About 10 years ago, deep in my tennis obsession, I played a one-day singles tournament near my home here in North Carolina. It was a brutal day, like a lot of brutal summer days, but I thought if I drank a ton of water, I'd be fine. And I drank a ton of water. By the fifth match of the afternoon—yes, I was stupid—I hit a wall. I told my opponent that I was sorry, but there was no way I could keep going. When I got home, practically stumbling, I weighed myself to see how much I'd lost over the course of the day. To my surprise, I weighed three pounds more than I had in the morning. Soon after that, I started shivering, and the nausea and headaches kicked in. For the next 14 hours or so, I huddled on my couch, unable to sleep, and—TMI alert—visited the bathroom frequently. By the morning, bleary-eyed and depleted, I weighed myself again and had lost 14 pounds.
I probably should have been in a hospital that night—drinking a lot of water when you're sweating can have the effect of depleting your salt content and making things worse. It’s a state called hyponatremia, and my body was desperately trying to correct by getting rid of as much water as possible. So now I know what heat stroke feels like. It sucks.
I mention all this because the summer months are upon us and essentially everywhere in the U.S. it’s getting hot—hot enough to be cautious about heat stroke. In golf, the ability to take a cart can be the difference between playing or not. Mind you, whenever you talk about riding in a golf cart, you risk invoking the wrath of the walk cultists. I love walking, too, and do it 100 percent of the time when it's feasible. If you're in great shape, there may be no conditions too hot or humid for you (although I have watched friends in much better shape than me go down, so be careful with that logic). But here in North Carolina, there are at least three months of the year where it's a real decision with real consequences, and nobody should be shamed for riding when the alternative could be dangerous.
So, if you're like me and you want to walk whenever possible, how do you make the call on those marginal days?
The answer, of course, is "check the forecast," but that's not as simple as it sounds. For a very long time, I looked at temperature and humidity. And what I saw almost every time was that humidity decreased as the day went along. It might be 85 percent in the morning, but only 50 percent at midday. I thought that meant the moisture in the air was burning off; conditions would be hotter, sure, but also drier.
Richard Hamilton Smith
Boy was that wrong! You may already know this—and you may consider me an idiot for not—but the measure we call "humidity" is actually "relative humidity," and is a function of the current temperature. The hotter it gets, the more humid it can get, but that has the effect of making the actual percentage useless as a metric. Humidity will decrease as the temperature rises even when there's the exact same amount of moisture in the air. The number goes down because the temperature goes up, not because the moisture is actually decreasing. And if you wrongly believe that the 50 percent number means drier air, you could be in for a really bad time. (By the way, this is the same reason the "heat index" isn't all it's cracked up to be, because combining temperature plus relative humidity, the latter of which is already a function of temperature, leads to recursive data.)
OK, so if relative humidity and heat index don't tell us what we want to know, what should we use?
The first part of the answer is dew point. This is a measurement I'd heard about without knowing what it meant. I wrote it off as something for the kind of people who own a barometer and measure rainfall. In fact, dew point is extremely useful because it does what I thought relative humidity did, which is to tell me, in one number, how much moisture is in the air. It's all in the name—to quote Wikipedia, dew point is "the temperature the air needs to be cooled to (at constant pressure) in order to produce a relative humidity of 100 percent."
If that sounds confusing, it's fine, because you don't need to know the mechanics. All you need to know is that it's a constant number that tells you how soupy it will be outside. The higher the dew point, the worse the humidity. And unlike relative humidity, it doesn't rise and fall with the temperature. Here's the quick and dirty way to evaluate: When the dew point is 70 or higher, it's going to be gross outside.
As good as dew point is, though, it still needs to be evaluated in concert with temperature, because soupy air at 60 degrees is not the same as at 90 degrees (obviously). As it turns out, the answer can be as simple as adding the two numbers together. My friends and I call this the Kirschenfeld-Noodles Index, after the two guys who brought it to our attention (yes, one is named Noodles), and we use the verb "knoodling," as in "it's knoodling 160 right now." We did not invent it—some runners use it to dictate pace adjustments—but if it has another name, I can't find it.
As an absolute measure, the combined number gives a better indication of how it feels outside, and how I'll personally stand up to the weather, than anything I've ever come across. After a couple weeks of experimentation, here's my personal scale:
Below 150: Fine to walk.
150-155: Fine to walk, but a little unpleasant.
155-160: Still OK to walk, but definitely uncomfortable.
160-165: You can walk, but you will be soaked and depleted afterward.
165 and up: Ride.
The usual caveats apply—your numbers will likely be different than mine, and it's important to err on the side of caution, evaluate your own level of fatigue, stay hydrated, use electrolyte supplements and sun protection, and all that. Plus, you'll want to map out what the number will be throughout your round (I use the AccuWeather app), and not just at the start.
But right now, I feel like I've cracked a code. With the combination of dew point and temperature, I'm armed with the right kind of knowledge to know exactly how I'll react to the conditions. The decision to walk or ride is no longer a guessing game—it's backed up by reliable numbers. For me, a guy who has a lasting bad memory of sun stroke, it will have the effect of letting me walk more in the hottest months, because now I know exactly where my threshold is, and won't have to play it quite so safe. It takes fear out of the equation, and it's been a total game-changer.
