The summer is a great time to talk about overheating. All over the country and year-round, dogs are put in situations that are not safe due to temperature. This can happen even in cooler parts of the country and during the winter, so you should always use good judgment.
Much publicity has been given to never leaving dogs in closed cars even on cool days, but people still do it. No living thing should be left in a car except at night in cold weather (though even then it may not be a good idea, because it can be too cold). The sun can make the inside of a car unbearable or deadly hot even in the winter. At night in warmer climates and in the summer, the inside of a car still builds up heat. If you see a dog in a car that is too hot, you can call the police. They will usually help find the owner and embarrass them enough to prevent it in the future.
In my opinion, one of the worst heat-related tragedies is when people believe the old myth that long fur insulates a dog and keeps them cool. This is true for a few seconds, but after a minute or so, the coat begins to hold in body heat. Imagine wearing a fur coat in the heat and you will get an idea of what it is like. Long-haired dogs in warmer climates should be trimmed short. Even with air conditioning, these dogs can get ridiculously hot.
Running or hiking is great exercise for dogs, and they love it. If you treat your dog the same way that you treat yourself, it should also be safe. If you are wearing light clothes, keep your dog shaved. If you need to stop to take a drink, so does your dog. If you are feeling hot, your dog probably is also, so pour some water on their chest. The best places to cool a dog down are on the neck, pads of the feet, and belly. Watch Cesar demonstrate the best way to cool down an overheated dog here.
If your dog wants to slow down, assume that there is a reason and allow it. Try to hike where there are streams along the way to jump in. You know your dogs; if they are the types to keep going and never stop, be sure that they jump in that stream. Remember you are the human, so you need to be the one to anticipate the dangers and not take a chance. If you are far away from help, the results can be tragic.
Smushed-faced dogs, such as bulldogs, should not exercise or be left out in hot weather without the permission of a veterinarian. These dogs often have small tracheas and long soft palates, which decrease their ability to cool themselves. You can also ask your vet about surgeries that can shorten the soft palate and increase the ability to exercise.
All muzzles other than greyhound muzzles are not acceptable on a dog that is hot or exercising. Much of a dog’s ability to cool down is based on panting, so eliminating panting can have disastrous consequences.
Dogs left in the yard need shade and preferably a small wading pool filled with cool water. Dog houses do not usually provide true shade, as they are often made to prevent air movement and can get very hot. Outdoor dogs will often rest under the house or deck, enjoy the shade of a large tree, or dig into the cool earth in shaded areas with air blowing through. A simple wood roof on four legs will also provide adequate shade. Again, I would say that if you are not comfortable in your yard, your dog won't be either.
Finally, whether it is hot or cold, every dog should always have an adequate supply of fresh drinking water.