Friday, July 27, 2012

Dew Point and Runners: What is it and why should we care?

Recently I have become interested in understanding the dew point. Having lived in Southern California, a hot but dry climate, I never had to worry much about things like humidity and "dew point."  Now, living and running in Michigan, these have become forces that I confront on a daily basis in the summer.

Apparently I am late to the "dew point" discussion because my friends Kate and Paul were already on top of this. The impetus for sharing this information with me was a really disastrous failed lactate threshold run I had this week that left me feeling pretty down. Leave it to my running friends to find a scientific answer to rationalize why I wimped out.

Most of us have heard the saying "It's not the heat -- it's the humidity."  Well apparently it is neither of these. Apparently the dew point is the thing that we runners need to be concerned about when running on these hot summer days.

What the heck is "dew point"? After doing some research of the explanations of dew point and relative humidity, I understand it only a bit better than when I started, but I will do my best to explain it here. Relative humidity tells the percentage of water vapor in the air related to the amount that is possible at that temperature. During a day, relative humidity can go up and down related to temperature rather than related to a change in water content in the atmosphere.  

The dew point reflects the actual amount of water that is in the air, regardless of temperature.  This amount is reported in degrees because the dew point represents the temperature to which air must be cooled for condensation (dew or frost) to occur.  Apparently, it is a more accurate measure of just how much humidity is in the air on a particular day.  The closer the dew point is to the actual air temperature, the more saturated the air is and the more uncomfortable it will be.

No matter what the temperature or relative humidity is, the dew point will always represent the actual amount of moisture in the air, so it is a better way for runners to think about the effect of humidity on their running performance. 

Why does dew point matter? Well, since dew point is the actual amount of water vapor in the air, it affects runners in two ways. The most significant one is that a high dew point means that the air is highly saturated with water, so sweat is less likely to evaporate from our bodies. Thus we can't cool ourselves effectively and our core temperature rises, interfering with our ability to perform and ultimately causing a physiological breakdown.  

Additionally, high water vapor content in the air affects our breathing.  When water vapor content is high, the air can feel "thick." Ironically, water molecules in the air displace some of the oxygen molecules, actually making the air less dense.  It is similar in some ways to breathing at altitude.

How is dew point related to running comfort?

Paul shared an article from Running Times that had a really great chart  to help runners know how the dew point would affect running performance.

How can I find the dew point:
You can find the dew point on most weather sites. For example, here is a screen shot from with the dew point prominently displayed.

Or, Kate showed me this really cool site which will calculate the dew point if you enter the temperature and relative humidity (

An example of dew point at work:
I was anxious to see how this might apply to my running. I have a marathon coming up in August (Leading Ladies Marathon) in Spearfish, South Dakota. I have not been to South Dakota and do not know what to expect.

If I look up today's weather for both Lansing and Spearfish for today, this is what I see:

Right now in Lansing, the temperature is only 71o while in Spearfish it is 84o. That looks bad at first until one looks at the dew point. In Lansing it is 63o while in Spearfish it is 52o.  When I look at the chart from Running Times, I find that while in Lansing this may be "uncomfortable for some people," in Spearfish, I would be in "PR Conditions."  Whoo hoo!!

So now, thanks to my friends Paul and Kate, and with a little help from Running Times, I will be keeping an eye on the dew point to help me know what to expect on a hot day.  It does not make me feel a little better about my failed workout -- but not much. 

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