The feeling after a hot and humid long run You've probably done this before: check the weather app before a run, see "65% humidity," and think, that's not so bad. Then you're a mile in, drenched, and gasping like you're running through soup. What happened? The problem isn't that the forecast was wrong. It's that you were looking at the wrong number. Relative humidity is a percentage that depends on temperature, and it shifts throughout the day even when the actual moisture in