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.