You'll ruin your knees!
""...the truth that someday, you will go for your last run. But not today—today you got to run." - Matt Crownover (after Western States)
Yeah, I get that often too... Now that you mention it, I think I may experience it more in cold weather? Perhaps because the cold air is also very dry so the sweat evaporates quickly.
Roads were made for journeys...
I'm running somewhere tomorrow. It's going to be beautiful. I can't wait.
You sweat. Sweat contains salt and water. Water evaporates, leaving salt. Sometimes it is humid out and the sweat does not evaporate as well (which keeps you from getting any benefit from evaporative cooling, which is why you stay warmer when it is humid). Sometimes it is dry out and the sweat evaporates very well, leaving noticable salt.
It's the humidity, not the heat