I will provide a comparison dog - human, along with explanations.
For a human, there are 2 ways for cooling:
- lowering the temperature of the environment; possible indoors with air conditioning, not possible under the full sun;
- perspiration + evaporation; the effect is amplified by applying ventilation, even without any active cooling. It works, because the water (perspiration) absorbs heat while evaporating.
For a dog:
- lowering the temperature of the environment.
Why is there only one option for dogs? Simply because dogs cannot sweat through their skin. The only mechanisms they have for biological cooling are:
- sweating through the paws and nose; remember the wet nose?
- evaporation of the saliva; remember the dogs keeping their mouth open and the tongue hanging, breathing heavily?
If the dog has his hairs not trimmed, then the dog will only have to deal with his natural internal heat. If the hairs are trimmed / shaved, the sun will heat the dog additionally, to levels which are not at all manageable by their biological processes. Additionally, if the hairs are trimmed excessively short they might even get severe sun-burns.
Bottom line: With all the fur, the dog will feel hot. Without all his fur, the dog will feel even hotter.
Bottom line 2: learn to NOT go against the specialists in areas where you are not a specialist - not before really doing your homework and fully understanding things.
From Wikipedia about dog odor:
Dogs only produce sweat on areas not covered with fur, such as the nose and paw pads, unlike humans who sweat almost everywhere.
I've had groomers tell me that a dog's fur keeps them cool in hot weather as well as keeping them warm in cold.
That is the poetic* way of expressing the facts. What actually happens is:
- during cold times, the hairs prevent the body heat to dissipate - therefore keeping the dog "warm";
- during hot times, the hairs prevent additional heat to reach the body - therefore keeping the dog "cool" - actually, keeping him just "not warmer than needed"
* poetic = easy to say and easy to understand by most people, even if it is not scientifically 100% true
I've measured the dog's skin temperature using a contact thermometer and an infrared thermometer (I'm an engineer and an ex-physics geek. I have these things lying around.)
Being an engineer using a infrared gadget is one thing. Not understanding the complexity of the situation (including biological facts) and drawing the wrong conclusion is another thing. Ignoring the advice of specialists, against the well being of your dog is the ultimate thing.
So unless someone can tell me how dog hair is somehow a heat sink, instead of an insulator in these situations
The hair is not a heat sink, it is an insulator, as you stated.
it's ridiculous to assert that dog hair is helping your dog keep cool
Yes it is ridiculous, because the assumption is wrong. The hair does not cool your dog, it just prevents the dogs body to be overwhelmed with the sun's heat. In this way, the body has to deal only with the internally generated heat.
For even more information, please search for "do dogs sweat?" in your favorite search engine.