Just got back in from a ride after work...98 here and the dew point 72. Nasty. I realized how useful my HRM is in these conditions. On a "normal" day I often go into zone 4 when riding along. Today when I looked down just cruising, feeling good, and I was in in zone 4 I told myself "Whoa there!" and backed it down 'till I was back in zone 3 and even 2. Staying in high zone 3 and low 4 for an extended time in these conditions will lead to bonking at the least, or heat exhaustion at the worst.