Today I went for a 6 Km run in Tokyo, the temperature was 27 degrees Celsius and the humidity 92%. It was much harder than usual today even though I am used to run the same distance 3 times a week at 75% humidity. I was sweating a lot and my tshirt was all wet but I also felt more tired somehow. Can it also be dangerous?
Runners are cooled by the surrouding air (when cooler than the body temperature), and by sweat vaporization. The higher the air temperature, the more we rely on vaporization for keeping cool.
If the humidity is high, the air is more saturated with water, and vaporization becomes less effective. The body responds by sweating more.
If both temperature and humidity are high, you get dehydrated faster.
A trick in these situations, besides drinking a lot of water, is to pour cool water over your head and body. Then your body won't have to sweat as much.