Hot and humid? Dread that stickiness? Did you know that when it rains, that is 100% humidity and most of us runners would welcome a nice cool rainstorm to work out in. A few years ago when Bob Carroll was the Checkers AC coach, he used to tell us to be careful when the dewpoint was over 60. In fact, I had to ask for a clarification and his response was…“Once you get over 60. If the dew point was over 65 I wouldn’t let Becky race.” Ok, so what is the deal with dew-point? Why that and not humidity.

Dew-point is the temperature of air which is needed for condensation or dew (at that particular temperature). If you take a glass of ice water and it develops condensation on the glass surface, the air on the glass has condensed to its dew-point and created dew. Dew-point actually measures how much water vapor is in the air.

Relative humidity is a bit difference. Relative humidity is a measure of how much water vapor the air actually could “hold” at a certain temperature.

One approach to explaining dew-point would be to say, dew-points above 60 make it feel sticky and humid outside while dew-points less than 60 F are comfortable with respect to the stickiness of the air. The higher the dewpoint is, the more moisture that is in the air. The higher the dew-point is above 60, the stickier it will feel outside (feels like you have to breathe in a bunch of moisture with each breath). 65 or above dew-point, the air really feels sticky and humid.

So Bob’s point goes to the extent of the body’s ability to sweat and have that moisture evaporate instead of it coating you. When it does that, you really feel it. That’s because the high moisture content in the air interferes with one’s sweat evaporation. Namely, when you sweat under high dew point conditions, the skin doesn’t get cooled off adequately and makes you more susceptible to overheating.

So what is the optimal for running? I found this on runnersworld.com. It seems to be a pretty good barometer for gauging your effort…

TEMP+DEW POINT:
===============
Below 100: Ideal
100-120: Not ideal, but ok to go for a fast time
120-130: Ok to go for a fast time in races up to 10K – adjust goals for slower pace in longer races
130-150: Adjust goals for slower pace in all race distances, especially longer races
Above 150: Forget pace – just focus on finishing

In any and all cases, hydrate, hydrate and HYDRATE.  Most importantly, listen to your body!