One reason performance declines on sultry, humid days is that working muscles have to compete with the skin for blood. Directing more blood to the skin removes body heat and helps keep your body’s temperature from rising to dangerous levels. But that can mean less blood reaches muscles. At the same time, when your body becomes hotter, muscle enzymes speed up, burning glycogen more rapidly, depleting stores of the sugar that the muscles use for fuel.
Very interesting article, Mimi. I've pondered the relationship of humidity vs heat vs pressure.

I currently cycle during the hottest part of the day to get acclimated for the hot August summers here. Last year I did the most mileage during the 2 week heatwave. I would ride with my Camelbak and 2 "shower bottles" to dump water on me as I cycled. It cooled me down for a little while but then I'd end up even hotter after a while. And I noticed on my rides when temps are above 90 F, that my speeds are slower.