It’s the Humidity

I may have reached the end of my rope with the summer humidity. It’s mid-September and the dew point is still over 60. I’ve spent way too much time thinking about it, fearing it, enduring it. The other day on The Paceline, the cycling podcast I do through The Cycling Independent, I took a deep dive on humidity, how to understand it, and how to survive it.

Maybe this is worth something to you. Maybe not.

If you read the weather report, you’ll be familiar with the idea of relative humidity. Today, here, it’s forecast to be 90 degrees with 77% humidity. This is where we get into people’s misconceptions of what that means. Many will read that and think, “Oh, only 77%, not too bad.” They think a high relative humidity correlates to a very humid day, but that’s not the case.

What 77% humidity means is that the dew point, a MUCH, MUCH better measure of humidity, will be 77% of 90 degrees, i.e. 70 degrees. The dew point is 70.

Now here’s the real skinny on humidity. If the dew point is over 60 degrees, it feels muggy. A 70 degree dew point, no matter what the air temperature is, feels oppressive. So, for example, if the temperature was 75, relatively cool, and the dew point was 70, then your relative humidity is 93%, and even though the air is cool, it’ll feel pretty bad outside.

As another example, if it’s 45 degrees air temp, and the dew point is 40, that’s 89% relative humidity, but you wouldn’t feel that heaviness in the air. A dew point of 40 just doesn’t register as humid.

So, to recap, humidity is about dew point, which is the temperature to which the air needs to be cooled to be saturated with water vapor. Over 60 degrees and it’s humid feeling. Over 70, and you’re pretty bummed out.

Whether you’re running or riding, the critical task on humid days is hydration.

I sweat excessively. This has something to do with having a high core temperature, but I also take an SSRI, an antidepressant, which makes me sweat more than I might normally. And of course, as you sweat you lose moisture, but more importantly, you lose sodium, magnesium, chloride, potassium and calcium. These turn out to be critical in your ability to maintain blood flow and pressure.

Several times this summer I’ve come up against a level of depletion that left me light-headed, unable to raise my heart rate, and generally miserable.

Sweat is how your body cools, so you really do want to sweat, but humidity keeps that sweat from evaporating efficiently, and that amplifies your sweating and depletes you faster. This is not a secret. I am not enlightening anyone.

What I’m getting to, admittedly much more slowly than you’d like, is just how important it is to replace those electrolytes, to run or ride with something more potent than water. In fact, on the hottest, most humid days this summer, I’ve been drinking electrolyte mix, sometimes doubled in strength, and also bringing along salt tabs. 

If you sweat like I do, electrolyte mix alone doesn’t keep up with the rapid fluid loss and mineral depletion you experience, and salt tabs are a fast way to replace. And they weigh nothing.

If on the other hand, you don’t sweat much, you like the summer, the heat, and this all sounds like a pile of mansplaining bullshit, I’m sorry. But also, I hate you. JK. You’re great.