When you exercise in hot weather, a lot of the energy produced by metabolism is taken up by keeping cool. Even standing still in warm weather, your resting heart rate will rise quite surprisingly as your bloodstream delivers the increased oxygen needed for these processes. If you start to get dehydrated, your heart rate will go up more.
Since I exercise with a heart monitor, and stay under a target level, I have to run much more slowly in warm weather. The result this morning was that my time for seven miles was 82:03, my slowest performance this season. But I was pleased that I was able to finish at all.