This could well be it. It never really occurred to me how radically the humidity changes throughout the day. I suspect this is because as the humidity rises the

This could well be it. It never really occurred to me how radically the humidity changes throughout the day. I suspect this is because as the humidity rises the temperature is in a low and vice versa. So it feels like a relative equilibrium, but it's not. It'll be interesting to check out the by-the-hour data the next time a particularly mean hot+wet window emerges.