What's the worst weather you've ever run in?

I built ArcPace after a summer of 75°-and-humid runs that felt like running in a sauna, even at an easy pace. The weather app said "nice morning," my heart rate disagreed. I got tired of doing the mental math on every factor that matters.

Curious what wrecks your runs most: heat, dew point, wind, or rain? And is there anything you'd want a forecast to tell you that weather apps don't?