The temperature in San Diego yesterday was about 91 degrees. That's HOT!!! While walking my client's dogs for their mid-day potty breaks and exercise I made some concessions due to the heat. Their walks were kept in the shade as much as possible. They were also shortened a bit and they got fresh cool water to drink before and after their walks. At this time of year when the temperatures can soar, it is extremely important to remember that dogs can suffer from heatstroke. NEVER EVER EVER leave a dog in a parked car, even with the windows cracked open, or the sun roof open, or anything similar to that. Not even for 5 minutes. It just takes a few minutes for a car's temperature to soar to well above 120 degrees on a hot day..
Dogs do not sweat. The way they cool themselves is through panting so in the heat do not use a muzzle or head devise that prevents the dog from being able to pant easily; otherwise they will be unable to regulate their own temperatures. Make walks shorter and exercise less vigorous on a hot day and ALWAYS have water with you for the dog so he/she doesn't get dehydrated. Learn what to watch for in heatstroke and if you suppect it, get your dog to a veterinarian immediately. DO NOT USE ICE COLD WATER to cool it down. Use a towel soaked in cool water as you transport the dog to the vet.
The best advice here is common sense. Watch your dog carefully to make sure it is not uncomfortable or suffering in the heat and if it is, go indoors.