. Saliva evaporation due to panting is less effective in rejecting body heat and worse yet this is now accompanied by heat flow through the coat reversing. Heat from the environment is now flowing into the "relatively cooler" body of the dog causing it to rise above it's normal level. The coat's insulating properties are slowing how fast that heat flows into the dog but nevertheless the overall energy balance is that heat is flowing into the dog forcing it's body temperature up.
The dog is now in a nasty, potentially fatal, escalating spiral; saliva evaporation is less effective - heat is flowing into the dog requiring more cooling - the dog has to pant more to cool but panting burns more fuel (food) which causes more internal heat driving the temperature up yet more - saliva evaporation is yet less effective...... As the surrounding temperature continues to rise there will come a point when the heat load entering the body exceeds that being rejected through panting-induced saliva evaporation and the dogs temperature then soars and can quickly become fatal."
Dogs and Heat Strokes | Cedar Breeze Holistic Pet Care Writer is Federally Certified for FEMA's Animals in Disaster program.
Should you shave dogs to keep them cool? Quote:"A Dog's Fur is Made to Protect the Dog from both Cold and Heat: TRUE. A dog's fur is made to insulate the dog from extreme temperatures, especially in heavy-coated breeds; however, that does not mean that shaving will not help keep your dog cool. If you live in a hot and humid climate and have a breed with a thick coat, it may be a good idea to shave them. The increased air flow across their back can cool them just as well (if not better) as the fur insulator, and they will feel much more comfortable without the extra fur.
Dogs Only Sweat through their Paws and by Panting: TRUE. Dogs do not sweat in the same way that humans do because the sweat would not normally be able to evaporate off of their skin through their fur. This DOES NOT, however, mean that shaving just the body of your dog will not help in their cooling off. As I mentioned before, the increased air blowing across their skin will help cool them and keep them comfortable."
