Short Wave - Why Dew Point Is This Summer's 'It Girl'

Episode Date: July 4, 2025

Happy Independence Day, Short Wavers! Do you have plans outdoors this weekend and want to figure out just how swampy it's gonna feel? For that kind of mental preparation, we're revisiting an episode in which some meteorologists are telling us to pay more attention to dew point temperature, not relative humidity. Interested in more weather episodes? Email us your question at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hey, shortwavers, Emily Kwong here. Okay, I know Summer just got here, but she is making her feelings known. There have been so many 90 and 100 degree days and afternoon thunderstorms, and then there was last month heat dome, which brought oppressive heat to the eastern half of the U.S. Experts forecast that heat waves will only become more intense and more frequent with climate change. So how can we proceed with summer? summer safely. Well, I'll let you in on a little secret. It's called the do-point. Former shortwave producer Thomas Liu reported this episode out a few years ago. You'll hear him

chatting with my former co-host, Maddie Safaya, about what the do-point is and why some meteorologists say it's the key to understanding how truly swampy it is outside. Stay cool and keep listening to Shortwave, the science podcast from NPR. Okay, Thomas Liu, Summer Hater. Here's a scenario. I wake up in the morning. I'm preparing to go on an afternoon bike ride on my hog around the city, check my weather app for the forecast. It's hot, you know, say about 85, but manageable. And the humidity is like 57%, let's say. Does that mean I'm still going to have like a nice bike ride or? Well, maybe, sort of. It's a little hard to hell with just humidity. Okay, but humidity is telling us how much water is in the air, right? Right, right. So yes, here's where it gets.

a little bit tricky. To understand this, we need to consider a couple of things, watering the air, temperature, and how these two interact with one another. So I called up someone I thought might have some answers. Hi, I'm Greg Jenkins. I'm a professor in the Department of Meteorology and Atmospheric Sciences at Penn State University. And Greg explained relative humidity like this. Relative humidity is this ratio or percentage of water vapor over a term that is related to water vapor in a saturated state. Okay, okay. So I'm going to oversimplify here, but relative humidity is the moisture content in the air compared to the maximum moisture content that could be in the air. Toaks, that's why it's called relative humidity. It's not an absolute measure of moisture.

Greg says a key factor in relative humidity is air temperature. You know, the number we usually look at when describing if it's going to be hot or cold out. Right, right. Warmer air can contain more moisture, while cooler air can contain less moisture. So over the course of a day... If you just had the amount of water vapor in the atmosphere fixed, and you let the temperature run its normal course, the relative humidity would go up and down just based on temperature.

Okay, so you can have the same amount of moisture all day, but the relative humidity will change as the day gets hotter or colder. Yep, indeed. So here's another way to think about it. Picture two drinking glasses, a pint glass and a juice glass. Okay. Since we know the warm air can hold more moisture, we'll use the larger pint glass to represent air that's 90 degrees Fahrenheit,

and the smaller juice glass to represent air that's 70 degrees of Fahrenheit. Got it. If you fill each glass with water about halfway, you'll have two different amounts of water in each glass. Oh, but you can still describe both glasses as having 50% relative humidity because they're both half full, even though one has much more water. Yes, exactly. That's exactly right. But I'm guessing that the 90 degree day or the bigger glass will probably feel a lot,

You'd probably be right, Maddie. But just seeing that percentage alone, you'll need to know a little more to help you decide how to dress for maximum comfort. Here, this is what Greg says. There are other terms that can be used to get an absolute sense of moisture in the atmosphere at any given time. Like dew point temperature,

which can be a little more helpful, if not a little more accurate, to gauge how much moisture is actually in the air at any given time. Yeah, dewpoint is the temperature at which any parcel of air would become saturated. So at this point, the amount of water vapor is at its max. You're really on this edge where physical droplets of water will kind of start to form. Oh, the dew and dew point.

Yes, exactly, the dew in dew point. I'm getting it. Basically, what's happening is that air has cooled to a point, the dew point, where it can no longer hold water in its gas form. The water molecules are slowing down and force out of the air, condensing around things like dust to form clouds. Or your car window will have dew all over it or the grass will have dew sitting on top of it. Condensation. Condensation, it is.

Okay, Thomas. So Dewpoint sounds, I don't know, more fun and a little bit more accurate. I'll say it. Oh, yeah, yeah. Totally, Maddie. So, you know, to get you a little more jazzed up. you know, like I am right now, you can totally find the dew point

in the same place you're looking at your daily forecasts on your phone or on your weather app. So it's not something that we really need to calculate. In fact, it's reported from all of our observations at most of the weather stations around the country. The dew point is more of a concrete number. So let's go back to our drinking glass metaphor. Both days had a relative humidity

of 50%. That doesn't really tell you much, right? Right. But when we look at dew point for the 90-degree day, the dew point was at 69 degrees. For the 75-degree day, the dew point was 51.

Yeah, that's easier to compare. I mean, that feels like a lot more helpful. Right, exactly. And that's the due point of it all. Okay. Here's a scale you can use based on the dew point to help you gauge

how comfortable you're feeling. Anything 55 and below is often described as dry or comfortable. Between 55 and 65, they'll start hearing words like sticky or damp. Dewpoints at 65 or above, people will start describing feeling like they're walking

through a swamp. When the dew point temperature starts to rise is telling us that the moisture in the environment is increasing. There's no doubt about that. And the closer, the higher it gets to the actual temperature, the worse is going to be in terms of how the body feels. Okay, Tilu, so we've chatted about relative humidity, the air temperature, and why dewpoint is a more absolute gauge of water in the atmosphere.

But, like, when it comes to how I feel, like, why exactly does it matter? Like, talk to me about some biology. Yeah. In high heat, like the days of summer, our bodies are constantly trying to keep cool. Oh, yeah. Through sweat. It's been a real sweaty summer out here, Thomas. Oh, it's been gross, Maddie.

I have been drenched all summer. But, you see, sweat is one way. our body cools itself off. And on hot, humid days, the air is already pretty saturated with water, which unfortunately means sweat evaporates more slowly. So this leaves our skin feeling damp and sticky, and, you know, it collects on our skin and keeps our body warmer. And moist and gross. You know what? Yep, that's exactly it, truly. But on a more serious note, Maddie, Greg told me it actually ties into climate change. So as the temperature rises, we can estimate what the heat stress is on the body,

just by looking at temperature and dew point on the heat index and on the human body. And that heat index he's referencing, it's a chart put out by the National Weather Service. We'll link to it in our episode notes. The chart takes into account temperature and moisture in the air to indicate how hot the weather feels to our body. In general, when the heat index goes above 90 degrees, you start to see health impacts. Okay? When a heat index goes beyond 105, the risk of heat exhaustion and cramping and sunstroke,

those go up and people start to show up in emergency rooms. If you go to the heat index beyond 130 degrees, you are likely going to see large numbers of fatalities. It's a little daunting to think about how much heat can affect our bodies. So on a day-to-day level, air temperatures, dewpoint, and relative humidity might help us decide what to wear or if we should go on a run or how much lotion to put on our bodies. But from a big picture level, it also helps us measure and predict the impact of heat, which is super important as the climate heats up. Okay, Thomas Liu, I had no idea about dewpoint.

I am a convert. I appreciate you. This was awesome. Thank you so much. Thank you. This episode was reported and produced by Thomas Liu, edited by Giselle Grayson and fact-checked by Indy Kara. The audio engineer for this episode was Quasey Lee. I'm Maddie Safaya. Thanks for listening to Shortwave from NPR.