If the dog has a dark coat, that absorbs the sun's rays in no time so even if the dog is lying around outside when it's stupid hot out there, he can all too easily suffer with Heat Stroke. I've seen it, twice, and the second time the dog didn't survive despite ours (me and my then neighbour) and her vet's every efforts. He went into a permanent state of fitting and could not be saved.
Just use common sense when the weather is hot (and that would be over 80F for me + humidity) and wait until the sun goes down, or play before it gets really hot. Reduce feed too as when it's hot, like us, dogs don't need a load of calories.
ps The two dogs I saw with this were a dark tricolour Basset, standing out in the heat when being judged and a dark coated Large Munsterlander who took off on my neighbour who insisted on exercising her two at around 1 pm every day, regardless of the weather. He was later found lying in a ditch by a passing woman on horseback.