Curious about how S. Fla.'s high temps and humidity were impacting my runs in the summer, I looked everywhere for this. More science: Above 70 degrees, heart rate increases 1 beat/min per degree F. Interestingly, the oxygen demand doesn’t increase in the heat (the sucking wind portion), but your cardio system must pump more blood to your skin to enhance heat dissipation and cool you down. Higher humidity also increases heart rate, because it decreases your sweat evaporation rate. Research shows a 10 beat increase in 90% humidity, compared to 50% humidity.
My heart works very, very hard on those 90/90 days, which is why it takes so long to recover. Treadmill lets me run faster longer, and recover quicker, without the knee pounding. Boring as it is.