Why Dew Point is Killing Your Pace

I've spent most of this summer running slow and trying to figure out how much was the weather and how much was simply my conditioning......or lack of.  I know that the increased fluid loss and energy used cooling the body takes away from performance but that just wasn't quite enough for me.  I did a bit of research and found the scientific answers but few things lost my attention faster than a weather man explaining the relationship between relative humidity, dew point, and temperature. 

Finally, after about 45 seconds reading a LetsRun thread about water vapor, ambient gas, air density, and something that seemed relevant about oxygen I decided it was time for me to figure it out and dumb it down so that people like me can understand what's going on.  So here goes.....

As a runner dew point is probably the most important measure affecting performance.  I've run in the dry heat of Arizona when it was near 100 and it was nothing like a 90 degree run in the dirty south.  Dew point reflects the actual amount of water in the air regardless of temperature.  The closer the air temperature and dew point are the more saturated the air is and the more uncomfortable your run will be.  Obviously when humidity is high running isn't going to be pleasant but humidity moves around a lot during the day as the air temperature increases and decreases.

A high dew point means that sweat is less likely to evaporate which is the body's cooling mechanism.  The result is that your core temperature rises and performance suffers accordingly.  The saturated or 'thick' air also makes breathing more difficult.  One reason the poor man's altitude analogy is often used is because having more water in the air displaces some of the oxygen molecules which makes the air less dense as you might experience in higher altitudes.  Training through the summer sweat baths eventually makes your body more efficient with delivering oxygen and blood to working muscles so your focus should be more on effort than pace. 

I found a number of charts illustrating the perceived impact on running at various dew points and included the information from Running Times below:

Dew Point____________________________________________________________________
50-54         Very Comfortable =>   PR Conditions
55-59         Comfortable =>  Hard efforts likely not affected
60-64         Uncomfortable for some =>  Slower than optimal conditions
65-69         Uncomfortable for most =>  Easy training could feel OK but hard efforts are difficult
70-74         Very humid and uncomfortable =>  Expect pace to suffer greatly
75 +           Extremely oppressive =>  Skip it or dramatically alter goal

That's all fine and dandy but really all I wanted to know is exactly, to the second, how much is weather affecting my performance.  I mean is that too much to ask?  That way when I come home after a disastrous long run, or a slow run that was supposed to be fast, at least there's some mathematical consolation.  Not that it will help that much but at least it's something....

So apparently some 'real' runners and coaches like to use a measure of Temperature plus Dew Point to determine pacing adjustments......and viola, I found my answer.  OK not really but it's enough of an understanding for me to stop using Google to compare my desired pace to my actual pace.  So for what it's worth I created the chart below to show the impact of the Temperature + Dew Point and the amount the 'experts' suggest your pace could be impacted.  Obviously it depends on your conditioning, acclimation to running in similar conditions, etc. but you get the idea.

And finally, I did the math of the impact to pacing at certain levels.  

My take away from all of this is pretty much the same as it was when I started; summer in the south is hot and humid so deal with it or go swimming :)