I try to avoid riding in really hot weather for fun... but since I've been trying to stay committed to commuting once or twice a week, I end up riding home when it is in the high 90's or even 100 sometimes. It's about 10 miles, but I usually stop halfway and refill my water bottle with ice at a convenience store.

The last few weeks I've noticed that on those occasional hot commutes home, my heart rate seems to be higher than I expect based on my effort otherwise. On a similar ride in the morning, my heart rate might be in the 130's or 140's, but according to my HRM, it has been up in the 150's.

Any one else notice this?