Remember that when it is hot for you, it is even hotter for them. Cats and dogs do not sweat through their skin. They cool themselves by panting or rapid breathing, which means animals must work hard to stay cool.
Too much heat can be extremely dangerous or even fatal. If your best friend has a shorter nose, like Persian cats and English Bulldogs, he is more susceptible to heatstroke than breeds with longer noses. This can occur very quickly for those breeds.