Can thick fur isolate against heat? I started writing this question on pet.SE but realised after a while that I think it has more to do with physics than pets but if some admin disagrees, please move it back to pets.
I have heard several people claim that their dog's (we are talking about breeds such as spitz') thick fur isolate against heat in the summer and trimming it would actually make the dog worse off (that is, having bigger problems with the heat).
The context here are summers in Northern Europe so it is rare that the temperature in the shadow is above 25 °C.
Is this really possible? A dog's normal body temperature is around 38 °C and my intuition says that as long as the ambient air temperature is colder than the body temperature the dog will lose heat. With a thick fur, that heat loss is slow and inefficient while if the dog was "naked" it would lose heat much faster.
Are there any properties of a fur that makes my intuition wrong?
 A: It would be difficult to construct a mathematical model of heat loss from a dog without some experimental input as there are so many variables (though it wouldn't surprise me to discover some enterprising thermodynamicist had done this) so the best I can do here is make some general observations.
If the dog is in the shade then you are correct that provided the external temperature is lower than the dog's temperature the less hair the better. However I believe a dog's skin is not heavily vascularised because they lose heat mostly by evaporation from their mouths. So even if you shaved the dog it isn't clear that heat loss through the skin would be that great. Humans lose heat through their skin by flushing and sweating, but dogs don't do this. And if you didn't shave the dog completely, but left even a thin layer of hair, this could prove an effective enough insulator to make heat loss through the skin negligible.
If the dog is in direct sunlight this is a completely different situation as direct sunlight can heat surfaces to much hotter than the ambient air temperature. In this case a thick layer of fur would be a considerable advantage as it would prevent the sunlight from heating the dog's skin while still allowing heat loss through the mouth from panting. (It is for just this reason that the Bedouin wear heavy robes in the heat of the Sahara.)
So there are plausible arguments for both outcomes, and it's impossible to say what would be best for keeping the dog cool without actually doing the experiment. Given that dog owners have been doing the experiment for many generations of dogs I would be inclined to go along with the accepted wisdom, though I'd give more weight to a breed society's advice than that of some random acquaintance.
A: Your intuition is correct. As long as the dog's temperature is higher than the air temperature, the air is cooling the dog, so fur is a hindrance to cooling, not a help. 
