Figure 1. An infrared image of a Pembroke Welsh Corgi in the hot Arizona sun. Areas with thinner coat are noticeably warmer. Source: Doug Klassen, MyCorgi.com
When people see an infrared image of a dog in the heat (Figure 1), they tend to think the fur is keeping the dog cool because the fur appears colder than the surroundings. This is because the coat insulates the dog from the hot temperatures, trapping cool air against the body and slowing heat transfer to the skin (Figure 2). Unfortunately, this interpretation is incomplete and has caused widespread misconception about the role of fur in keeping cool.
Figure 2. Concept drawing of how fur insulates the dog from environmental heat.
Underneath all that fur is a dog that's trying hard to maintain a constant temperature. In order to maintain that temperature, the dog must release body heat to the environment. It does this in a number of ways (Figure 3). Note that insulation from outside heat is only a minor part of keeping the dog cool (compare Figures 2 and 3). Actual cooling effects, like air blowing through the coat (convection), panting (evaporation), or direct contact with cool surfaces (conduction) are much more important when maintaining normal body temperature in hot climates.
Figure 3. Concept drawing of the full heat environment of the dog. In order for the dog to maintain its core body temperature, metabolic and environmental heat gains must be equal to heat losses via convection, evaporation, and conduction.
Much like a thermos keeping a cup of coffee hot for hours, the fur inhibits these cooling mechanisms and greatly slows the rate of heat loss to the environment. This is great in cold temperatures but not good in hot ones. A thick fur coat may reduce the discomfort of stepping from a cool house into the hot summer sun, but over time a thick-coated dog is going to have more trouble staying cool than a thin-coated dog. A thick-coated dog can't release heat as easily as a thin-coated one.
With a better understanding of all the processes that affect the dog's body temperature, we can properly evaluate infrared images. In the infrared image showing a partially shaved dog (Figure 4), the skin is bright and hot not because the sun is beating down on it, but because the shaved part is radiating body heat much more easily than the furred part! The shaved area is actually significantly more effective at cooling the dog because there's no insulation to block body heat from escaping.
Figure 4. An infrared image showing the heat radiating from the coated and shaved parts of a dog. Source: unknown, Facebook.
The coat is important for a few reasons and the dog needs some insulation, so shaving to the skin is not necessary or recommended for summer comfort. That said, increasing air flow to the skin is necessary for efficient heat loss. Trimming hair to a reasonable length will go a long way in keeping the dog comfortable (aesthetic and coat health impacts notwithstanding). Without trimming the fur, the best way to help a thick-coated dog deal with the heat is to keep the fur well-brushed and as free from loose hair as possible.
This post was brought to you by basic thermodynamics. I apologize for the quality of the concept drawings; they were rescued from a failed hard drive and I haven't redone them.