Tag Archives: dew point

What is the dew point, and why does it matter?

Having lived in Chicago since 2014, I’ve not only experienced the gamut of weather (from frigid cold to stifling triple digit heat), but I’ve discovered the nuances of summer dew points and how they factor into humid conditions… not to mention affect your running.

What is a dew point, and why does it matter?

The dew point is the temperature at which water vapor condenses into physical liquid or solid condensation (dew, frost). For example, in winter you see frost when the temperature falls below the dew point at a given time, e.g. if the dew point is 31 degrees you’ll begin to see frost once the temperature is 30 degrees Fahrenheit or less.

However, dew point becomes a more important factor in hot weather. As the air humidity rises, typically so does the dew point. You will tend to notice stickier, more humid conditions when the weather is currently at a higher dew point. The more humid the air, the less cooling benefit you get from sweating due to the air already being full of hot moisture. This can exacerbate the overheating you experience during exercise as the compiled sweat plus the external humidity may provide an added seal for your internal heat instead of evaporating and cooling you as intended.

This didn’t matter to me for most of my life. Growing up in the hot dry desert of Las Vegas, I typically never saw high dew points, and rarely did the temperature drop low enough to trigger dew or frost.

Living in Seattle for 10 years, while it did get warm during summer, the dew points typically never exceeded the 50’s Fahrenheit. While we would see frost or dew during winter, when the dew point would drop to the 30’s, it rarely got seriously muggy.

Chicago is where life with summer humidity got serious for me. And after I recently learned more from Jonathan Savage about the effects of heat on running, I realized that dew point provides a loose but reliable barometer of how the humidity affects you, not to mention your running.

Runner’s World first published a primer on dew point’s effect several years back. While their scale is general and crude, it does provide a reliable barometer for how uncomfortable the outdoor heat can feel. And yes, the actual temperature is still important (a dry 105 degree day in the desert may have a safe dew point but obviously 100+ degrees is still dangerously hot), as does your physical condition and how acclimated you are to current temperatures.

A typical Chicago summer day, regardless of true temperature, sits at a dew point in the 60’s Fahrenheit. Low 60’s is (provided acclimation) reasonably comfortable, even if there’s enough humidity to soak through your top.

The mid to high 60’s is where any amount of extended activity begins to overheat you more quickly. And once the dew point reaches the 70’s, that’s some seriously uncomfortable humidity.

20180902 Weather

Nowadays when I look at the weather report (something I’ve done religiously throughout life, because I commute on foot and run a lot), I don’t just look at rain forecasts or temperature, but I pay attention to the dew point. I may not be able to count on the wildly-fluctuating forecasts for high and low temps, but the trend of the dew point will tell me when it’s going to get real hot or cool down.

For example in the above-pictured forecast, the dew point (the dark green line) sits pretty in the low 70’s. So I know the next few days are going to feel uncomfortably humid. In these conditions I keep my daily runs real slow, focus less on pushing the pace and more on maintaining steady continuous form… and most of all be willing to rest for a couple minutes if I feel like I really should take one. (And go figure that, with the forecasted rain, the dew point is forecasted to go down as well… since obviously a lot of the humidity in the air will become rain on the ground)

Savage’s Perceived Temperature For Runners calculator is a great guide to how given conditions will really feel when running at a given pace for your frame. The temps this week may only be in the low 80’s, but for me running a slow-ish 11:00 mile pace the combination of heat and humidity feels like the equivalent of 140-150 degrees to a normal person standing still. If I think I need a break 30 minutes into a run in such sauna-like conditions, I’m probably right.

In the most recent London Marathon, runners dropped like flies in 75 degree conditions, drawing derision from the general public since 73 degrees is supposed to be room temperature. How can runners struggle so much in room temperature conditions?

Realize that when indoors we’re probably not exercising, let alone with any intensity. Even the “Feels Like” temperature provided in weather reports assumes a normal-framed person doing no more than walking at a normal pace of 3 mph.

Of course 73 degrees feels comfortable if you’re just sitting down or otherwise hanging out. Once you’re running, you’re expending a lot more heat and energy, and suddenly even 50-60 degrees can feel uncomfortably warm.

(And this is not just something casual observers screw up. Most experienced runners don’t realize this either. Many go full speed into workouts in 65-70+ degree dewpoint conditions, and then wonder why they’re having such a hard time.)

A 5’10”, 160 pound marathon runner running 8:00 miles would have felt the equivalent of 130 degrees running that pace in 75 degrees Fahrenheit with a reasonable 50% humidity. Add any more humidity to that and it feels even hotter. It’s no surprise that runners indulged on fluids so heavily that the race organizers ran out of water, which exacerbated the dangerous conditions further for slower runners.

A general rule of thumb given by many when running is to take whatever the temperature is, and assume you will feel 20 degrees warmer. I’ve found this to be true, especially in winter where I’ve gone for outdoor runs in 0-20 degree weather and felt perfectly warm within a couple miles. Expending significant energy to run does heat you up considerably. Your perfect 70 degree weather as a sedentary pedestrian can become very dangerous once you go on a simple run.

But I digress. Not every 70 degree run is going to feel equal. But the dew point is going to give you a more nuanced idea of how it will feel, and how you should plan to run it.

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