High temperatures are harder on your dog than they are on you.
It's easy to forget this, because they are stronger than us in so many other ways. Generally speaking, the shorter your dog's nose, the more caution should be taken in hot weather. A dog's primary method of cooling is panting; exchanging the hot air in their lungs for cooler, dryer outside air. When the air temperature approaches the 90's, the difference between that and the average dog's 100F° breath is just not enough to help keep them cool. Add a high humidity and a steady trot alongside your brisk walk, and it's easy for them to overheat when it's hot outside. When the heat index goes over 100 - well, taking a black dog out in direct sunlight on hot pavement when the index is over 99 is risking heat stroke within 15 minutes.
Don't expect your dog to refuse to go for a walk, or avoid burning pavement, or insist upon water; your dog will follow you to the ends of the earth and beyond, far beyond his or her abilities or safety. Unconditional love. Counting on you to be the one with the big brain, who can check the weather before you go for that walk.
You can sweat, so a breeze helps cool you, so does a misting tent, or a splash of water, since your pores will cool as the water evaporates off your skin. Water on a dog's coat, however, becomes just another layer of insulation, helping to keep their already elevated inner temperature high. If you're going to get a dog wet when it's hot out, make sure they stay in cool water until their breathing has slowed, and then keep them in shade or inside until dry.
How can you tell when your dog is experiencing heat stress?
Look at the expression, listen to the breathing, watch for feet shifting to lift sore pads off a hot surface. If your hand is uncomfortable after a minute on the ground, don't make your dog stand on it. A stressed dog will open eyes wide, often darting around without making eye contact. Mouth open, lips stretched, tongue hanging, sometimes a slight wheezing or whining sound with each breath. Your normally obedient, attentive dog just doesn't respond to you, seems distracted. If the dog is already in danger, the eyes may remain fixed, staring, and the dog may walk in a lurching, stiffened gait. If you see any of these symptoms, get the dog into shade or inside in cool moving air as swiftly as possible, with plenty of clear water available. Laying on iced towels would help bring the temperature back down as well.
Do not wet the dog down unless it is in front of a fan or in a creek or pool where it can stay immersed until it has completely cooled off. More specific emergency info here.
If you typically walk your dog with a Gentle Leader, hotter weather makes it even more important that you properly fit it to your dog's face and neck each time it goes on. An improperly fitting Gentle Leader can slide around on your dog and hold the muzzle closed, severely interfering with breathing, and more importantly - panting. Your dog's mouth should freely open wide enough to hold her favorite toy.
Here are some simple directions.
By now, surely, it does not need to be explained why we don't leave our dogs in cars?
Just don't do it!
Remember: Inside or in shade, plenty of water, dry fur, no hot pavement, never in a car!